Java Collections 2

1.What is meant by compatible equals() and hashCode() methods ?
In order for the Java Collections to work properly (and everything else in Java), the equals() and hashCode() methods must be compatible. Here, compatible means that if equals() reports that two instances are the same, then the hashCode() of both instances must be the same value.

2.Since Properties extends Hashtable, can I use the Hashtable methods to add elements to a Properties list ?
Technically speaking you can. However, you have to make sure you only add key-value pairs where both are strings. If you add something other than a String, the listing, loading, and saving methods won’t work as expected.
Like the Stack/Vector subclass relationship, Properties/Hashtable should be a has-a relationship, not an is-a/subclass relationship.

3.When I wrap a collection to be read-only or synchronized, why can’t I call any of the collection methods via reflection without getting an IllegalAccessException ?
When you wrap a collection through the static methods of the Collections class, this creates an instance of a package-private (default access) class. Because you don’t have access to these classes, you can’t call their methods via reflection (though you can call their methods directly through the appropriate interface).

4.What is a weak reference and what are they used for ?
Normally the Java garbage collector plays safe. It will only free up the memory used by an object when that object can no longer be accessed by the program. Once an object become impossible to reach it is eligible for collection, and eventually its memory will be reclaimed.
This eliminates one of the most common programming errors in some other languages (like C++), where code accidentally tries to access an object that has been freed. Unfortunately it can lead to another problem, where you leave open a potential access route to an object that you don’t need any more. Memory fills up, and the program slows down or reports an “Out of Memory” error.

To avoid this, you can be very careful to close off access paths to an object once you have finished using it. Java 2 introduces another alternative, the weak reference. Weak references provide access to an object without preventing it from being freed. When you use a weak reference you have to accept that the object referred to may have disappeared, which results in the reference being automatically set to null. On the other hand, the weak reference will not hold the object in memory once it is inaccessible via normal references (or via “soft” references – see below). Weak references are not appropriate in all circumstances, but sometimes they can make code easier to write and understand.

The most common use of weak references is indirect – they are used internally by the WeakHashMap class. Like HashMap, WeakHashMap associates key objects with values. However, once the key object becomes inaccessible via stronger references it becomes eligible for garbage collection. When it is freed, the map entry magically disappears. The assumption here is that if you are not using the key anywhere other than in the map you will have no need to look it up, so it should be freed.

Other specialist references are soft references (which inhibit collection until memory runs short), and phantom references (used for cleanup when objects are freed).

5.What is the minimum number of key-value pairs for which it makes sense to use a HashMap, as opposed to using a pair of arrays (one for keys, the other for values) with brute-force key searches ?

Many people often need maps for very small numbers (2-5) of key-value pairs. When does it make sense to forgo the convenience of the HashMap to avoid the associated overhead?
Well, is there really that much of a performance loss using a HashMap? There is no synchronization penalty (unless you impose your own). You can tune the sizing by adjusting the initial size and load factor. Plus, do you really want to be responsible for “rolling your own” code to handle the dynamic resizing of the key and value arrays, inserting/removing data from these arrays, optimizing the searching algorithm, etc. Yuck!

In general, the performance hit associated with using a general purpose Map (such as the HashMap) is far outweighed by the benefits of using a simple interface backed by a tested algorithm.

The only reason I could see wanting to use arrays is to guaruntee the type of your key/values to add type checking and avoid casting. Still, if this is a critical aspect of your application, you can wrap your HashMap in another object to provide type-safety, and the casting overhead should be minimal.

Another alternative to creating a custom solution is to explore other collection classes, such as ObjectSpaces’s JGL Libraries. There may be something there that would suit your needs.

So, to answer your question, I would say that the fewer the key-value pairs you have, the more reason you have to use a HashMap. Since the fewer the keys, the faster the search, why not use it for 2-5 key-value pairs. I would think that only when you get to many pairs (tens of thousands) and there is a performance problem you should consider an alternative. Basically, exhaust your search of tried-and-true collections before you try a custom solution. Let other people create these collections so you can focus on your application.

6.How does ArrayList increase its capacity ?
Unlike Vector where you can specify a capacity increment, ArrayList doesn’t support this. Instead, ArrayList will increase capacity by about a half when it runs out of space. The refernece implementation uses the forumla:
newCapacity = (oldCapacity * 3)/2 + 1
though, this isn’t part of the class definition so others can implement it differently.

7.What is the default initial size for a Hashtable / HashMap ?
This depends on what version of Java you are using. For JDK 1.2, the size was 101. For JDK 1.3, the size changed to 11.

8.How do you create a multi-dimensional List ?
Since the elements of a List are objects, in order to make the List multi-dimensional, each object in the outer list needs to be a List, too. For instance, …

List list = new ArrayList(10);
for (int i=0; i<10; i++) {
list.set(i, new LinkedList());
}

Then just fill up each inner list with items.

9.In a TreeMap, can I use a sorting algorithm other than the natural sorting for the keys ?
You can pass a Comparator to the TreeMap constructor to use a sorting order other than the natural order.

10.What are the differences between HashMap and Hashtable ?
Both provide key-value access to data. The Hashtable is one of the original collection classes in Java. HashMap is part of the new Collections Framework, added with Java 2, v1.2.
The key difference between the two is that access to the Hashtable is synchronized on the table while access to the HashMap isn’t. You can add it, but it isn’t there by default.

Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn’t. If you change the map while iterating, you’ll know.

And, a third difference is that HashMap permits null values in it, while Hashtable doesn’t.

For new code, I would tend to always use HashMap.

11.What are the differences between Vector and ArrayList? Which is best to use ?
Vector and ArrayList are very similar. Both of them represent a ‘growable array’, where you access to the elements in it through an index.

ArrayList it’s part of the Java Collection Framework, and has been added with version 1.2, while Vector it’s an object that is present since the first version of the JDK. Vector, anyway, has been retrofitted to implement the List interface.

The main difference is that Vector it’s a synchronized object, while ArrayList it’s not.

While the iterator that are returned by both classes are fail-fast (they cleanly thrown a ConcurrentModificationException when the orignal object has been modified), the Enumeration returned by Vector are not.

Unless you have strong reason to use a Vector, the suggestion is to use the ArrayList.

12.How should I implement object comparisons in a flexible manner? For example, I have a Person class and sometimes I will compare based on name and sometimes I will compare based on age.
Instead of having the Person class implement the Comparable interface, you could delegate the comparing to another class. Perhaps you could have a PersonComparator interface that you could implement for the various types of comparisons. For example:
public interface Person {
public String getName();
public int getAge();
}

public interface PersonComparator {
public int compare(Person p1, Person p2);
}

public class AgeComparator implements PersonComparator {
public int compare(Person p1, Person p2) {
if (p1.getAge() == p2.getAge()) return 0;
return p1.getAge() > p2.getAge() ? 1 : -1;
}
}

public class NameComparator implements PersonComparator {
public int compare(Person p1, Person p2) {
return p1.getName().compareTo(p2.getName());
}
}

This is a very simple example of the Strategy Pattern. This allows your comparisons and your object to change independent of one another.

13.Does the equals() method of an array do element-level checking ?
If you have two arrays in memory with the same elements, and ask first.equals(second), this does not do an element-by-element comparison. Instead, it behaves just like Object’s equals() method, essentially asking if the variables point to the same place in memory:
int a[] = {1, 2, 3};
int b[] = {1, 2, 3};
// This prints false
System.out.println(a.equals(b));

To check for equality of two arrays, use Arrays.equals().
// This prints true
System.out.println(Arrays.equals(a,b));

14.How can I retrieve the items in my HashSet / HashMap in the order they were added ?
Prior to Java 1.4, you had to manage a separate insertion order list yourself. Starting with Java 1.4, you can use the new LinkedHashMap / LinkedHashSet classes. The iterators you get back from them return the items in insertion order.

15.How do you sort an ArrayList (or any list) of user-defined objects ?
Create an implementation of the java.lang.Comparable interface that knows how to order your objects and pass it to java.util.Collections.sort(List, Comparator).

16.How can you get the hash code for an instance of a class if the class overrode hashCode() ?
The System class method identityHashCode() allows you to get this information:
int code = System.identityHashCode(anObject);

17.How can I easily shift the elements in a List / Vector such that all the elements rotate n elements ?
The Java 1.4 API adds a rotate() method to the Collections class: rotate(List list, int distance) that will shift the elements for you.

18.What’s the most optimum way of swapping two elements in a List ?
The 1.4 version of Collections has a swap() method to do this for you. However, for earlier version of Java, you can swap two elements w/o an intermediate variable with:
list.set(index1, list.set(index2, list.get(index1)));
This works because the set() method returns the original element.

19.What’s the purpose of the IdentityHashMap ?
The IdentityHashMap uses == for equality checking instead of equals(). This can be used for both performance reasons, if you know that two different elements will never be equals and for preventing spoofing, where an object tries to imitate another.

20.How do I convert an old-style Enumeration to something in the Collections Framework ?
Prior to Java 1.4, any conversion had to be manually done. With the introduction of 1.4, you can call Collections.list(enumeration) to automatically convert the Enumeration to an ArrayList.

21.How do I retrieve the values of a Hashtable/HashMap in sorted order ?
Basically, you can’t directly do this. What you can do is get the Collection of values() back from the map and create a sorted collection, or maintain two maps, one in each direction, and keep the second map sorted by being a TreeMap. Which you use depends on the frequency you must sort the elements.

22.How can I add a Collection to another Collection ?
The java.util.Collection interface includes an addAll(Collection c) method to add one collection to another.

23.How can I use two iterators to go through a collection ?
Just get a separate iterator for each loop:
Collection l = …;
for(Iterator i = l.iterator(); …) {
for(Iterator j = l.iterator();…) {
}
}

24.How do I traverse a map backwards ?
Just keep getting the last key and the head map before it:
if (!map.isEmpty()) {
Object last = map.lastKey();
boolean first = true;
do {
if (!first) {
System.out.print(“, “);
}
System.out.print(last);
last=map.headMap(last).lastKey();
first=false;
} while (last != map.firstKey());
System.out.println();
}

25.How do I traverse a sorted set backwards ?
Just keep getting the last element and the head set before it:
if (!set.isEmpty()) {
Object last = set.last();
boolean first = true;
do {
if (!first) {
System.out.print(“, “);
}
System.out.print(last);
last=set.headSet(last).last();
first=false;
} while (last != set.first());
System.out.println();
}

26.How can I go through an Iterator mulitple times ?
There is no direct support for this. You’ll need to create your own caching mechanism. For instance, as you go through the Iterator the first time, add the elements to a LinkedList. Then, you can just get an Iterator from the LinkedList for the second pass through.

27.What’s new to the Collections Framework in Java 1.4 ?
There are three new implementations:
LinkedHashSet
LinkedHashMap
IdentityHashMap
One marker interface:
RandomAccess
And six new utility methods for the Collections class:
rotate(List list, int distance)
replaceAll(List list, Object oldVal, Object newVal)
indexOfSubList(List source, List target)
lastIndexOfSubList(List source, List target)
swap(List list, int i, int j)
list(Enumeration e)

28.How can I add an array of objects to a collection ?
First you need to convert the array to a Collection. This can be done with Arrays.asList(objectArray). Once you have the array as a List, you can add it to another Collection with theCollection.addAll(theList).

29.Is Vector’s clone method thread-safe ?
Sure it is, since it is a Vector which is thread-safe.

30.How do I load property settings with the Properties class ?
java.util.Properties objects can load values from a file using the method load(InputStream).
Here is the code you need:

Properties props = new Properties();
props.load(new FileInputStream(“propertyfile.properties”));
String value = props.getProperty(“propertyname”);

//Just a trick: in a web archive (war) you can get the InputStream inside the war archive using
ClassLoader cl = this.getClass().getClassLoader();
InputStream is = cl.getResourceAsStream(“it/package/application.properties”);

This is better than using a FileInputStream, because you are loading the file within the archive as it was a resource. You should use this.getClass().getClassLoader() to use the same ClassLoader as the one used the servlet container to load your JSP/Servlet. This code is snipped from a JSP page inside Tomcat.

31.How do I save properties settings with the Properties class ?
Try this:
Properties prop = new Properties();
FileOutputStream output = new FileOutputStream(“Test.properties”);
prop.store(output,”my testproperties”);
output.flush();
output.close();

You’ll need to catch an IOException.

32.What happens if two threads perform a get of one hashmap at the same time ?
Synchronization needs to be done only when there is a chance of changing the data from different threads simultaneously. In your case, it is simply going to be a read, the synchronization is not required. If you need to remove or modify the values in the hashmap, then you [may] need to synchronize that.

For synchronizing a HashMap, you can use Collections.synchronizedMap() which will return a synchronized map for you, which is thread-safe.

Remember, synchronization will cause a performance problem. So, it needs to be used carefully, when really required.

33.How can I convert a Collection to an Array then back to a Collection ?
The Collection interface provides a mechanism to turn a Collection into an Array using the methods T[] toArray(T[] a) or Object[] toArray(). The first method will return a Array containing all the elements of the Collection with the array being that of the type provided in the method call. The second method just returns an array being of an Object[] type.

The Arrays class provides the opposite. A way to turn an array into a List using the List asList(Array[] a) method. The List returned is of a fixed length with any attempts to add an element throwing an UnsupportedOperationException.

import java.util.*;

public class G{
public static void main(String[] args){
List sun = new ArrayList();
sun.add(“Feel”);
sun.add(“the”);
sun.add(“power”);
sun.add(“of”);
sun.add(“the”);
sun.add(“Sun”);
String[] s1 = sun.toArray(new String[0]); //Collection to array
for(int i = 0; i < s1.length; ++i){
String contents = s1[i];
System.out.print(contents);
}
System.out.println();
List sun2 = Arrays.asList(s1); //Array back to Collection
for(String s2: sun2){
String s3 = s2; System.out.print(s3);
}
//sun2.add(new String(“Hello”)); // throws UnsupportedOperationException
}
}

34.How can I create a Collection based on another Collection ?
Every concrete implementation provides a constructor, which takes a Collection as an argument. Care must be taken when creating a Collection based another Collection, this is because depending on the target concrete implementation being created, specific rules regarding duplicates may be be enforced. Such as creating a Set based on a List.

The following is a short list of the constructors provided by some of the concrete Classes of the JCF (Java Collections Framework), which enable the creation of a Collection based an another implementation.

ArrayList(Collection c)
LinkedList(Collection c)
Vector(Collection c)
TreeSet(Collection c)

Creating a Collection based on another Collection is quite easy. The hard part is knowing which Collection to use based on performance and other issues.

For example the ArrayList created will contain the same elements in the same order as the first ArrayList created.

List slist = new ArrayList();
slist.add(“g”);
slist.add(“a”);
slist.add(“d”);
slist.add(“a”);
slist.add(“f”);
slist.add(“e”);
slist.add(“c”);
slist.add(“b”);
for(String s : slist){
System.out.print(s + “\t”);
}
System.out.println();
List s2list = new ArrayList(slist);
for(String s : s2list){
System.out.print(s + “\t”);
}

When creating a Set based on an existing List the Set will be void of duplicates.
List slist = new ArrayList();
slist.add(“g”);
slist.add(“a”);
slist.add(“d”);
slist.add(“a”);
slist.add(“f”);
slist.add(“e”);
slist.add(“c”);
slist.add(“b”);
for(String s : slist){
System.out.print(s + “\t”);
}
System.out.println();
Set s2set = new TreeSet(slist);
for(String s : s2set){
System.out.print(s + “\t”);
}

35.How can I define my own Comparable type so that it can be naturally sorted within a List ?
When taking a peek at the Java docs you will notice certain classes implement an interface named Comparable. Take a look at some of the subclasses of Number such as Byte, Integer, Long, Float or some of the classes like String and Date. What the Comparable interface provides is a way for a class to be sorted by it’s natural ordering. So what do we mean by natural ordering? Depending on the type wishing to be sorted the natural ordering can be different things. If we are sorting Strings the ordering is lexicographic or alphabetic if we are sorting Dates the ordering is chronological if we are sorting Integers the ordering is numerical.

Comparable only contains one method that needs to be implemented by the class wishing to be sorted naturally. Remember if you try and sort a list that contains elements that do not implement the Comparable interface then Collections.sort() will throw an exception specifically a ClassCastException.

public interface Comparable{
public int compareTo(T o);
}

The following is a short example on how to implement the Comparable interface and use the compareTo(T o) method.
import java.util.*;

public final class Alpha implements Comparable{
public static void main(String[] args){
List alpha = new ArrayList();
alpha.add(new Alpha(“z”));
alpha.add(new Alpha(“g”));
alpha.add(new Alpha(“k”));
alpha.add(new Alpha(“q”));
alpha.add(new Alpha(“a”));
alpha.add(new Alpha(“b”));
alpha.add(new Alpha(“o”));
alpha.add(new Alpha(“v”));
alpha.add(new Alpha(“c”));
Collections.sort(alpha);
System.out.println(alpha);
}
private String letter;
public Alpha(String letter){
if(letter == null){throw new NullPointerException();}
this.letter = letter;
}
public String toString(){return letter;}
public boolean equals(Alpha a){
if(!(a instanceof Alpha))
return false;
return letter.equals(a.letter);
}
public int compareTo(Alpha a){
int i = letter.compareTo(a.letter);
return i;
}
}

More complex examples might included sorting on multiple fields. Most things that you would have to sort probably have more then one part like a name for instance (First:Middle:Last) or maybe you have to sort in (Brand:Model) order.
import java.util.*;

public final class Car implements Comparable{
public static void main(String[] args){
Car[] cararry = {new Car(“Toyota”,”Celica”), new Car(“Honda”,”Civic”),
new Car(“Ford”,”Mustang”), new Car(“Lexus”,”ES”), new Car(“Acura”,”Integra”),
new Car(“Honda”,”Accord”), new Car(“Acura”,”RL”), new Car(“Toyota”,”Avalon”)
};
List car = Arrays.asList(cararry);
Collections.sort(car);
System.out.println(car);
}
private String brand;
private String model;
public Car(String brand, String model){
if(brand == null || model == null){throw new NullPointerException();}
this.brand = brand;
this.model = model;
}
public String toString(){return brand + ” ” + model;}
public boolean equals(Car car){
if(!(car instanceof Car))
return false;
boolean samebrand = brand.equals(car.brand);
return samebrand != true ? samebrand: model.equals(car.model);
}
public int compareTo(Car car){
int i = brand.compareTo(car.brand);
return(i != 0 ? i : model.compareTo(car.model));
}
}

36.What are Generics and how can I use them ?
One of the biggest additions since the creation of the Collections framework is Generics.This long awaited update to the type system is a welcomed feature, which C++ developers have had in their toolbox for years using the STL. A generic type is defined using one or more type variables with it’s contained methods using that type variable as a place holder to mark a parameter or return type. For instance the interface List has now been defined as.

public interface List{
public add(E e);
Iterator iterator();
}
public interface Iterator{
E next();
boolean hasNext();
}

Type Safe Collections

So you might ask. What are Generics and why should I use them? Generics are a way to restrict a data structure to hold only a specific type thus providing compile time type checking. One of the added bonuses is that it is no longer necessary to cast a returned Object to a specific type because the compiler is aware of what type is being held by the Collection and what type is to be returned.

Set s = new SortedSet();
s.add(new String(“Java”));
String j = (String) s.get(0); // cast required;

// java 5
Set s = new SortedSet();
s.addElement(new String(“String Type”));
String s = s.get(0); // no cast required!

Having a type safe system not only obviates the need to cast to a specific type but shields the programmer against miss-casting or casting to an unrelated type at runtime.

String s = “Java, write once run anywhere”
List l = new ArrayList();
l.add(new String(“Java”));
Integer i = (Integer)l.get(0); // Runtime exception! ClassCastException!

// Using Generics the compiler catches
String s = “Java. Write once run anywhere!”
List l = new ArrayList();
l.add(new String(“Java”));
Integer i = l.get(0);

Generics and Subtyping

Generics do not share some of the things that are commonly held true in the Java language and one of them is subtyping. One would assume the following perfectly legal since it is true that Object is a supertype of String. But the following is in fact illegal and will cause a compile time error.

List

Java Collections 1

1.How to find the maximum size of heap used in the memory in java ?
maxMemory(): Returns the maximum amount of memory that the Java virtual machine will attempt to use
totalMemory(): Returns the total amount of memory in the Java virtual machine.
Also you can use -Xmx parameter to set the max heap size.

2.What is the difference between marshalling and unmarshalling ?
marshling is convert from byte code to network understand able format
unmarshsling is converting from network understand able fromat to byte code convertion.

3.What is the Collections API ?
The Collections API is a set of classes and interfaces that support operations on collections of objects.

4.What is the List interface ?
The List interface provides support for ordered collections of objects.

5.What is the Vector class ?
The Vector class provides the capability to implement a growable array of objects.

6.What is an Iterator interface ?
The Iterator interface is used to step through the elements of a Collection .

7.Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.

8.What is the GregorianCalendar class ?
The GregorianCalendar provides support for traditional Western calendars

9.What is the Locale class ?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region .

10.What is the SimpleTimeZone class ?
The SimpleTimeZone class provides support for a Gregorian calendar .

11.What is the Map interface ?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

12.What is the highest-level event class of the event-delegation model ?
The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.

13.What is the Collection interface ?
The Collection interface provides support for the implementation of a mathematical bag – an unordered collection of objects that may contain duplicates.

14.What is the Set interface ?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

15.What is the typical use of Hashtable ?
Whenever a program wants to store a key value pair, one can use Hashtable.

16.I am trying to store an object using a key in a Hashtable. And some other object already exists in that location, then what will happen? The existing object will be overwritten? Or the new object will be stored elsewhere?
The existing object will be overwritten and thus it will be lost.

17.What is the difference between the size and capacity of a Vector?
The size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time.

18.Can a vector contain heterogenous objects ?
Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object.

19.Can a ArrayList contain heterogenous objects ?
Yes a ArrayList can contain heterogenous objects. Because a ArrayList stores everything in terms of Object.

20.What is an enumeration ?
An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection.

21.Considering the basic properties of Vector and ArrayList, where will you use Vector and where will you use ArrayList ?
The basic difference between a Vector and an ArrayList is that, vector is synchronized while ArrayList is not. Thus whenever there is a possibility of multiple threads accessing the same instance, one should use Vector. While if not multiple threads are going to access the same instance then use ArrayList. Non synchronized data structure will give better performance than the synchronized one.

22.Can a vector contain heterogenous objects ?
Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object

23.How do I use an Iterator to go through a Collection ?
Collection collection = …;
Iterator iterator = collection.iterator();
while (iterator.hasNext()) {
Object element = iterator.next();
// Do something with element
}
}

24.How do I use a ListIterator to go through a List backwards ?
List list = …;
ListIterator iterator = list.listIterator(list.size());
while (iterator.hasPrevious()) {
Object element = iterator.previous();
// Process element
}

25.How do I count the frequency of some word/object ?
The Map interface can be used to count the number of times a word/object appears. The following program demonstrates counting word frequency from the command line:

import java.util.*;

public class MapExample {
public static void main(String args[]) {
Map map = new HashMap();
Integer ONE = new Integer(1);
for (int i=0, n=args.length; i

26. How do I sort objects into their reverse natural ordering ?
The Collections.reverseOrder() method returns a Comparator that sorts objects that implement the Comparable interface in reverse order.

27.How do I use Enumeration to iterate through a collection ?
Enumeration enum = …;
while (enum.hasMoreElements()) {
Object element = iterator.nextElement();
// process element
}

28.How do I make an array larger ?
You cannot directly make an array larger. You must make a new (larger) array and copy the original elements into it, usually with System.arraycopy(). If you find yourself frequently doing this, the Vector class does this automatically for you, as long as your arrays are not of primitive data types.

29.How do you store a primitive data type within a Vector or other collections class ?
You need to wrap the primitive data type into one of the wrapper classes found in the java.lang package, like Integer, Float, or Double, as in:

Integer in = new Integer(5);

30.How do I create linked lists if there are no pointers ?
No pointers does not mean no reference variables. You just can’t deference them as you can in C/C++ or perform pointer arithmetic. You can still use abstract data types that require dynamic data structures. See the LinkedList class for an example of a linked list implementation.

31.How do I use an array with the Collections Framework ?
The Arrays.asList() method provides a fixed-length List view of an array, where changes to the List are stored in the original array. The Arrays class also provides additional support methods for sorting and searching an array.

32.Which is faster, synchronizing a HashMap or using a Hashtable for thread-safe access ?
Because a synchronized HashMap requires an extra method call, a Hashtable is faster for synchronized access.

33.Which is the preferred collection class to use for storing database result sets ?
When retrieving database results, the best collection implementation to use is the LinkedList.
The benefits include:
Retains the original retrieval order
Has quick insertion at the head/tail
Doesn’t have an internal size limitation like a Vector where when the size is exceeded a new internal structure is created (or you have to find out size beforehand to size properly)
Permits user-controlled synchronization unlike the pre-Collections Vector which is always synchronized

Basically:

ResultSet result = stmt.executeQuery(“…”);
List list = new LinkedList();
while(result.next()) {
list.add(result.getString(“col”));
}

If there are multiple columns in the result set, you’ll have to combine them into their own data structure for each row. Arrays work well for that as you know the size, though a custom class might be best so you can convert the contents to the proper type when extracting from databse, instead of later.

34.Why doesn’t the Iterator interface extend the Enumeration interface ?
If the Iterator interface extended the Enumeration interface, the Iterator interface would end up with five methods where two methods just called other methods in the interface. The designers of the framework wanted to get rid of the cumbersome Enumeration method names so had the Iterator interface stand on its own with new shorter method names.

35.How do I print a Collection ?
The Collection Framework implementation classes override the toString() method to print out all the elements of the collection. If you create your own custom implementation, as long as your class is a subclass of AbstractMap or AbstractCollection you’ll inherit this behavior. (Keep in mind that AbstractList and AbstractSet subclass AbstractCollection.)

36.How do I synchronize a collection ?
With the Collections Framework, the new implementations are all unsynchronized by default. If you need synchronized access, you must synchronize things yourself. The Collections class offers a wrapper method for each of the six core collection interfaces that add synchronization to an arbitrary collections implementation. To ensure thread-safety, direct access to the original backing collection must be avoided.
For example, the following will synchronize an arbitrary List and lose the original reference so you can’t access it directly:

List list = …;
list = Collections.synchronizedList(list);

37.How I do a case-sensitive sort in a language-insensitive manner ?
If you have an array of primitives or an array of equivalent objects that implement the Comparable interface, all you need to do is call the sort() method of the java.util.Arrays class. If the class doesn’t implement Comparable, you need to provide your own Comparator implementation to the sort() method.

38.How do I get the length of an array ?
To avoid getting an ArrayIndexOutOfBoundsException, you can check for the array length from either the length instance variable or using reflection and calling java.lang.reflect.Array.getLength(), passing the array as an argument to the method.
int length = args.length;
// or
int length2 = Array.getLength(args);

39.How can I speed up array accesses and turn off array bounds checking ?
You cannot. It is part of the security architecture of the Java runtime to ensure never accessing invalid memory space.

40.How do I get the list of system properties that tell me things like which version of Java a user is running and their platform-specific line separator ?
The System.getProperties() method will return the standard property set. However, in untrusted applets, you can only ask for specific properties, as in System.getProperty(“java.version”).

41.How do I sort an array ?
The Arrays class in java.util provides a series of sort() methods for sorting arrays. If the array is an array of primitives or an array of a class that implements Comparable then you can just call the method directly:

Arrays.sort(theArray);

If, however, it is an array of objects that don’t implement the Comparable interface then you need to provide a custom Comparator to help you sort the elements in the array.

Arrays.sort(theArray, theComparator);

42.How can I implement a List (ordered collection) that keeps an index (i.e. a Map) of its contents ?
You can’t. Each of the Map and List interfaces define a remove(Object o) method. Each method returns a different type (Map returns an Object while List returns a boolean). Because the compiler doesn’t permit overloaded methods that differ by only return type, you cannot create a class that implements both the List and Map interface.

If you need a Map that maintains insertion order, see the LinkedHashMap added in Java 1.4.

43.Is there a way to create a homogenous collection in Java? How do I make a collection where all the elements within it are a specific data type ?
You can wait for Generics to be added to Java or…

You’d have to build one yourself. Basically, you’re creating a class that is a Proxy (Gang Of Four design pattern) around the actual Collection.

This works similarly to the way the (hidden) Synchronized versions in Collections work. They contain a reference to the original collection object that does the “real” work, but restrict access to it by synchronizing all the access methods.

What you’d be doing in this case is restricting access by keeping a reference to the Class object of the class you want to restrict your collection to contain, and throw IllegalArgumentExceptions whenever there’s an attempt to add an object not of that class.

There’s no way to enforce compile-time type safety in this manner. The API using “Object” references can’t be changed in Collection.

You can also wrap a collection by a Proxy that provides the alternative API that has type-safe method signatures. This is what’s done by the StringTokenizer API (which implements Enumeration, and wraps Enumeration with String versions of the same methods).

Otherwise, create your own class with its own signatures, and keep the Collection strictly private. This is best done if you are designing a class for others to use.

44.How does a Hashtable internally maintain the key-value pairs ?
The Hashtable class uses an internal (private) class named Entry to hold the key-value pairs. All entries of the Hashtable are stored in an array of Entry objects with the hash value of the key serving as the index. If two or more different keys have the same hash value these entries are stored as a linked list under the same index.

45.How do I look through each element of a HashMap ?
To go through all the elements of a HashMap, or any class that implements the Map interface, call the entrySet() or keySet() methods than loop through what is returned. The entrySet() returns a group of Map.Entry elements, whereas the keySet() method just returns a Set of key elements. If you then want what the key refers to, you’d have to look them up.
Once you have a Set to work with, you would then use an Iterator to go through all its elements. The following demonstrates:

Map map = some hash map
Set set = map.keySet();
Iterator it = set.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}

46.How do I create a read-only collection ?
The Collections class has six methods to help out here:
unmodifiableCollection(Collection c)
unmodifiableList(List list)
unmodifiableMap(Map m)
unmodifiableSet(Set s)
unmodifiableSortedMap(SortedMap m)
unmodifiableSortedSet(SortedSet s)

If you then get an Iterator from one of these unmodifiable collections, when you call remove() it will throw an UnsupportedOperationException

47.How can I process through the keys of a Hashtable in sorted order ?
In order to get all the keys for a Hashtable, you use the keys() method to get an Enumeration or the keySet() method to get a Set. If you are using Java 2, and can use the collections framework, what you should do is get the key set of the Hashtable and create a TreeSet from it. You can then get an iterator() from the created TreeSet that will have the keys in order. If you can’t use the collections framework, you’ll have the sort the Enumeration you get back from keys() yourself.

48.Which collections in Java are synchronized and which aren’t ?
The original collection classes in Java are all synchronized: Vector and Hashtable, along with their subclasses Stack and Properties. Those classes introduced with the Java 2 Collections Framework are all NOT synchronized by default, the sets, lists, and maps

49.What are the differences between ArrayList and LinkedList ?
An ArrayList is a List implementation backed by a Java array, similar to the Vector class. As the number of elements in the collection increases, the internal array grows to fit them. If there are lots of growth periods, performance degrades as the old array needs to be copied into the new array. However, random access is very quick as it uses an array index to access.
With a LinkedList, the List implementation is backed by a doubly linked list data structure, allowing easy inserts/deletions anywhere in the structure, but really slow random accesses as the access must start at an end to get to the specific position.

Which you use really depends on the type of operations you need to support.

50.How do I read input from a stream “one word at a time” ? What you need to do is use the java.util.StringTokenizer or java.io.StreamTokenizer to parse your input into words. Each has a default set of delimiters like white space that can be changed. The following demonstrates the using of StringTokenizer to count words in a file.

import java.io.*;
import java.util.*;

public class Test {
static final Integer ONE = new Integer(1);

public static void main (String args[])
throws IOException {

Map map = new TreeMap();
FileReader fr = new FileReader(args[0]);
BufferedReader br = new BufferedReader(fr);
String line;
while ((line = br.readLine()) != null) {
processLine(line, map);
}
printMap(map);
}
static void processLine(String line, Map map) {
StringTokenizer st = new StringTokenizer(line);
while (st.hasMoreTokens()) {
addWord(map, st.nextToken());
}
}
static void addWord(Map map, String word) {
Object obj = map.get(word);
if (obj == null) {
map.put(word, ONE);
} else {
int i = ((Integer)obj).intValue() + 1;
map.put(word, new Integer(i));
}
}
static void printMap(Map map) {
Set set = map.entrySet();
Iterator it = set.iterator();
while (it.hasNext()) {
Map.Entry entry = (Map.Entry)it.next();
System.out.println(entry.getKey() +
“: ” + entry.getValue());
}
}
}

51.Do the keys() and elements() methods of a Hashtable enumerate things in the same order ?
There is no requirement that the elements are returned in the same order, only that all of them are returned in the Enumeration. Your best bet for getting the element for a key is to loop through the keys() returned and fetch the element for each. Or, if you can use a HashMap instead of a Hashtable, work with the Map.Entry that is returned by the keySet(). This includes both the key and value together, not requiring a separate lookup.

52.How do I treat an object I get out of a Vector (collection) as the type I put into it ?
When you get an object out of a Vector (or any collection), the object is returned as being of type Object. You need to cast it back into the object type you put into the data structure if you need to call or treat the object as the original type.
For instance, if you add an array to a vector:

String args[] = {“1″, “2″, “3″};
Vector v = new Vector();
v.addElement(args);

Then, when you get the object out of the vector, you need to cast it back to the original type:
String args2[] = (String[])v.firstElement();
System.out.println(args2.length);

53.What is the difference between a singly linked list and doubley linked list ?
A singly linked list is one that has only a pointer/reference to the next element in the list. A doubley linked list has both a previous and next pointer/reference.

54.What is meant by natural ordering of objects in the context of the collections framework ?
Java documentation refers to the natural ordering of objects when describing various algorithms and data structures in the collections framework. Object ordering is obviously needed by the sorting algorithms. It is also needed by the specifications of the interfaces such as SortedSet, SortedMap, etc., and the data structures used for container classes such as TreeSet, TreeMap, etc. Unless instructed otherwise via a Comparator object supplied as an argument to the constructor, the default behavior of a class such as TreeSet is to store its objects in an ascending natural order.
The objects of a class exhibit natural ordering if the class has implemented the java.lang.Comparable interface. Such a class must provide an implementation for the compareTo method — referred to as the class’s natural comparison method — that can then be used by the algorithms and the data structures for comparing data objects. The compareTo method must return a negative integer, a zero, or a positive integer if the object on which it is invoked is less than, equal to, or greater than the argument object.

It is strongly recommended that a class’s natural ordering as dictated by the implementation of the compareTo method be consistent with equals. This consistency is achieved if and only if e1.compareTo( (Object) e2 ) == 0 has the same boolean value as e1.equals( (Object) e2 ) for every pair of objects e1 and e2 of the class. Lack of this consistency could elicit strange behavior from the data structures that need to compare objects.

Many of the system supplied classes possess natural ordering. These include String, Integer, Float, Double, Date, File and many others. For the String class, the natural order is lexicographic; it is chronological for the Date class; lexicographic on the pathname for the File class, etc.

55.What is a fail-fast iterator ?
An iterator is considered fail-fast if it throws a ConcurrentModificationException under either of the following two conditions:
In multithreaded processing: if one thread is trying to modify a Collection while another thread is iterating over it.

In single-threaded or in multithreaded processing: if after the creation of the Iterator, the container is modified at any time by any method other than the Iterator’s own remove or add methods.
Note in particular what is implied by the second condition: After we create a container’s iterator, during a loop that iterates over the container we must only use the remove (and when applicable add) methods defined for the iterator and that we must NOT use the same methods defined for the container itself. To illustrate this point, suppose we declare and initialize a List in the following manner
List list = new ArrayList();
list.add(“Peter”);
list.add(“Paul”);
list.add(“Mary”);

Let’s say we wish to iterate over this list. We’d need to declare a ListIterator as follows:
ListIterator iter = list.listIterator();

Having created this iterator, we could now set up a loop like:
while(iter1.hasNext()){
String str = iter1.next();
// do something with str
}

Because iter is fail-fast, we are not allowed to invoke List’s add or remove methods inside the loop. Inside the loop, we are only allowed to use ListIterator’s add and remove methods. This makes sense because it is the Iterator object that knows where it is in a List as the List is being scanned. The List object itself would have no idea of that.
The Iterators supported by all the work-horse container classes, such as ArrayList, LinkedList, TreeSet, and HashSet, are fail-fast. The Iterator type retrofitted to the older container class Vector is also fail-fast. For associative containers, such as HashMap and the older HashTable, the Iterator type for the Collections corresponding to either the keys or the values or the pairs are fail-fast with respect to the container itself. That means that even if you are iterating over, say, just the keys of the container, any illegal concurrent modifications to the underlying container would be detected.

One final note regarding iterators versus enumerations: It is also possible to use an Enumeration object returned by the elements() method for iterating over the older container types such as Vector. However, Enumerations do not provide a fail-fast method. On the other hand, the more modern Iterator returned by a Vector’s iterator() and listIterator() methods are fail-fast. Hence, iterators are recommended over enumerations for iterating over the elements of the older container types.

56.How do you sort the elements of a vector ?
Since the Vector class implements the List interface, you can call the Collections.sort() method to sort the elements in place. You can also manually insert elements into a Vector to keep the vector sorted. Of course, if keeping sorted access is such a big deal, you should consider using a different, more appropriate data structure like a TreeSet.
For 1.1 Java users, you’ll need to sort the elements yourself. There is no built-in support for this. A better option might be to sort the elements as you insert each element.

57.What is a WeakHashMap? What is its use and when should you use it ?
A WeakHashMap is a special Map implementation where the keys of the map are stored in a java.lang.ref.WeakReference. By storing the keys in a weak reference, key-value pairs can dynamically be dropped from the map when the only reference to the key is from the weak reference. This makes the WeakHashMap an excellent implementation for a weakly referenced list, where entries that aren’t used elsewhere may be dropped with no side effects. Also, just because a key may be dropped, doesn’t mean it will immediately be dropped. If the system has sufficient resources, the weak key reference that isn’t externally referenced could stay around for a long time.

58.What is the function of a load factor in a Hashtable ?
The load factor determines how full a hashtable may get before it expands its capacity. I think that the comments on the Hashtable API docs explain it very well:
The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hashtable exceeds the product of the load factor and the current capacity, the capacity is increased by calling the rehash method.

Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most Hashtable operations, including get and put).

59.How do you control growth of vectors when their internal arrays are full ?
The vector constructor can include either an initial capacity or a capacity and growth increment. When not specified, the initial size of the vector is 10 and growth will double when necessary. Otherwise, initialize size and growth will grow when needed as specified by the arguments to the constructor.
If the argument to the constructor is a collection, the initial size of the internal structure is 10% larger than the collection. Since there is no second argument to control growth, the capacity will double when necessary.

60.How to sort the messages in JavaMail ?
Within the JavaMail classes there is no support for this. However, once you get the array of messages back from a folder, you can call the Arrays.sort() method in the collections framework to sort the messges. Since MimeMessage doesn’t implement Comparable, you’ll need to provide your own Comparator specifying how you want the messages to be sorted.

61.When did Strings start caching their hash codes ?
Starting with the 1.3 release of Java, the java.lang.String class will only calculate the hashcode once, when its first needed. Future calls to hashCode() will return the previously calculated value.

62.How can you retrieve the Hashtable’s load factor ?
There is no public interface to access the load factor setting.
Some choices you can do to expose this value…

Subclass Hashtable and add a read-only accessor method
Use reflection to access the private field (only possible in trusted environment)
Serialize the Hashtable and get the value from the stream (when reading back the stream, you’ll have to figure out where the appropriate field is)

63.Why can’t I add a collection to itself ?
This will cause a stack overflow exception to be generated on calls to methods like toString() and hashCode(), which recursively call the method on the elements of the collection.