More Java Questions

1. Is “abc” a primitive value? - The String literal “abc” is not a primitive value. It is a String object.
2. What restrictions are placed on the values of each case of a switch statement? - During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
3. What modifiers may be used with an interface declaration? - An interface may be declared as public or abstract.
4. Is a class a subclass of itself? - A class is a subclass of itself.
5. What is the difference between a while statement and a do statement? - A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
6. What modifiers can be used with a local inner class? - A local inner class may be final or abstract.
7. What is the purpose of the File class? - The File class is used to create objects that provide access to the files and directories of a local file system.
8. Can an exception be rethrown? - Yes, an exception can be rethrown.
9. When does the compiler supply a default constructor for a class? - The compiler supplies a default constructor for a class if no other constructors are provided.
10. If a method is declared as protected, where may the method be accessed? - A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
11. Which non-Unicode letter characters may be used as the first character of an identifier? - The non-Unicode letter characters $ and _ may appear as the first character of an identifier
12. What restrictions are placed on method overloading? - Two methods may not have the same name and argument list but different return types.
13. What is casting? - There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
14. What is the return type of a program’s main() method? - A program’s main() method has a void return type.
15. What class of exceptions are generated by the Java run-time system? - The Java runtime system generates RuntimeException and Error exceptions.
16. What class allows you to read objects directly from a stream? - The ObjectInputStream class supports the reading of objects from input streams.
17. What is the difference between a field variable and a local variable? - A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.
18. How are this() and super() used with constructors? - this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
19. What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution? - A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.
20. Why are the methods of the Math class static? - So they can be invoked as if they are a mathematical code library.
21. What are the legal operands of the instanceof operator? - The left operand is an object reference or null value and the right operand is a class, interface, or array type.
22. What an I/O filter? - An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
23. If an object is garbage collected, can it become reachable again? - Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.
24. What are E and PI? - E is the base of the natural logarithm and PI is mathematical value pi.
25. Are true and false keywords? - The values true and false are not keywords.
26. What is the difference between the File and RandomAccessFile classes? - The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.
27. What happens when you add a double value to a String? - The result is a String object.
28. What is your platform’s default character encoding? - If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1.
29. Which package is always imported by default? - The java.lang package is always imported by default.
30. What interface must an object implement before it can be written to a stream as an object? - An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
31. How can my application get to know when a HttpSession is removed? - Define a Class HttpSessionNotifier which implements HttpSessionBindingListener and implement the functionality what you need in valueUnbound() method. Create an instance of that class and put that instance in HttpSession.
32. Whats the difference between notify() and notifyAll()? - notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a “writer” lock on a file might permit all “readers” to resume).
33. Why can’t I say just abs() or sin() instead of Math.abs() and Math.sin()? - The import statement does not bring methods into your local name space. It lets you abbreviate class names, but not get rid of them altogether. That’s just the way it works, you’ll get used to it. It’s really a lot safer this way.
    However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn’t need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can’t use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can’t use it on java.lang.Math at all, because Math is a “final” class which means it can’t be extended.
34. Why are there no global variables in Java? - Global variables are considered bad form for a variety of reasons: Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables), State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state, When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once. For these reasons, Java decided to ban global variables.
35. What does it mean that a class or member is final? - A final class can no longer be subclassed. Mostly this is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. Methods may be declared final as well. This means they may not be overridden in a subclass. Fields can be declared final, too. However, this has a completely different meaning. A final field cannot be changed after it’s initialized, and it must include an initializer statement where it’s declared. For example, public final double c = 2.998; It’s also possible to make a static field final to get the effect of C++’s const statement or some uses of C’s #define, e.g. public static final double c = 2.998;
36. What does it mean that a method or class is abstract? - An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate that a class is abstract with the abstract keyword like this:

     public abstract class Container extends Component {
    

    Abstract classes may contain abstract methods. A method declared abstract is not actually implemented in the current class. It exists only to be overridden in subclasses. It has no body. For example,

     public abstract float price();
    

    Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or itself be declared abstract.

37. What is a transient variable? - transient variable is a variable that may not be serialized.
38. How are Observer and Observable used? - Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
39. Can a lock be acquired on a class? - Yes, a lock can be acquired on a class. This lock is acquired on the class’s Class object.
40. What state does a thread enter when it terminates its processing? - When a thread terminates its processing, it enters the dead state.
41. How does Java handle integer overflows and underflows? - It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
42. What is the difference between the >> and >>> operators? - The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
43. Is sizeof a keyword? - The sizeof operator is not a keyword.
44. Does garbage collection guarantee that a program will not run out of memory? - Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection
45. Can an object’s finalize() method be invoked while it is reachable? - An object’s finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object’s finalize() method may be invoked by other objects.
46. What value does readLine() return when it has reached the end of a file? - The readLine() method returns null when it has reached the end of a file.
47. Can a for statement loop indefinitely? - Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;
48. To what value is a variable of the String type automatically initialized? - The default value of an String type is null.
49. What is a task’s priority and how is it used in scheduling? - A task’s priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.
50. What is the range of the short type? - The range of the short type is -(2^15) to 2^15 - 1.
51. What is the purpose of garbage collection? - The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.
52. What do you understand by private, protected and public? - These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.
53. What is Downcasting ? - Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy
54. Can a method be overloaded based on different return type but same argument type ? - No, because the methods can be called without using their return type in which case there is ambiquity for the compiler
55. What happens to a static var that is defined within a method of a class ? - Can’t do it. You’ll get a compilation error
56. How many static init can you have ? - As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.
57. What is the difference amongst JVM Spec, JVM Implementation, JVM Runtime ? - The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM implementation is the actual implementation of the spec by a vendor and the JVM runtime is the actual running instance of a JVM implementation
58. Describe what happens when an object is created in Java? - Several things happen in a particular order to ensure the object is constructed properly: Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data. The instance variables of the objects are initialized to their default values. The constructor for the most derived class is invoked. The first thing a constructor does is call the consctructor for its superclasses. This process continues until the constrcutor for java.lang.Object is called, as java.lang.Object is the base class for all objects in java. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.
59. What does the “final” keyword mean in front of a variable? A method? A class? - FINAL for a variable: value is constant. FINAL for a method: cannot be overridden. FINAL for a class: cannot be derived
60. What is the difference between instanceof and isInstance? - instanceof is used to check to see if an object can be cast into a specified type without throwing a cast class exception. isInstance() Determines if the specified Object is assignment-compatible with the object represented by this Class. This method is the dynamic equivalent of the Java language instanceof operator. The method returns true if the specified Object argument is non-null and can be cast to the reference type represented by this Class object without raising a ClassCastException. It returns false otherwise.
61. Why does it take so much time to access an Applet having Swing Components the first time? - Because behind every swing component are many Java objects and resources. This takes time to create them in memory. JDK 1.3 from Sun has some improvements which may lead to faster execution of Swing applications.

Java Questions

1. What is garbage collection? What is the process that is responsible for doing that in java? - Reclaiming the unused memory by the invalid objects. Garbage collector is responsible for this process
2. What kind of thread is the Garbage collector thread? - It is a daemon thread.
3. What is a daemon thread? - These are the threads which can run without user intervention. The JVM can exit when there are daemon thread by killing them abruptly.
4. How will you invoke any external process in Java? - Runtime.getRuntime().exec(….)
5. What is the finalize method do? - Before the invalid objects get garbage collected, the JVM give the user a chance to clean up some resources before it got garbage collected.
6. What is mutable object and immutable object? - If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer, …) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float, …)
7. What is the basic difference between string and stringbuffer object? - String is an immutable object. StringBuffer is a mutable object.
8. What is the purpose of Void class? - The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the primitive Java type void.
9. What is reflection? - Reflection allows programmatic access to information about the fields, methods and constructors of loaded classes, and the use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.
10. What is the base class for Error and Exception? - Throwable
11. What is the byte range? -128 to 127
12. What is the implementation of destroy method in java.. is it native or java code? - This method is not implemented.
13. What is a package? - To group set of classes into a single unit is known as packaging. Packages provides wide namespace ability.
14. What are the approaches that you will follow for making a program very efficient? - By avoiding too much of static methods avoiding the excessive and unnecessary use of synchronized methods Selection of related classes based on the application (meaning synchronized classes for multiuser and non-synchronized classes for single user) Usage of appropriate design patterns Using cache methodologies for remote invocations Avoiding creation of variables within a loop and lot more.
15. What is a DatabaseMetaData? - Comprehensive information about the database as a whole.
16. What is Locale? - A Locale object represents a specific geographical, political, or cultural region
17. How will you load a specific locale? - Using ResourceBundle.getBundle(…);
18. What is JIT and its use? - Really, just a very fast compiler… In this incarnation, pretty much a one-pass compiler — no offline computations. So you can’t look at the whole method, rank the expressions according to which ones are re-used the most, and then generate code. In theory terms, it’s an on-line problem.
19. Is JVM a compiler or an interpreter? - Interpreter
20. When you think about optimization, what is the best way to findout the time/memory consuming process? - Using profiler
21. What is the purpose of assert keyword used in JDK1.4.x? - In order to validate certain expressions. It effectively replaces the if block and automatically throws the AssertionError on failure. This keyword should be used for the critical arguments. Meaning, without that the method does nothing.
22. How will you get the platform dependent values like line separator, path separator, etc., ? - Using Sytem.getProperty(…) (line.separator, path.separator, …)
23. What is skeleton and stub? what is the purpose of those? - Stub is a client side representation of the server, which takes care of communicating with the remote server. Skeleton is the server side representation. But that is no more in use… it is deprecated long before in JDK.
24. What is the final keyword denotes? - final keyword denotes that it is the final implementation for that method or variable or class. You can’t override that method/variable/class any more.
25. What is the significance of ListIterator? - You can iterate back and forth.
26. What is the major difference between LinkedList and ArrayList? - LinkedList are meant for sequential accessing. ArrayList are meant for random accessing.
27. What is nested class? - If all the methods of a inner class is static then it is a nested class.
28. What is inner class? - If the methods of the inner class can only be accessed via the instance of the inner class, then it is called inner class.
29. What is composition? - Holding the reference of the other class within some other class is known as composition.
30. What is aggregation? - It is a special type of composition. If you expose all the methods of a composite class and route the method call to the composite method through its reference, then it is called aggregation.
31. What are the methods in Object? - clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString
32. Can you instantiate the Math class? - You can’t instantiate the math class. All the methods in this class are static. And the constructor is not public.
33. What is singleton? - It is one of the design pattern. This falls in the creational pattern of the design pattern. There will be only one instance for that entire JVM. You can achieve this by having the private constructor in the class. For eg., public class Singleton { private static final Singleton s = new Singleton(); private Singleton() { } public static Singleton getInstance() { return s; } // all non static methods … }
34. What is DriverManager? - The basic service to manage set of JDBC drivers.
35. What is Class.forName() does and how it is useful? - It loads the class into the ClassLoader. It returns the Class. Using that you can get the instance ( “class-instance”.newInstance() ).
36. Inq adds a question: Expain the reason for each keyword of
    

    public static void main(String args[])

Java Collections Questions

What is HashMap and Map?

Map is Interface and Hashmap is class that implements this interface.

What is the significance of ListIterator?

Or

What is the difference b/w Iterator and ListIterator?

Iterator : Enables you to cycle through a collection in the forward direction only, for obtaining or removing elements

ListIterator : It extends Iterator, allow bidirectional traversal of list and the modification of elements

Difference between HashMap and HashTable? Can we make hashmap synchronized?

1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).
2. HashMap does not guarantee that the order of the map will remain constant over time.
3. HashMap is non synchronized whereas Hashtable is synchronized.
4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn’t.

Note on Some Important Terms
1)Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.

2)Fail-safe is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object “structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke “set” method since it doesn’t modify the collection “structurally”. However, if prior to calling “set”, the collection has been modified structurally, “IllegalArgumentException” will be thrown.

HashMap can be synchronized by

Map m = Collections.synchronizeMap(hashMap);

What is the difference between set and list?

A Set stores elements in an unordered way and does not contain duplicate elements, whereas a list stores elements in an ordered way but may contain duplicate elements.

Difference between Vector and ArrayList? What is the Vector class?

Vector is synchronized whereas ArrayList is not. The Vector class provides the capability to implement a growable array of objects. ArrayList and Vector class both implement the List interface. Both classes are implemented using dynamically resizable arrays, providing fast random access and fast traversal. In vector the data is retrieved using the elementAt() method while in ArrayList, it is done using the get() method. ArrayList has no default size while vector has a default size of 10. when you want programs to run in multithreading environment then use concept of vector because it is synchronized. But ArrayList is not synchronized so, avoid use of it in a multithreading environment.

What is an Iterator interface? Is Iterator a Class or Interface? What is its use?

The Iterator is an interface, used to traverse through the elements of a Collection. It is not advisable to modify the collection itself while traversing an Iterator.

What is the Collections API?

The Collections API is a set of classes and interfaces that support operations on collections of objects.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.

What is the List interface?

The List interface provides support for ordered collections of objects.

How can we access elements of a collection?

We can access the elements of a collection using the following ways:
1.Every collection object has get(index) method to get the element of the object. This method will return Object.
2.Collection provide Enumeration or Iterator object so that we can get the objects of a collection one by one.

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.

What’s the difference between a queue and a stack?

Stack is a data structure that is based on last-in-first-out rule (LIFO), while queues are based on First-in-first-out (FIFO) rule.

What is the Map interface?

The Map interface is used associate keys with values.

What is the Properties class?

The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.

Which implementation of the List interface provides for the fastest insertion of a new element into the middle of the list?

a. Vector
b. ArrayList
c. LinkedList
d. None of the above

ArrayList and Vector both use an array to store the elements of the list. When an element is inserted into the middle of the list the elements that follow the insertion point must be shifted to make room for the new element. The LinkedList is implemented using a doubly linked list; an insertion requires only the updating of the links at the point of insertion. Therefore, the LinkedList allows for fast insertions and deletions.

How can we use hashset in collection interface?

This class implements the set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the Null element.

This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets.

What are differences between Enumeration, ArrayList, Hashtable and Collections and Collection?

Enumeration: It is series of elements. It can be use to enumerate through the elements of a vector, keys or values of a hashtable. You can not remove elements from Enumeration.

ArrayList: It is re-sizable array implementation. Belongs to ‘List’ group in collection. It permits all elements, including null. It is not thread -safe.

Hashtable: It maps key to value. You can use non-null value for key or value. It is part of group Map in collection.

Collections: It implements Polymorphic algorithms which operate on collections.

Collection: It is the root interface in the collection hierarchy.

What is difference between array & arraylist?

An ArrayList is resizable, where as, an array is not. ArrayList is a part of the Collection Framework. We can store any type of objects, and we can deal with only objects. It is growable. Array is collection of similar data items. We can have array of primitives or objects. It is of fixed size. We can have multi dimensional arrays.

Array: can store primitive ArrayList: Stores object only

Array: fix size ArrayList: resizable

Array: can have multi dimensional

Array: lang ArrayList: Collection framework

Can you limit the initial capacity of vector in java?

Yes you can limit the initial capacity. We can construct an empty vector with specified initial capacity

public vector(int initialcapacity)

What method should the key class of Hashmap override?

The methods to override are equals() and hashCode().

What is the difference between Enumeration and Iterator?

The functionality of Enumeration interface is duplicated by the Iterator interface. Iterator has a remove() method while Enumeration doesn’t. Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects, where as using Iterator we can manipulate the objects also like adding and removing the objects.

So Enumeration is used when ever we want to make Collection objects as Read-only.