33 Java Interview Questions

Typical HR Questions

1. What are the thread states in java ?

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State	Description
NEW	A thread that has not yet started is in this state.
RUNNABLE	A thread executing in the Java virtual machine is in this state.
BLOCKED	A thread that is blocked waiting for a monitor lock is in this state.
WAITING	A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
TIMED_WAITING	A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
TERMINATED	A thread that has exited is in this state.

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2. List all the types of nested class ?

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Class Type	Description
static inner class	the static inner class can only access the static fields of the enclosing class and define its own static fields.
non static inner class	the inner class can access all the fields of the enclosing class, they can define there own fields but these can not be static.
local class	This is a class inside a method, the local inner class can access all the fields from the enclosing class and all final parameters from the enclosing method, they can define there own fields but these can not be static.
anonymous class

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3. How to increase the heap memory of the JVM ?

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-Xmx (maximum heap size)

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4. Name three JVMs ?

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some of them are:
HotSpot
JRockit
Windows JVM

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5. What is the quickest type of Map ?

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 IdentityHashMap()

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6. How do you redirect the System.out ( "standard" output stream ) to a file ?

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System.setOut(...)

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7. A class that supports serializarion must implement serializable, if its base class is not serizable then what must it have. ?

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A default constructor

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8. Which method should you override when writing a Customer Class Loader ?

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you should override the findClass() method, see example below :

  class CustomClassLoader extends ClassLoader {
         String host;
         int port;
 
         public Class findClass(String name) {
             byte[] b = loadClassData(name);
             return defineClass(name, b, 0, b.length);
         }
 
         private byte[] loadClassData(String name) {
             // load the class data from the connection
         }
    }
 
  // then to load your class
  Class r = new CustomClassLoader().findClass("com.x.MyClass");
 
 or 
 
 Class.forName("com.x.MyClass",true,new CustomClassLoader());

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9. Name the standard class loaders ?

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10. What is the signature of readResolve() and what is it used for ?

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The read resolve method will be called ( if exists ) to create the object. Note : the constructor of the object does not get called if readResolve() is called ( so this is one way to create an object without calling its constructor )

ANY-ACCESS-MODIFIER Object readResolve()
            throws ObjectStreamException;

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11. Other than using externizable, how can you handle custom serilization ?

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Just provide the following methods in your class being serialized. note: the private access modifiers.

 private void writeObject(java.io.ObjectOutputStream out)
     throws IOException
 private void readObject(java.io.ObjectInputStream in)
     throws IOException, ClassNotFoundException;

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12. In java 6, how can you see the assembly code that the JIT is creating ?

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-XX:+PrintOptoAssembly

more details

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13. Name 10 Ways to Reduce Lock Contention in Threaded Programs ?

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1.Protect data, not code.
2.Get rid of expensive calculations while in locks.
3.Use lock striping.
4.Employ interlocked / atomic operations.
5.Use synchronized data structures.
6.Use Reader-Writer Locks where applicable.
7.Use Read-Only data whenever possible.
8.Avoid Object Pooling.
9.Use local variables or thread-local storage.
10. Avoid hotspots.

more details

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14. List 4 Collection Classes and 4 Collection Interfaces ?

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(some of them are listed below )

Classes
ArrayList
HashSet
HashMap
LinkedList
LinkedHashSet
LinkedHashMap
ConcurrentHashMap
ArrayDeque
Stack
TreeSet
ConcurrentSkipListMap

Interfaces
Iterable
Collection
List
Set
Map
NavitableSet
NavitableMap
Deque
Queue
SortedSet

For a UML diagram of the collections framework see [http://en.wikibooks.org/wiki/File:Java_collection_implementation.jpg]

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15. How do you create an array using reflection

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use Array.newInstance(String.class, 10);

import java.lang.reflect.*;
 
public class array1 {
      public static void main(String args[])
      {
         try {
            Class cls = Class.forName(
              "java.lang.String");
            Object arr = Array.newInstance(cls, 10);
            Array.set(arr, 5, "this is a test");
            String s = (String)Array.get(arr, 5);
            System.out.println(s);
         }
         catch (Throwable e) {
            System.err.println(e);
         }
      }
}

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16. Describethe following exceptions : CloneNotSupportedException, ClassNotFoundException, ArrayStoreException, ClassCastException, ConcurrentModificationException, IllegalMonitorStateException, UnsupportedOperationException.

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Checked Exceptions
CloneNotSupportedException Thrown to indicate that the clone method in class Object has been called to clone an object, but that the object's class does not implement the Cloneable interface.
ClassNotFoundException Thrown when an application tries to load in a class through its string name, and this fails.

Runtime Exceptions
ArrayStoreException Thrown to indicate that an attempt has been made to store the wrong type of object into an array of objects.
ClassCastException Thrown to indicate that the code has attempted to cast an object to a subclass of which it is not an instance.
ConcurrentModificationException
IllegalMonitorStateException Thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.
UnsupportedOperationException Thrown to indicate that the requested operation is not supported.

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17 With java collections, whats the difference between Offer(E e) and Add(E e).

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generally the following is true :

Throws exception	Special value
add	offer
remove	poll
get	peek

Add(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an Exception if no space is currently available.

Offer() Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. When using a capacity-restricted queue, this method is generally preferable to add(E), which can fail to insert an element only by throwing an exception. It is implemented in Queue and Deque.

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18 Can a top level class be have a protected class modifier

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No, only inner class can have protected class modifier access. The top level class can not be private or protected.

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19 Does this produce a runtime error

public class Test {

    public static void main(String... args) {

        Object o = new Object() {
            public boolean equals(Object t) {
                return t.equals("hi");
            }
        };

        System.out.println(o.equals("hi"));
    }
}

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No, it prints "true"

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20 What is the difference between weak and soft references

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A soft reference is exactly like a weak reference, except that it is less eager to throw away the object to which it refers. An object which is only weakly reachable (the strongest references to it are WeakReferences) will be discarded at the next garbage collection cycle, but an object which is softly reachable will generally stick around for a while.

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21 What is the difference between NoClassDefFoundError and a ClassNotFoundException

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ClassNotFoundException is thrown when an application tries to load in a class through its string name using:

The forName method in class Class.
The findSystemClass method in class ClassLoader .
The loadClass method in class ClassLoader.

NoClassDefFoundError usually happens when the class that you compiled against is not available at run time. For example the Jar is not on your class path.

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22 When should you use Serializable and when should you use Externalizable

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If you wish to do custom serialization then its is preferable to use Externalizable over Serializable, as Externalizable gives you more control.

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23 How do you create a singleton

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import org.apache.log4j.Logger;

public final class Singleton {

    private static final Logger LOG = Logger.getLogger(Singleton.class);

    private static Singleton instance;

    // we create a holder tha lazy loads the class
    private static class Holder {
        static {
            LOG.info("instance created");
            instance = new Singleton();
        }

        public static Singleton getInstance() {
            return instance;
        }

    }

    private Singleton() {
    }

    public static Singleton getInstance() {
        return Holder.getInstance();
    }

    public static String helloWorld() {
        return "hello";
    }

    public static void main(String... args) {
        LOG.info(Singleton.helloWorld());

        Singleton.getInstance();
        Singleton.getInstance();
    }
}

30-06-10 12:32:35 [INFO ] [main] Singleton - hello
30-06-10 12:32:35 [INFO ] [main] Singleton - instance created

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24 What is the difference between parameters and arguments

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parameters - make up the method signature ( these are what you are expecting )
arguments - are what you are sending.

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25 what is escape analysis

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[[http://en.wikipedia.org/wiki/Java_performance#Escape_analysis_and_lock_coarsening]]

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26 what are memory barriers

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http://www.infoq.com/articles/memory_barriers_jvm_concurrency

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27 how/when does instruction reodering occur in java

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28 what is object orientation

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Programming techniques may include features such as data abstraction, encapsulation, modularity, polymorphism, and inheritance

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29 explain double checked locking

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30 is there ever a case when a singleton may return a differnt instance

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yes when the singleton is loaded with a different class loader.

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30 what is instruction reordering and when / where is it used

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31 If the compareTo method of a comparable object returns zero are they equal

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Usually but not always, virtually all Java core classes that implement comparable have natural orderings that are consistent with equals. One exception is java.math.BigDecimal, whose natural ordering equates BigDecimal objects with equal values and different precisions (such as 4.0 and 4.00).

It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave "strangely" when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method.

32 what are synthetic bridge methods

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Synthetic methods are often referred to as bridge methods in Java generics world. These methods are created by the Java compiler as a result of type erasure when the signature of the inherited methods change when a type extends or implements parameterized classes or interfaces. The compiler generates synthetic methods in subtypes of parameterized supertypes to make sure that subtyping works as expected.

33 write a class that implements the following :

• if the object has been seen before, returns the same integer
• if the object has not been seen before returns a different integer.

show a possible answer

import java.util.Queue;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;

public class UniqueNumber {

    final static int MAX_SIZE = 50000;

    final ConcurrentMap<Object, Integer> cache = new ConcurrentHashMap<Object, Integer>();
    final Queue<Integer> values = new ConcurrentLinkedQueue<Integer>();

    public UniqueNumber() {

        // pre fill the queue
        for (int i = 0; i < MAX_SIZE; i++) {
            values.add(i);
        }

    }

    /**
     *
     * if the object has been seen before, returns the same integer
     * if the object has not been seen before returns a different integer.
     *
     * can hold upto 50000 values
     *
     * @param o an object
     * @return returns an int where equal objects return the same int
     */
    public Integer getUniqueIntValue(Object o) {

        final Integer result = cache.get(o);

        if (result != null)
            return result;

        final Integer candidateValue = values.poll();

        final Integer prevValue = cache.putIfAbsent(result, candidateValue);

        // we will use the new candidateValue from the queue
        if (prevValue == null)
            return candidateValue;

        // we already have a prevValue in the map so lets use that and return the candidateValue back to the queue for reuse
        values.add(candidateValue);

        return prevValue;

    }

}