Top 15 Core Java pre-interview technical written test questions and answers

Written tests can help short-list candidates and often used by recruitment agencies  and prospective employers. Here are some popular written test questions and answers.

Note: Multiple choice questions may have more than one right answers.

Q1. What will be the output of the following code snippet?

package com.writtentest1;

public class Base {
 int i;

 public Base() {
   add(1);
 }

 public void add(int j) {
   i += j;
 }

}

package com.writtentest1;

public class Extension extends Base {

 public Extension() {
   add(2);
 }

 public void add(int j) {
   i += j*2;
 }
}

package com.writtentest1;

public class Main {

 public static void main(String[] args) {
   exec(new Extension());
 }

 private static void exec(Base base) {
   base.add(8);
   System.out.println(base.i);
 }

}

a) 21
b) 22
c) 20
d) 16
e) 17


A1. The answers is b, i.e 22.

Step 1: When new Extension( ) is invoked it invokes the constructor Extension( ).
Step 2: The trick here is that default constructors implicitly call super( ), so Base( ) constructor will be invoked next.
Step 3: Base( ) --> add(1) method on Extension class as the stored object is of type Extension and polymorphic will kick in. The value of i becomes 0 + 1*2 = 2.
Step 4: add(2) is invoked from Extension constructor. The i becomes 2 + 2*2 = 6;
Step 5: Finally, add(8) is invoked from Main --> exec, and i becomes 6 + 2*8 = 22.

Q2. What will be happen when you run the following code?

package com.writtentest2;

public class ThreadMain {

 public static void main(String[] args) {
  
  Runnable runnable = new Runnable() {
   
   @Override
   public void run() {
     try {
       System.out.print("Going to sleep. "); 
       Thread.sleep(5000);
     } catch (InterruptedException e) {
       e.printStackTrace();
     } 
     System.out.print("Has woken up. "); 
   }
  };
  
  Thread thread = new Thread(runnable);
  thread.start();
  
  
  try {
    thread.join();
  } catch (InterruptedException e) {
    e.printStackTrace();
  }
  
   System.out.print("Completed.");
 }

}

a) prints: Going to sleep. Has woken up. Completed.
b) InterruptedException is thrown with a stack trace.
c) prints: Completed. Going to sleep. Has woken up.
d) prints: Completed.

A2. The answer is a.

There are 2 threads involved in this. Main thread and the worker thread "thread". The main thread spawns the new thread and when thread.start( ) is invoked the run( ) method inside the anonymous inner class for Runnable interface is executed and prints "Going to sleep. ". In the mean time the Main thread is blocked due to thread.join( ) statement. The worker thread sleeps for at least 5 seconds, and then prints "Has woken up. " and it completes its execution. Once the worker thread "thread" has finished, the main thread continues and prints "Completed."

Try commenting out thread.join( ); and its try/catch block see what happens to the output.

Q3: Given the following interface,

package com.writtentest3;

public interface Ixy {
    abstract String get();
    abstract void set(String value);
}

Which of the following statements are correct?

a)

package com.writtentest3;

public  abstract class IxyImpl implements Ixy {

   public  void calc(String value){}
}

b)

package com.writtentest3;

public  class IxyImpl implements Ixy {

 @Override
 public String get() {
    return null;
 }

 public  void calc(String value){}

}

c)

package com.writtentest3;

public  class IxyImpl {

 @Override
 public String get() {
    return null;
 }

 public  void calc(String value){}

}

d)

package com.writtentest3;

public  class IxyImpl {

 public String get() {
    return null;
 }

 public  void calc(String value){}

}

A3. Answer is: a and d.

a: No compile error since the class is abstract, even though it does not implement all the methods from the interface.
b: Compile error because it is a concrete class, and does not implement the abstract method void set(String value).
c: Compile error because it does not implement the interface Ixy, but the method public String get( ) is still annotated with @Override
d: No compile error as the @Override annotation is no longer there.


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