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Home Java Programming Inner Classes See What Others Are Saying!
  • Question
  • Which one create an anonymous inner class from within class Bar?
    class Boo 
    {
        Boo(String s) { }
        Boo() { }
    }
    class Bar extends Boo 
    {
        Bar() { }
        Bar(String s) {super(s);}
        void zoo() 
        {
        // insert code here
        }
    }
    


  • Options
  • A. Boo f = new Boo(24) { };
  • B. Boo f = new Bar() { };
  • C. Bar f = new Boo(String s) { };
  • D. Boo f = new Boo.Bar(String s) { };

  • Correct Answer
  • Boo f = new Bar() { }; 

    Explanation
    Option B is correct because anonymous inner classes are no different from any other class when it comes to polymorphism. That means you are always allowed to declare a reference variable of the superclass type and have that reference variable refer to an instance of a subclass type, which in this case is an anonymous subclass of Bar. Since Bar is a subclass of Boo, it all works.

    Option A is incorrect because it passes an int to the Boo constructor, and there is no matching constructor in the Boo class.

    Option C is incorrect because it violates the rules of polymorphism—you cannot refer to a superclass type using a reference variable declared as the subclass type. The superclass is not guaranteed to have everything the subclass has.

    Option D uses incorrect syntax.


  • More questions

    • 1. When is the B object, created in line 3, eligible for garbage collection?
      void start() {  
          A a = new A(); 
          B b = new B(); 
          a.s(b);  
          b = null; /* Line 5 */
          a = null;  /* Line 6 */
          System.out.println("start completed"); /* Line 7 */
      } 
      

    • Options
    • A. after line 5
    • B. after line 6
    • C. after line 7
    • D. There is no way to be absolutely certain.
    • Discuss
    • 2. Which is a reserved word in the Java programming language?

    • Options
    • A. method
    • B. native
    • C. subclasses
    • D. reference
    • E. array
    • Discuss
    • 3. Which statement is true for the class java.util.ArrayList?

    • Options
    • A. The elements in the collection are ordered.
    • B. The collection is guaranteed to be immutable.
    • C. The elements in the collection are guaranteed to be unique.
    • D. The elements in the collection are accessed using a unique key.
    • Discuss
    • 4. What will be the output of the program?
      public class Test 
      { 
          public static void main(String[] args) 
          {
              final StringBuffer a = new StringBuffer(); 
              final StringBuffer b = new StringBuffer(); 
      
              new Thread() 
              { 
                  public void run() 
                  {
                      System.out.print(a.append("A")); 
                      synchronized(b) 
                      { 
                          System.out.print(b.append("B")); 
                      } 
                  } 
              }.start(); 
                  
              new Thread() 
              {
                  public void run() 
                  {
                      System.out.print(b.append("C")); 
                      synchronized(a) 
                      {
                          System.out.print(a.append("D")); 
                      } 
                  } 
              }.start(); 
          } 
      }
      

    • Options
    • A. ACCBAD
    • B. ABBCAD
    • C. CDDACB
    • D. Indeterminate output
    • Discuss
    • 5. Which is true about a method-local inner class?

    • Options
    • A. It must be marked final.
    • B. It can be marked abstract.
    • C. It can be marked public.
    • D. It can be marked static.
    • Discuss
    • 6. Given that all methods of class FileOutputStream, including close(), throw an IOException, which of these is true?
      import java.io.*;
      public class MyProgram 
      {
          public static void main(String args[])
          {
              FileOutputStream out = null;
              try 
              {
                  out = new FileOutputStream("test.txt");
                  out.write(122);
              }
              catch(IOException io) 
              {
                  System.out.println("IO Error.");
              }
              finally 
              {
                  out.close();
              }
          }
      }
      

    • Options
    • A. This program will compile successfully.
    • B. This program fails to compile due to an error at line 4.
    • C. This program fails to compile due to an error at line 6.
    • D. This program fails to compile due to an error at line 18.
    • Discuss
    • 7. What will be the output of the program?
      public class Test 
      {
          public static void main(String [] args) 
          {
              int I = 1;
              do while ( I < 1 )
              System.out.print("I is " + I);
              while ( I > 1 ) ;
          }
      }
      

    • Options
    • A. I is 1
    • B. I is 1 I is 1
    • C. No output is produced.
    • D. Compilation error
    • Discuss
    • 8. What will be the output of the program?
      class MyThread extends Thread 
      {
          public static void main(String [] args) 
          {
              MyThread t = new MyThread();
              Thread x = new Thread(t);
              x.start(); /* Line 7 */
          }
          public void run() 
          {
              for(int i = 0; i < 3; ++i) 
              {
                  System.out.print(i + "..");
              }
          }
      }
      

    • Options
    • A. Compilation fails.
    • B. 1..2..3..
    • C. 0..1..2..3..
    • D. 0..1..2..
    • Discuss
    • 9. Which of the following is/are legal method declarations?

      1. protected abstract void m1();
      2. static final void m1(){}
      3. synchronized public final void m1() {}
      4. private native void m1();

    • Options
    • A. 1 and 3
    • B. 2 and 4
    • C. 1 only
    • D. All of them are legal declarations.
    • Discuss
    • 10. Which three statements are true?
      1. f1 == f2
      2. f1 == f3
      3. f2 == f1[1]
      4. x == f1[0]
      5. f == f1[0]
      import java.awt.Button;
      class CompareReference 
      {
          public static void main(String [] args) 
          {
              float f = 42.0f;
              float [] f1 = new float[2];
              float [] f2 = new float[2];
              float [] f3 = f1;
              long x = 42;
              f1[0] = 42.0f;
          }
      }
      

    • Options
    • A. 1, 2 and 3
    • B. 2, 4 and 5
    • C. 3, 4 and 5
    • D. 1, 4 and 5
    • Discuss


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