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  • Question
  • What will be the output of the program?
    public class WrapTest 
    {
        public static void main(String [] args) 
        {
            int result = 0;
            short s = 42;
            Long x = new Long("42");
            Long y = new Long(42);
            Short z = new Short("42");
            Short x2 = new Short(s);
            Integer y2 = new Integer("42");
            Integer z2 = new Integer(42);
    
            if (x == y) /* Line 13 */
                result = 1;
            if (x.equals(y) ) /* Line 15 */
                result = result + 10;
            if (x.equals(z) ) /* Line 17 */
                result = result + 100;
            if (x.equals(x2) ) /* Line 19 */
                result = result + 1000;
            if (x.equals(z2) ) /* Line 21 */
                result = result + 10000;
    
            System.out.println("result = " + result);
        }
    }
    


  • Options
  • A. result = 1
  • B. result = 10
  • C. result = 11
  • D. result = 11010

  • Correct Answer
  • result = 10 

    Explanation
    Line 13 fails because == compares reference values, not object values. Line 15 succeeds because both String and primitive wrapper constructors resolve to the same value (except for the Character wrapper). Lines 17, 19, and 21 fail because the equals() method fails if the object classes being compared are different and not in the same tree hierarchy.

    More questions

    • 1. Which statement is true?

    • Options
    • A. The notifyAll() method must be called from a synchronized context.
    • B. To call wait(), an object must own the lock on the thread.
    • C. The notify() method is defined in class java.lang.Thread.
    • D. The notify() method causes a thread to immediately release its locks.
    • Discuss
    • 2. Which statement is true?
      class Test1 
      {
          public int value;
          public int hashCode() { return 42; }
      }
      class Test2 
      {
          public int value;
          public int hashcode() { return (int)(value^5); }
      }
      

    • Options
    • A. class Test1 will not compile.
    • B. The Test1 hashCode() method is more efficient than the Test2 hashCode() method.
    • C. The Test1 hashCode() method is less efficient than the Test2 hashCode() method.
    • D. class Test2 will not compile.
    • Discuss
    • 3. Assuming that the equals() and hashCode() methods are properly implemented, if the output is "x = 1111", which of the following statements will always be true?
      x = 0;
      if (x1.hashCode() != x2.hashCode() )  x = x + 1;
      if (x3.equals(x4) )  x = x + 10;
      if (!x5.equals(x6) ) x = x + 100;
      if (x7.hashCode() == x8.hashCode() )  x = x + 1000;
      System.out.println("x = " + x);
      

    • Options
    • A. x2.equals(x1)
    • B. x3.hashCode() == x4.hashCode()
    • C. x5.hashCode() != x6.hashCode()
    • D. x8.equals(x7)
    • Discuss
    • 4. The static method Thread.currentThread() returns a reference to the currently executing Thread object. What is the result of this code?
      class Test 
      {
          public static void main(String [] args) 
          {
              printAll(args);
          }
      
          public static void printAll(String[] lines) 
          {
              for(int i = 0; i < lines.length; i++)
              {
                  System.out.println(lines[i]);
                  Thread.currentThread().sleep(1000);
              }
          }
      }
      

    • Options
    • A. Each String in the array lines will output, with a 1-second pause.
    • B. Each String in the array lines will output, with no pause in between because this method is not executed in a Thread.
    • C. Each String in the array lines will output, and there is no guarantee there will be a pause because currentThread() may not retrieve this thread.
    • D. This code will not compile.
    • Discuss
    • 5. Which answer most closely indicates the behavior of the program?
      public class MyProgram 
      {
          public static void throwit() 
          {
              throw new RuntimeException();
          }
          public static void main(String args[])
          {
              try 
              {
                  System.out.println("Hello world ");
                  throwit();
                  System.out.println("Done with try block ");
              }
              finally 
              {
                  System.out.println("Finally executing ");
              }
          }
      }
      

    • Options
    • A. The program will not compile.
    • B. The program will print Hello world, then will print that a RuntimeException has occurred, then will print Done with try block, and then will print Finally executing.
    • C. The program will print Hello world, then will print that a RuntimeException has occurred, and then will print Finally executing.
    • D. The program will print Hello world, then will print Finally executing, then will print that a RuntimeException has occurred.
    • Discuss
    • 6. Which two code fragments will compile?
      1. interface Base2 implements Base {}
      2. abstract class Class2 extends Base
        { public boolean m1(){ return true; }}
      3. abstract class Class2 implements Base {}
      4. abstract class Class2 implements Base
        { public boolean m1(){ return (7 > 4); }}
      5. abstract class Class2 implements Base
        { protected boolean m1(){ return (5 > 7) }}
      interface Base 
      {
          boolean m1 ();
          byte m2(short s);
      }
      

    • Options
    • A. 1 and 2
    • B. 2 and 3
    • C. 3 and 4
    • D. 1 and 5
    • Discuss
    • 7. Which statement, if placed in a class other than MyOuter or MyInner, instantiates an instance of the nested class?
      public class MyOuter 
      {
          public static class MyInner 
          {
              public static void foo() { }
          }
      }
      

    • Options
    • A. MyOuter.MyInner m = new MyOuter.MyInner();
    • B. MyOuter.MyInner mi = new MyInner();
    • C. MyOuter m = new MyOuter();

      MyOuter.MyInner mi = m.new MyOuter.MyInner();

    • D. MyInner mi = new MyOuter.MyInner();
    • Discuss
    • 8. What will be the output of the program, if this code is executed with the command line:

      > java F0091 world

      public class F0091 
      {    
          public void main( String[] args ) 
          {  
              System.out.println( "Hello" + args[0] ); 
          } 
      }
      

    • Options
    • A. Hello
    • B. Hello Foo91
    • C. Hello world
    • D. The code does not run.
    • Discuss
    • 9. At what point is the Bar object, created on line 6, eligible for garbage collection?
      class Bar { } 
      class Test 
      {  
          Bar doBar() 
          {
              Bar b = new Bar(); /* Line 6 */
              return b; /* Line 7 */
          } 
          public static void main (String args[]) 
          { 
              Test t = new Test();  /* Line 11 */
              Bar newBar = t.doBar();  /* Line 12 */
              System.out.println("newBar"); 
              newBar = new Bar(); /* Line 14 */
              System.out.println("finishing"); /* Line 15 */
          } 
      }
      

    • Options
    • A. after line 12
    • B. after line 14
    • C. after line 7, when doBar() completes
    • D. after line 15, when main() completes
    • Discuss
    • 10. At Point X on line 5, which code is necessary to make the code compile?
      public class ExceptionTest 
      { 
          class TestException extends Exception {} 
          public void runTest() throws TestException {} 
          public void test() /* Point X */ 
          { 
              runTest(); 
          } 
      }
      

    • Options
    • A. No code is necessary.
    • B. throws Exception
    • C. catch ( Exception e )
    • D. throws RuntimeException
    • Discuss


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