Object oriented programing in Java

QUESTION : 1

 What is the result of compiling and running this program?

      class Mammal{
         void eat(Mammal m){
         System.out.println("Mammal eats food");
         }
      }
      class Cattle extends Mammal{
         void eat(Cattle c){
         System.out.println("Cattle eats hay");
         }
      }

      class Horse extends Cattle{
          void eat(Horse h){
          System.out.println("Horse eats hay");
          }
      }
      public class Test{
        public static void main(String[] args){
          Mammal h = new Horse();
          Cattle c = new Horse();
          c.eat(h);
        }
      }

1. prints "Mammal eats food"
2. prints "Cattle eats hay"
3. prints "Horse eats hay"
4. Class cast Exception at runtime.

ANS : 1

    The method that will be called is the one
    from class Mammal. The reasons are quite obvious.

QUESTION : 2

 Comsider the following class hierarchy.
1. interface A{
2. public void method1();
3. }
4. class One implements A{
5. public void method1(){
6. System.out.println("hello");
7. }
8. }
9. class Two extends One{}
10. public class Test extends Two{
11. public static void main(String[] args)
12. {
13. A a;
14. Two t = new Two();
15. a = t;
16. a.method1();
17. }
18. }
What will be the outcome on attempting to compile and run this ?

1. Compiles and runs printing out "hello".
2. Compilation error at line 16.
3. The compiler raises an objection to the assignment at line 15.
4. Throws a NoSuchMethodException at runtime.  

ANS : 1

    Object reference conversion is possible here.
    The old type which is class can be assigned
    to an interface type as long as the class implements
    that interface.

QUESTION : 3
  What will happen if you try to compile and run this ?
 interface A{
 public void innerMeth();
 }
        public class Test {
   A a;
   int memVar = 1;
          void aMethod(){
                 a = new A(){
                 public void innerMeth(){
                 System.out.println(memVar);
                 } };
    }

           public static void main(String[] args){
            Test t = new Test();
             t.a.innerMeth();
            }
          }

1. Compiler error.
2. NoSuchMethodException at runtime.
3. Compiles and runs printing 1
4. Throws a NullPointerException at runtime.

ANS : 4
     You will get a NullPointerException because the
     inner class object gets assigned to the reference a
     only after the aMethod() runs. You can prevent
     the exception by calling t.aMethod() before the
     inner anonymous class method is called.

QUESTION : 4

       What will happen if you try to compile and run this code.

         class Rectangle{
           public int area(int length , int width) {
             return  length * width;
           }
          }

         class Square extends Rectangle{
            public int area(long length , long width) {
            return  (int) Math.pow(length ,2);
           }
         }
        class Test{
          public static void main(String args[]) {
           Square r = new Square();
           System.out.println(r.area(5 , 4));
          }
        }

1. Will not compile.
2. Will compile and run printing out 20
3. Runtime error
4. Will compile and run printing out 25

ANS : 1

    This code will fail to compile because the
    compiler cannot resolve the method call here.

QUESTION : 5

   What will be the result of attempting to compile and run this.

     class Base{}
     class Derived extends Base{}
     public class Test {
       public static void main(String[] args){
         Derived d = (Derived) new Base();
       }
     }

1. Will not compile
2. Compiles and runs without error.
3. Runtime error

ANS : 3

QUESTION : 6

   What will this program print out ?
      class Base{
        int value = 0;
        Base(){
        addValue();
        }
        void addValue(){
        value += 10;
        }
       int getValue(){
         return value;
       }
     }
    class Derived extends Base{
      Derived(){
       addValue();
      }
     void addValue(){
     value +=  20;
      }
    }
    public class Test {
      public static void main(String[] args){
          Base b = new Derived();
          System.out.println(b.getValue());
      }
    }

1. 10
2. 20
3. 30
4. 40

ANS : 4

QUESTION : 7

   Almost the same code as in the previous question.
   The only difference is the methods are static now.
   What will it print now?

   class Base{
      static  int value = 0;
        Base(){
        addValue();
        }
    static void addValue(){
        value += 10;
        }
       int getValue(){
         return value;
       }
     }
    class Derived extends Base{
      Derived(){
       addValue();
      }
    static void addValue(){
     value +=  20;
      }
    }
    public class Test {
      public static void main(String[] args){
          Base b = new Derived();
          System.out.println(b.getValue());
      }
    }

1. 10
2. 20
3. 30
4. 40

ANS : 3

QUESTION : 8

     What is the result of attempting to compile and run this ?
      interface ITest{
           public void setVal();
      }
      public class Test {
          private String a;
          void aMethod(){
          final String b;
          ITest it = new ITest() {
               public void setVal(){
               a = "Hello";
               b = " World";
               }};
          it.setVal();
          System.out.println(a + b);
          }
          public static void main(String[] args) {
          Test t = new Test();
          t.aMethod();
          }
       }

1. Code will not compile
2. Run time error
3. Will compile and run printing "Hello"
4. Will compile and run without any output

ANS : 1

QUESTION : 9

   What is the result of attempting to compile and run this ?

         class Base{
              String s = "Base";
              String show(){
              return s;
              }
         }
         class Derived extends Base{
              String s = "Derived";
              }
         public class Test {
              void print(Base b){
              System.out.println(b.show());
              }
              void print(Derived d){
              System.out.println(d.show());
              }
              public static void main(String[] args){
              Test t = new Test();
              Base b = new Derived();
              t.print(b);
              }
         }

1. Code will not compile
2. Run time error
3. Will compile and run printing "Derived"
4. Will compile and run printing "Base"

ANS : 4

QUESTION : 10

   What is the result of attempting to compile and run this ?
    interface ITest{
     public void setVal();
    }
    public class Test {
      private String a;
      void aMethod(){
      final String b = " World";
      ITest it = new ITest() {
                   public void setVal(){
                   a = "Hello" + b;
                   }};
      it.setVal();
      System.out.println(a);
      }
      public static void main(String[] args) {
      Test t = new Test();
      t.aMethod();
      }
    }

1. Code will not compile
2. Run time error
3. Will compile and run printing "Hello World"
4. Will compile and run printing "Hello"

ANS : 3