Inner Classes - Finding the output Questions and Answers

Q: What will be the output of the program? public class TestObj { public static void main (String [] args) { Object o = new Object() /* Line 5 */ { public boolean equals(Object obj) { return true; } } /* Line 11 */ System.out.println(o.equals("Fred")); } }

Correct answer: Compilation failsThe anonymous class is an expression, so the statement assigning it must end in a semicolon. The closing brace on line 11 is not followed by one, and the compiler reports the error there. Had the semicolon been present the program would have printed true, since the overridden equals returns true for any argument - which is what the first option describes.

Q: What will be the output of the program? public class HorseTest { public static void main (String [] args) { class Horse { public String name; /* Line 7 */ public Horse(String s) { name = s; } } /* class Horse ends */ Object obj = new Horse("Zippo"); /* Line 13 */ Horse h = (Horse) obj; /* Line 14 */ System.out.println(h.name); } } /* class HorseTest ends */

The code in the HorseTest class is perfectly legal. Line 13 creates an instance of the method-local inner class Horse, using a reference variable declared as type Object. Line 14 casts the Horse object to a Horse reference variable, which allows line 15 to compile. If line 14 were removed, the HorseTest code would not compile, because class Object does not have a name variable.

Q: What will be the output of the program? public abstract class AbstractTest { public int getNum() { return 45; } public abstract class Bar { public int getNum() { return 38; } } public static void main (String [] args) { AbstractTest t = new AbstractTest() { public int getNum() { return 22; } }; AbstractTest.Bar f = t.new Bar() { public int getNum() { return 57; } }; System.out.println(f.getNum() + " " + t.getNum()); } }

You can define an inner class as abstract, which means you can instantiate only concrete subclasses of the abstract inner class. The object referenced by the variable t is an instance of an anonymous subclass of AbstractTest, and the anonymous class overrides the getNum() method to return 22. The variable referenced by f is an instance of an anonymous subclass of Bar, and the anonymous Bar subclass also overrides the getNum() method (to return 57). Remember that to instantiate a Bar instance, we need an instance of the enclosing AbstractTest class to tie to the new Bar inner class instance. AbstractTest can't be instantiated because it's abstract, so we created an anonymous subclass (non-abstract) and then used the instance of that anonymous subclass to tie to the new Bar subclass instance.

Q: What will be the output of the program? public class Foo { Foo() { System.out.print("foo"); } class Bar { Bar() { System.out.print("bar"); } public void go() { System.out.print("hi"); } } /* class Bar ends */ public static void main (String [] args) { Foo f = new Foo(); f.makeBar(); } void makeBar() { (new Bar() {}).go(); } }/* class Foo ends */

Option C is correct because first the Foo instance is created, which means the Foo constructor runs and prints "foo". Next, the makeBar() method is invoked which creates a Bar, which means the Bar constructor runs and prints "bar", and finally the go() method is invoked on the new Bar instance, which means the go() method prints "hi".

What will be the output of the program?

public class TestObj 
{
    public static void main (String [] args) 
    {
        Object o = new Object() /* Line 5 */
        {
            public boolean equals(Object obj) 
            {
                return true;
            } 
        }      /* Line 11 */
        
        System.out.println(o.equals("Fred"));
    }
}

A.It prints "true".
B.It prints "Fred".
C.An exception occurs at runtime.
D.Compilation fails

Answer: D

Correct answer: Compilation fails

The anonymous class is an expression, so the statement assigning it must end in a semicolon. The closing brace on line 11 is not followed by one, and the compiler reports the error there. Had the semicolon been present the program would have printed true, since the overridden equals returns true for any argument - which is what the first option describes.

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What will be the output of the program?

public class HorseTest 
{
    public static void main (String [] args) 
    {
        class Horse 
        {
            public String name; /* Line 7 */
            public Horse(String s) 
            {
                name = s;
            }
        } /* class Horse ends */
        
        Object obj = new Horse("Zippo"); /* Line 13 */
        Horse h = (Horse) obj; /* Line 14 */
        System.out.println(h.name);
    }
} /* class HorseTest ends */

A.An exception occurs at runtime at line 10.
B.It prints "Zippo".
C.Compilation fails because of an error on line 7.
D.Compilation fails because of an error on line 13.

Answer: B

The code in the HorseTest class is perfectly legal. Line 13 creates an instance of the method-local inner class Horse, using a reference variable declared as type Object. Line 14 casts the Horse object to a Horse reference variable, which allows line 15 to compile. If line 14 were removed, the HorseTest code would not compile, because class Object does not have a name variable.

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What will be the output of the program?

public abstract class AbstractTest 
{
    public int getNum() 
    {
        return 45;
    }
    public abstract class Bar 
    {
        public int getNum() 
        {
            return 38;
        }
    }
    public static void main (String [] args) 
    {
        AbstractTest t = new AbstractTest() 
        {
            public int getNum() 
            {
                return 22;
            }
        };
        AbstractTest.Bar f = t.new Bar() 
        {
            public int getNum() 
            {
                return 57;
            }
        };
        
        System.out.println(f.getNum() + " " + t.getNum());
    }
}

A.57 22
B.45 38
C.45 57
D.An exception occurs at runtime.

Answer: A

You can define an inner class as abstract, which means you can instantiate only concrete subclasses of the abstract inner class. The object referenced by the variable t is an instance of an anonymous subclass of AbstractTest, and the anonymous class overrides the getNum() method to return 22. The variable referenced by f is an instance of an anonymous subclass of Bar, and the anonymous Bar subclass also overrides the getNum() method (to return 57). Remember that to instantiate a Bar instance, we need an instance of the enclosing AbstractTest class to tie to the new Bar inner class instance. AbstractTest can't be instantiated because it's abstract, so we created an anonymous subclass (non-abstract) and then used the instance of that anonymous subclass to tie to the new Bar subclass instance.

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What will be the output of the program?

public class Foo 
{
    Foo() 
    {
        System.out.print("foo");
    }
    
class Bar
{
    Bar() 
    {
        System.out.print("bar");
    }
    public void go() 
    {
        System.out.print("hi");
    }
} /* class Bar ends */

    public static void main (String [] args) 
    {
        Foo f = new Foo();
        f.makeBar();
    }
    void makeBar() 
    {
        (new Bar() {}).go();
    }
}/* class Foo ends */

A.Compilation fails.
B.An error occurs at runtime.
C.It prints "foobarhi"
D.It prints "barhi"

Answer: C

Option C is correct because first the Foo instance is created, which means the Foo constructor runs and prints "foo". Next, the makeBar() method is invoked which creates a Bar, which means the Bar constructor runs and prints "bar", and finally the go() method is invoked on the new Bar instance, which means the go() method prints "hi".

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