Overriding Methods Practice

Sample questions

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1. Sample 1difficulty 2/5

   public class Point {
       private int x, y;
       public Point(int x, int y) { this.x = x; this.y = y; }
       public String toString() { return "(" + x + "," + y + ")"; }
   }
   System.out.println(new Point(3, 4));

   What is printed?

   • A

     (3,4)

     check_circle
   • B

     Point@<hash>

   • C

     3,4

   • D

     Compile-time error

   Why

   <code>println(Object)</code> calls <code>toString()</code>. The override returns "(3,4)".

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2. Sample 2difficulty 2/5

   public class Point {
       private int x, y;
       public Point(int a, int b) { x = a; y = b; }
       public String toString() {
           return "(" + x + "," + y + ")";
       }
   }
   // Usage:
   Point p = new Point(3, 4);
   System.out.print(p);

   What is printed?

   • A

     Point@hashcode

   • B

     3 4

   • C

     Compile error

   • D

     (3,4)

     check_circle

   Why

   System.out.print on an object calls toString(). The overridden toString returns "(3,4)".

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3. Sample 3difficulty 2/5

   public abstract class Shape {
       public abstract double area();
   }
   public class Circle extends Shape {
       private double r;
       public Circle(double radius) { r = radius; }
       // no area method defined
   }

   What is the result of compiling Circle?

   • A

     Compiles fine; area() returns 0

   • B

     Compiles but throws RuntimeException

   • C

     Compile error: Circle must implement area() or be declared abstract

     check_circle
   • D

     Compiles only with a warning

   Why

   A non-abstract subclass of an abstract class must implement all inherited abstract methods. Circle does not implement area(), so it must either implement it or be declared abstract.

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4. Sample 4difficulty 3/5

   public class Shape {
       public double area() { return 0.0; }
       public String describe() { return "Area=" + area(); }
   }
   public class Square extends Shape {
       public double area() { return 25.0; }
   }
   Shape s = new Square();
   System.out.println(s.describe());

   What is printed?

   • A

     Area=0.0

   • B

     Area=25.0

     check_circle
   • C

     Compile-time error

   • D

     Area=

   Why

   <code>describe()</code> calls <code>area()</code>. Through dynamic dispatch, <code>Square.area()</code> is invoked, returning 25.0.

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5. Sample 5difficulty 3/5

   public class Point {
       private int x, y;
       public Point(int a, int b) { x = a; y = b; }
       // no equals override
   }
   // Usage:
   Point a = new Point(1, 2);
   Point b = new Point(1, 2);
   System.out.print(a.equals(b));

   What is printed?

   • A

     0

   • B

     true

   • C

     Compile error

   • D

     false

     check_circle

   Why

   Without overriding equals, the inherited Object.equals compares references. a and b are different objects, so equals returns false.

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