Object Superclass Practice

Sample questions

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1. Sample 1difficulty 2/5

   public class Foo {
   }
   // ...
   Foo f = new Foo();
   Object o = f;

   Which statement is true?

   • A

     Object is a primitive type

   • B

     Foo cannot be assigned to an Object reference

   • C

     Every class implicitly extends Object, so Foo is-a Object

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   • D

     Foo cannot inherit from Object because Foo has no superclass

   Why

   All Java classes (other than Object itself) implicitly extend Object, so a Foo reference is assignable to an Object reference.

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2. Sample 2difficulty 3/5

   public class P {
       private int x;
       public P(int x) { this.x = x; }
       public boolean equals(Object o) {
           if (!(o instanceof P)) return false;
           return ((P) o).x == this.x;
       }
   }
   // ...
   P a = new P(3);
   String s = "3";
   System.out.println(a.equals(s));

   What is printed?

   • A

     false

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   • B

     true

   • C

     Compile-time error

   • D

     ClassCastException

   Why

   Since s is not an instance of P, equals returns false at the first check.

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3. Sample 3difficulty 3/5

   public class P {
       private int x;
       public P(int x) { this.x = x; }
       public boolean equals(Object o) {
           if (!(o instanceof P)) return false;
           return ((P) o).x == this.x;
       }
   }
   // ...
   P a = new P(3);
   P b = new P(4);
   System.out.println(a.equals(b));

   What is printed?

   • A

     false

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   • B

     true

   • C

     3

   • D

     4

   Why

   The x values 3 and 4 differ, so equals returns false.

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4. Sample 4difficulty 3/5

   public class Plain {
       private int x = 5;
   }
   // ...
   Plain p = new Plain();
   String s = p.toString();
   System.out.println(s.startsWith("Plain@"));

   What is printed?

   • A

     Plain@5

   • B

     Compile-time error

   • C

     true

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   • D

     false

   Why

   Without overriding, Object's toString returns the form ClassName@hash, so the string starts with "Plain@".

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5. Sample 5difficulty 3/5

   Methods inherited from Object include

   • A

     main, exit

   • B

     add, remove

   • C

     size, length

   • D

     toString, equals, hashCode

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   Why

   Object provides toString(), equals(Object), hashCode(), and others.

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