this Keyword Practice

Sample questions

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1. Sample 1difficulty 2/5

   public class Person {
       private String name;
       public Person(String name) {
           name = name;
       }
       public String getName() { return name; }
   }
   // ...
   Person p = new Person("Bo");
   System.out.println(p.getName());

   What is printed?

   • A

     null

     check_circle
   • B

     Bo

   • C

     name

   • D

     Compile-time error

   Why

   Without this.name = name the assignment only sets the parameter to itself; the instance variable name remains its default null.

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2. Sample 2difficulty 2/5

   public class Pair {
       private int a, b;
       public Pair(int a, int b) {
           this.a = a;
           this.b = b;
       }
       public int sum() { return a + b; }
   }
   // ...
   Pair p = new Pair(3, 4);
   System.out.println(p.sum());

   What is printed?

   • A

     7

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   • B

     0

   • C

     12

   • D

     Compile-time error

   Why

   this.a and this.b are set to 3 and 4; sum returns 7.

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3. Sample 3difficulty 3/5

   public class Point {
       private int x, y;
       public Point(int x, int y) {
           /* missing */
           this.y = y;
       }
   }

   Which line replaces /* missing */?

   • A

     this.x = this.x;

   • B

     x = x;

   • C

     this.x = x;

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   • D

     x = this.x;

   Why

   Use this.x to refer to the field shadowed by the parameter; otherwise the assignment is parameter-to-itself.

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4. Sample 4difficulty 3/5

   public class Box {
       private int width;
       public Box(int width) {
           width = width;
       }
   }

   What is wrong with the constructor?

   • A

     Nothing is wrong

   • B

     private fields cannot be set in a constructor

   • C

     The parameter shadows the field; assignment has no effect on the field

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   • D

     Constructors cannot have parameters

   Why

   The parameter width shadows the instance field of the same name. The assignment width = width copies the parameter to itself; it should be this.width = width.

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5. Sample 5difficulty 3/5

   public class A {
       public A() {
           System.out.print("hi ");
           this(5);
       }
       public A(int n) {}
   }
   // ...

   What is the result?

   • A

     Nothing is printed

   • B

     Run-time error

   • C

     Compile-time error: this(...) must be the first statement

     check_circle
   • D

     hi printed

   Why

   A call to this(...) (or super(...)) must appear as the very first statement of a constructor.

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