Writing Constructors for Subclasses Practice

Sample questions

5 of 19 — sign in to practice the rest with adaptive difficulty and mastery tracking.

1. Sample 1difficulty 2/5

   public class Animal {
       private String name;
       public Animal(String n) { name = n; }
       public String getName() { return name; }
   }
   public class Dog extends Animal {
       private String breed;
       public Dog(String n, String b) {
           super(n);
           breed = b;
       }
   }
   // Usage:
   Dog d = new Dog("Rex", "Lab");
   System.out.print(d.getName());

   What is printed?

   • A

     Lab

   • B

     RexLab

   • C

     Rex

     check_circle
   • D

     Compile error: Dog has no getName

   Why

   Dog inherits getName() from Animal. The super(n) call passes "Rex" to Animal's constructor, which assigns it to name. Calling d.getName() returns "Rex".

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2. Sample 2difficulty 2/5

   public class A {
       public A() { System.out.print("A "); }
   }
   public class B extends A {
       public B() { System.out.print("B "); }
   }
   new B();

   What is printed when <code>new B()</code> runs?

   • A

     A B

     check_circle
   • B

     B A

   • C

     B

   • D

     A

   Why

   Every subclass constructor implicitly calls <code>super()</code> first. So <code>A()</code> runs, printing "A ", then <code>B()</code> prints "B ".

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3. Sample 3difficulty 2/5

   public class Base {
       public Base() { System.out.print("B "); }
   }
   public class Child extends Base {
       public Child() { System.out.print("C "); }
   }
   // ...
   new Child();

   What is printed?

   • A

     B

   • B

     C

   • C

     C B

   • D

     B C

     check_circle

   Why

   Java implicitly calls super() before the body of the subclass constructor, so Base prints first.

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4. Sample 4difficulty 2/5

   public class Person {
       private String name;
       public Person(String n) { name = n; }
       public String getName() { return name; }
   }
   public class Student extends Person {
       private int id;
       public Student(String n, int i) {
           super(n);
           id = i;
       }
   }
   // Usage:
   Student s = new Student("Maya", 7);
   System.out.print(s.getName());

   What is printed?

   • A

     Compile error

   • B

     null

   • C

     7

   • D

     Maya

     check_circle

   Why

   super("Maya") sets name in Person. getName returns "Maya".

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5. Sample 5difficulty 3/5

   public class A {
       protected int x;
       public A() { x = 1; }
       public A(int v) { x = v; }
   }
   public class B extends A {
       public B() { super(5); }
   }
   // Usage:
   B b = new B();
   System.out.print(b.x);

   What is printed?

   • A

     5

     check_circle
   • B

     1

   • C

     Compile error

   • D

     0

   Why

   super(5) calls A's parameterized constructor, setting x to 5.

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