Super keyword in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of super keyword in constructor call

What is the output of this Java program that uses super to call the parent class constructor?

Java

class Parent {
    Parent() {
        System.out.println("Parent constructor");
    }
}

class Child extends Parent {
    Child() {
        super();
        System.out.println("Child constructor");
    }
}

public class Test {
    public static void main(String[] args) {
        new Child();
    }
}

A Child constructor

B Child constructor\nParent constructor

C Parent constructor

D Parent constructor\nChild constructor

💡 Hint

Remember that super() calls the parent constructor before the child constructor body runs.

The super() call in the Child constructor invokes the Parent constructor first, printing "Parent constructor". Then the Child constructor prints "Child constructor".

❓ Predict Output

intermediate

Using super to access parent method

What is the output of this Java program that uses super to call a parent class method?

Java

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
    void printSounds() {
        sound();
        super.sound();
    }
}

public class Test {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.printSounds();
    }
}

A Animal sound\nDog barks

B Dog barks\nAnimal sound

C Dog barks\nDog barks

D Animal sound\nAnimal sound

💡 Hint

The super keyword calls the parent class method, while calling the method normally calls the child class version.

Calling sound() inside printSounds() calls the overridden method in Dog, printing "Dog barks". Calling super.sound() calls the Animal class method, printing "Animal sound".

❓ Predict Output

advanced

Output when super keyword used with variables

What is the output of this Java program that uses super to access a parent class variable?

Java

class Base {
    int x = 10;
}

class Derived extends Base {
    int x = 20;
    void printX() {
        System.out.println(x);
        System.out.println(super.x);
    }
}

public class Test {
    public static void main(String[] args) {
        Derived d = new Derived();
        d.printX();
    }
}

A 20\n10

B 10\n20

C 10\n10

D 20\n20

💡 Hint

The variable x in Derived hides the one in Base. Use super.x to access the parent's variable.

The x in Derived is 20, so System.out.println(x) prints 20. The super.x accesses the Base class variable, which is 10.

❓ Predict Output

advanced

Output of super in multi-level inheritance

What is the output of this Java program using super in a multi-level inheritance chain?

Java

class A {
    void show() {
        System.out.println("A show");
    }
}

class B extends A {
    void show() {
        System.out.println("B show");
    }
}

class C extends B {
    void show() {
        super.show();
        System.out.println("C show");
    }
}

public class Test {
    public static void main(String[] args) {
        C c = new C();
        c.show();
    }
}

A B show\nC show

B A show\nC show

C C show\nB show

D A show\nB show\nC show

💡 Hint

In class C, super.show() calls the immediate parent class B's show() method.

The super.show() in C calls B's show(), which prints "B show". Then C prints "C show".

🧠 Conceptual

expert

Error caused by incorrect use of super

Which option will cause a compile-time error due to incorrect use of super in Java?

A Using <code>super</code> to call a parent class method from a child instance method

B Using <code>super</code> to access a parent class variable when it is hidden by child variable

C Using <code>super()</code> inside a static method

D Using <code>super()</code> as the first statement in a child constructor

💡 Hint

Remember that super relates to instance context, not static context.

Using super() inside a static method causes a compile-time error because static methods do not belong to an instance, so super cannot be used there.

Practice

1. What does the super keyword do in Java?

easy

A. It defines a static method in the class.

B. It creates a new object of the child class.

C. It accesses methods and variables from the parent class.

D. It terminates the program execution.

Solution

1. Step 1: Understand the role of super

   The super keyword is used to refer to the parent class's members (methods or variables) from a child class.

2. Step 2: Compare options with definition

   Only It accesses methods and variables from the parent class. correctly describes this behavior. Other options describe unrelated actions.

3. Final Answer:

   It accesses methods and variables from the parent class. -> Option C

4. Quick Check:

   super accesses parent members = A [OK]

Hint: Remember: super means parent class access [OK]

Common Mistakes:

• Thinking super creates new objects
• Confusing super with this keyword
• Assuming super ends program

2. Which of the following is the correct way to call a parent class constructor in Java?

easy

A. super();

B. this();

C. parent();

D. base();

Solution

1. Step 1: Recall syntax for parent constructor call

   In Java, super() is used inside a child constructor to call the parent class constructor.

2. Step 2: Evaluate options

   Only super(); uses the correct keyword super(). Others are invalid or refer to different concepts.

3. Final Answer:

   super(); -> Option A

4. Quick Check:

   Parent constructor call = super() [OK]

Hint: Use super() to call parent constructor [OK]

Common Mistakes:

• Using this() instead of super()
• Trying to call parent() which is invalid
• Confusing base() with super()

3. What will be the output of the following code?

class Parent {
  int x = 10;
}
class Child extends Parent {
  int x = 20;
  void printX() {
    System.out.println(super.x);
  }
}
public class Test {
  public static void main(String[] args) {
    Child c = new Child();
    c.printX();
  }
}

medium

A. 20

B. Compilation error

C. 0

D. 10

Solution

1. Step 1: Understand variable hiding and super usage

   The child class has its own x = 20, but super.x accesses the parent's x which is 10.

2. Step 2: Trace the print statement

   The method printX() prints super.x, so it prints 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   super.x accesses parent variable = 10 [OK]

Hint: super.variable accesses parent class variable [OK]

Common Mistakes:

• Printing child variable instead of parent
• Confusing super.x with this.x
• Expecting compilation error

4. Identify the error in this code snippet:

class Parent {
  void show() {
    System.out.println("Parent show");
  }
}
class Child extends Parent {
  void show() {
    super.show();
    System.out.println("Child show");
  }
  void display() {
    super();
  }
}

medium

A. super.show() is invalid inside child class

B. super() cannot be called like a method in display()

C. Child class cannot override show() method

D. No error, code is correct

Solution

1. Step 1: Check usage of super in methods

   Calling super.show() inside overridden method is valid to call parent method.

2. Step 2: Analyze super() call in display()

   super() can only be used to call parent constructor inside child constructor, not as a method call elsewhere.

3. Final Answer:

   super() cannot be called like a method in display() -> Option B

4. Quick Check:

   super() only in constructor = D [OK]

Hint: super() only calls parent constructor inside child constructor [OK]

Common Mistakes:

• Using super() outside constructor
• Thinking super.show() is invalid
• Believing overriding is not allowed

5. Given these classes:

class Animal {
  String name;
  Animal(String name) {
    this.name = name;
  }
  void sound() {
    System.out.println("Animal sound");
  }
}
class Dog extends Animal {
  Dog() {
    super("Dog");
  }
  void sound() {
    super.sound();
    System.out.println("Bark");
  }
}
public class Test {
  public static void main(String[] args) {
    Dog d = new Dog();
    d.sound();
    System.out.println(d.name);
  }
}

What is the output when running Test.main()?

hard

A. Animal sound
Bark
Dog

B. Bark
Animal sound
Dog

C. Animal sound
Dog
Bark

D. Compilation error due to super()

Solution

1. Step 1: Understand constructor chaining

   The Dog constructor calls super("Dog"), setting name to "Dog" in Animal.

2. Step 2: Trace the sound() method call

   Dog.sound() calls super.sound() which prints "Animal sound", then prints "Bark".

3. Step 3: Print the name field

   Printing d.name outputs "Dog".

4. Final Answer:

   Animal sound Bark Dog -> Option A

5. Quick Check:

   super() sets name, super.sound() prints parent sound = A [OK]

Hint: super() sets parent state; super.method() calls parent method [OK]

Common Mistakes:

• Expecting Dog before Animal sound
• Confusing order of prints
• Thinking super() causes error