Super keyword in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the parent class constructor.

Java

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

class Child extends Parent {
    Child() {
        [1]();
        System.out.println("Child constructor called");
    }
}

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

Drag options to blanks, or click blank then click option'

Asuper

Bthis

Cparent

Dbase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this()' instead of 'super()' to call the parent constructor.

Trying to call 'parent()' which is not a valid keyword.

The keyword 'super' is used to call the parent class constructor in Java.

2fill in blank

medium

Complete the code to access the parent class method from the child class.

Java

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

class Child extends Parent {
    void show() {
        [1].show();
        System.out.println("Child show method");
    }
}

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

Drag options to blanks, or click blank then click option'

Athis

Bparent

Csuper

Dbase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this.show()' which calls the child method recursively.

Using 'parent.show()' which is not valid in Java.

The 'super' keyword is used to call the parent class method from the child class.

3fill in blank

hard

Fix the error in the constructor call to the parent class.

Java

class Parent {
    Parent(int x) {
        System.out.println("Parent constructor with value: " + x);
    }
}

class Child extends Parent {
    Child() {
        [1](10);
        System.out.println("Child constructor");
    }
}

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

Drag options to blanks, or click blank then click option'

Abase

Bthis

Cparent

Dsuper

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this(10);' which calls the same class constructor and causes recursion.

Using 'parent(10);' which is not a valid keyword.

Use 'super(10);' to call the parent constructor with an argument.

4fill in blank

hard

Fill the blank to override a method and call the parent method inside it.

Java

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

class Child extends Parent {
    void display() {
        [1].display();
        System.out.println("Child display");
    }
}

public class Main {
    public static void main(String[] args) {
        Child c = new Child();
        c.display();
    }
}

Drag options to blanks, or click blank then click option'

Asuper

Bthis

Cparent

Dbase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this.display();' which causes recursive call.

Using 'parent.display();' which is invalid.

The 'super' keyword is used to call the parent class method from the overridden method in the child class.

5fill in blank

hard

Fill both blanks to use super keyword for constructor and method calls.

Java

class Parent {
    Parent() {
        System.out.println("Parent constructor");
    }
    void greet() {
        System.out.println("Hello from Parent");
    }
}

class Child extends Parent {
    Child() {
        [1]();
        System.out.println("Child constructor");
    }
    void greet() {
        [2].greet();
        System.out.println("Hello from Child");
    }
}

public class Main {
    public static void main(String[] args) {
        Child c = new Child();
        c.greet();
    }
}

Drag options to blanks, or click blank then click option'

Athis

Bsuper

Cparent

Dbase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this()' in constructor to call itself causing recursion.

Using 'parent' or 'base' which are invalid keywords.

Use 'super()' to call the parent constructor and 'super.greet()' to call the parent method.

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