Super keyword in Java - Mini Project: Build & Apply

Understanding the Super Keyword in Java

📖 Scenario: Imagine you are creating a simple program to represent vehicles. You have a basic Vehicle class and a more specific Car class that inherits from Vehicle. You want to use the super keyword to access the parent class's constructor and methods.

🎯 Goal: You will build two classes: Vehicle and Car. You will use the super keyword to call the parent class constructor and method from the child class. Finally, you will print the details of the car using these calls.

📋 What You'll Learn

Create a class called Vehicle with a constructor that takes a String parameter called brand and stores it in an instance variable.

Create a method in Vehicle called displayInfo() that prints the vehicle brand.

Create a class called Car that extends Vehicle and has an additional instance variable model.

Use the super keyword in Car's constructor to call the Vehicle constructor.

Override the displayInfo() method in Car and use super.displayInfo() to print the brand before printing the model.

Create an instance of Car and call its displayInfo() method to show the output.

💡 Why This Matters

🌍 Real World

Using <code>super</code> is common when you want to build new classes based on existing ones, like creating different types of vehicles or employees with shared and unique features.

💼 Career

Understanding inheritance and the <code>super</code> keyword is essential for Java developers to write clean, reusable, and maintainable code in many software projects.

1

Create the Vehicle class with a brand variable and constructor

Create a class called Vehicle with a String instance variable called brand. Add a constructor that takes a String brand parameter and assigns it to the instance variable brand.

Java

class Vehicle {
    // Your code here
}

Hint

Remember to create a constructor with the same name as the class and assign the parameter to the instance variable.

2

Add a displayInfo() method to Vehicle

Inside the Vehicle class, add a method called displayInfo() that prints the text "Vehicle brand: " followed by the brand variable.

Java

class Vehicle {
    String brand;

    Vehicle(String brand) {
        this.brand = brand;
    }

    // Your code here
}

Hint

Use System.out.println to print the brand with the label.

3

Create the Car class that extends Vehicle with a model variable

Create a class called Car that extends Vehicle. Add a String instance variable called model. Create a constructor for Car that takes two String parameters: brand and model. Use the super keyword to call the Vehicle constructor with brand, and assign model to the instance variable.

Java

class Vehicle {
    String brand;

    Vehicle(String brand) {
        this.brand = brand;
    }

    void displayInfo() {
        System.out.println("Vehicle brand: " + brand);
    }
}

// Your code here

Hint

Use super(brand); inside the Car constructor to call the parent constructor.

4

Override displayInfo() in Car and print brand and model

In the Car class, override the displayInfo() method. Inside it, first call super.displayInfo() to print the brand. Then print "Car model: " followed by the model variable. Finally, create a Car object with brand "Toyota" and model "Corolla", and call its displayInfo() method.

Java

class Vehicle {
    String brand;

    Vehicle(String brand) {
        this.brand = brand;
    }

    void displayInfo() {
        System.out.println("Vehicle brand: " + brand);
    }
}

class Car extends Vehicle {
    String model;

    Car(String brand, String model) {
        super(brand);
        this.model = model;
    }

    // Your code here
}

// Create a Car object and call displayInfo()
// Your code here

Hint

Use super.displayInfo(); to reuse the parent method, then print the model. Create the Car object and call displayInfo() in the main 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