Inheritance

Inheritance is one of the advantages of Object Oriented Programming. It allows us to inherit the characters and actions from its parent and add its features to them if needed. One class can only inherit from one parent and if the parent has a parent the child can also inherit the characters and actions from the ancient.

List of Contents

• Structures
• Inheritance
  • Super()
  • Override
• Compositions

Structures

The object is the prime parent for all the classes and every class created inherits the object class implicitly. To explicitly inherit (for all other parent and child relationships) we use extends keyword.

Inheritance

▶ Super Class

package com.example.inheritencedemo.entities;

public class Vehicle {    
    private String dateProduced;
    private String speed;    

    public String getDateProduced() {
        return dateProduced;
    }

    public void setDateProduced(String dateProduced) {
        this.dateProduced = dateProduced;
    }

    public String getSpeed() {
        return speed;
    }

    public void setSpeed(String speed) {
        this.speed = speed;
    }

    public void move() {
        System.out.println("moving");
    }
}

▶ Sub Class

package com.example.inheritencedemo.entities;

public class Car extends Vehicle {
    private String model;
}

When we instantiate and run them, we can see that a child can use what is in the parent

Super()

super() is a keyword that calls the parent from a child class. It is similar to the this() keyword and since they both need to be on the top line of the code they cannot be used together. If the parent has only an argument constructor defined a child must call the constructor with parameters using the super() when calling it from its constructor.

super() is used to set common values in the parent and add what should be different for each child using the constructor.

▶ Super Class

package com.example.inheritencedemo.entities;

public class Vehicle {
    private String dateProduced;
    private String speed;

    public Vehicle(String dateProduced, String speed) {
        this.dateProduced = dateProduced;
        this.speed = speed;
    }

    public String getDateProduced() {
        return dateProduced;
    }

    public void setDateProduced(String dateProduced) {
        this.dateProduced = dateProduced;
    }

    public String getSpeed() {
        return speed;
    }

    public void setSpeed(String speed) {
        this.speed = speed;
    }

    public void move() {
        System.out.println("moving");
    }
}

▶ Sub Class

package com.example.inheritencedemo.entities;

public class Car extends Vehicle {
    private String model;

    public Car(String model) {
        super("2027", "good");
        this.model = model;
    }
}

Override

A child can override the method of its parent. Click Ctrl + O to display the list of the inherited methods

▶ Sub Class

package com.example.inheritencedemo.entities;

public class Car extends Vehicle {
    private String model;

    public Car(String model) {
        super("2027", "good");
        this.model = model;
    }

    @Override
    public void move() {
        super.move();
    }
}

Note that when a child has to access a file of a parent directly we can use the modifier protected

▶ Super Class

package com.example.inheritencedemo.entities;

public class Vehicle {
    private String dateProduced;
    private String speed;
    protected String type;

    public Vehicle(String dateProduced, String speed) {
        this.dateProduced = dateProduced;
        this.speed = speed;
    }

    public String getDateProduced() {
        return dateProduced;
    }

    public void setDateProduced(String dateProduced) {
        this.dateProduced = dateProduced;
    }

    public String getSpeed() {
        return speed;
    }

    public void setSpeed(String speed) {
        this.speed = speed;
    }

    public void move() {
        System.out.println("moving");
    }
}

▶ Sub Class

package com.example.inheritencedemo.entities;

public class Car extends Vehicle {
    private String model;

    public Car(String model) {
        super("2027", "good");
        this.model = model;
    }

    @Override
    public void move() {
        super.move();
        System.out.println(type + model);
    }
}

Compositions

Composition is similar to inheritance but represents things that are a part of a class. Since the composition is more flexible than the inheritance it is recommended to use composition over inheritance.

So far, we have seen inheritance in Java.

---

References

https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Object.html

Object (Java SE 17 & JDK 17)