leejunkim

Inheritance in java

3 min read

Key

A mechanism where one class inherits fields and methods from another class.

  • Use inheritance when you need hierarchy among ur objects
  • Everything that is not private in the original class will show up in the new class
    • extends keyword
    • the new class ALWAYS have the same or more members than the super class
  • There are no multiple inheritance of classes in java (multiple parents)
  • scalability
// Superclass / parent class
public class Keyboard {
 
    int keys = 100;
    String color = "White";
 
    public void pressed()
    {
        System.out.println("Pressed the keyboard");
    }
 
    public void throwIt()
    {
        System.out.println("Got hit!");
    }
}
 
// Subclass / child class
class AdvKeyboard extends Keyboard
{
     public void hitNum()
     {
         System.out.println("Hit the keyboard");
     }
}
  • Used commonly to represent hierarchical structures (계층 구조를 위해 많이 쓰임)
    • Person > Programmer > Backend, Frontend, AI, etc
  • Superclass
    • the parent ⇒ super class is the official term
  • Subclass
    • If we don’t write any constructors, it has the default constructor which contains super()
      • so the constructors are not inherited, but just called through super()
  • Method Overriding (재정의)

type casting

Person p1 = new Programmer();
  • the type actually automatically changes to Person from Programmer
  • but you wont be able to access the fields/methods only in programmer
  • using instanceof

super

Referencing members

  • Can reference members of the super class
    • Useful if you have members with the same name with the sub & super class
public class Example {
    public static void main(String[] args) {
        SubClass sub = new SubClass();
        sub.callNum();
    }
}
 
class SuperClass {
    int count = 20; // super.count
}
 
class SubClass extends SuperClass {
    int count = 15; // this.count
 
    void callNum() {
        System.out.println("count = " + count);           // 15
        System.out.println("this.count = " + this.count); // 15
        System.out.println("super.count = " + super.count); // 20
    }
}

Calling constructor

  • You only put it at the first line
Class Person 
{
	String name;
	int age;
 
	public Person(String name, int age)
	{
		this.name = name;
		this.age = age;
	}
 
	void walk()
	{
		System.out.println("Walking");
	}
}
 
class Programmer extends Person
{
	public Programmer(String name, int age)
	{
		super(name, age);
	}
	public Programmer(String name, int age, String companyName)
	{
		this(name, age);
		this.companyName = companyName;
	}
}
  • Just like this(), you need to put it at the 1st line of ur constructor
  • It calls the constructor of your super class with the arguments you put in

Disadvantages

  • tight coupling
    • if the super class changes, then the subsequent subclass will also chang
    • 남용하면 오히려 유지보수가 어려울 수 있음
  • Hard to keep track with deeper layers
    • java의 packages들은 상속이 엄청 많이 되어있음
    • 상속을 공부하려면 java의 package들을 보자 << 생각보다 엄청 도움 됨

Object

  • all classes inherit from Object automatically
    • and they can use Object’s methods

Graph View

Backlinks