What is the constructor for?

What is a constructor?

A special method that runs automatically when an instance is created ** You can freely control the initial state of the instance! ** **

That said, let's actually execute the code.

What if there is no constructor?

** Default for each type is entered in the value ** For example, suppose you have a program that retrieves customer information.

	//Define customer information
	class CustomerCard {
		int id; //Customer id
		String name; //Customer name
	}

	class CustomerManager {
		public static void main(String[] args) {
			CustomerCard out = new CustomerCard(); //Instance creation
			System.out.println(out.id); //Output customer id
			System.out.println(out.name); //Output customer name
		}
	}

When I do this, the result is:

0 //Customer id
null //Customer name

Why this happens

When a class is instantiated (new), member variables that do not contain any value are automatically initialized. Remember that there is a rule that numeric types such as int are initialized to 0, boolean types are initialized to false, and reference types such as String are initialized to null.

Try to put in the constructor

Let's put the constructor in the previous program, set the ** initial state **, and execute it.

	class CustomerCard {
		int id; //Customer id
		String name; //Customer name

	//**Set the initial value in the constructor**
	CustomerCard() {
			this.id = 1111;
			this.name = "taylor";
		}
	}

	class CustomerManager {
		public static void main(String[] args) {
			CustomerCard out = new CustomerCard(); //Instance creation
			System.out.println(out.id); //Output customer id
			System.out.println(out.name); //Output customer name
		}
	}

The result is this.

1111 //Customer id
taylor //Customer name

Yeah, the value specified in the constructor is the initial value. It would be nice to understand roughly that the constructor can set the initial value of the instance like this.