Constructor & this

Constructor

Used when an instance of a class is created, initializes the attribute values

• if there is no constructor in the class, the compiler automatically makes a compiler that has no parameters
  • All classes have a constructor (whether explicitly made or not)
• roles
  • creates an instance
  • initializes the attribute values
• A constructor is called when using the new keyword
  • Java has 1 by default, you can explicitly create one
• Needs to be same name with the class

public class Person 
{
	private String name;
	private int age;
	private int cashAmount;
	
	public Person(String pName, int pAge, int pCashAmount) 
	{
		name = pName;
		age = pAge;
		cashAmount = pCashAmount;
	}
	// constructors can also have method overloading
	public Person(String pName, int pAge) 
	{
		name = pName;
		age = pAge;
		cashAmount = 0;
	}
}
 
// can use it like this
Person p1 = new Person("Leejun", 23, 300000);
Person p1 = new Person("Kirby", 23);

• Constructor overloading is possible!
  • Method overloading

this

Refers to the current object — the instance of the class you’re working inside

Referring to the Current Object

• Distinguishes between instance variables and parameters

public class Person {
    private String name;
 
    public Person(String name) {
        this.name = name; // 'this.name' refers to the instance variable
    }
}

Calling Another Constructor

• You can use this(...) to call another constructor in the same class
• Rules
  • Must be the first statement in the constructor.
  • Syntax: this(arg1, arg2, ...)

public Person(String name) {
    this(name, 12, 0); // 12살을 기본 나이로 설정, 초기 현금 보유액은 0원.
}
 
public Person(String name, int age) {
    this(name, age, 0); // 초기 현금 보유액은 0원.
}
 
public Person(String name, int age, int cashAmount) {
    if (age < 0) {
        this.age = 12;
    } else {
        this.age = age;
    }
 
    if (cashAmount < 0) {
        this.cashAmount = 0;
    } else {
        this.cashAmount = cashAmount;
    }
    this.name = name;
}

• Inside the shorter constructors, we’re calling the most complete constructor to reduce code duplication
• it’s effective if the constructors with the least parameters call the constructor with the most parameters