this

✅ this

• this refers to the field of the current class

• this is to distinguish which is field and which is paramter when the variable names are the same

• this.name: the name field of this class
• name: the nameparameter for method initMember

public class MemberInit {
    String name; //🔴
    int age;
    int grade;

    void initMember(String name //🔵, int age, int grade){
        this.name//🔴 = name; //🔵
        this.age = age;
        this.grade = grade;
    }

}

• in Main class to create a new member
• this code will create a new member, with name of member1

MemberInit member1 = new MemberInit2();
member1.initMember("member1", 15, 90);

• ⭐️ so if field name and parameter is different, no need for this

public class MemberThis {
    String nameField;

    void initMember(String nameParameter){
        nameField = nameParameter;
        //this.nameField = nameParameter; // this is also possible
    }
}

• ⭐️ this is to distinguish which is field and which is paramter
• ⭐️ if variable names were same in MemberThis the same code would look like this

public class MemberThis {
    String name;

    void initMember(String name){
        this.name = name;
    }
}

✅ Constructor

• 👍🏻 allows to create instance and set the fields at the same time
• 👍🏻 if instance is created and fields are not set, compile error

• (prevent ghost member with no fields being created)

• constuctor name == class name
• no return value

public class MemberConstruct {
    String name;
    int age;
    int grade;

    MemberConstruct(String name, int age, int grade){ //constuctor name == class name
        this.name = name;
        this.age = age;
        this.grade = grade;
    }
}

• 👎🏻 before

    MemberConstruct member1 = new MemberConstruct();
    member2.name = "member1";

• 👍🏻 after

    MemberConstruct member1 = new MemberConstruct("member1", 15, 90);
    //use constructor, 그러면
    //한번에 create class and field 가능

✅ Default contructor

• we need a constructor to create an instance

MemberDefault member = new MemberDefault();

• basic default constructor is automatically created by Java
• has no field

public class MemberDefault {
    String name;

    MemberDefault(){} //basic default constructor
}

• if there is any other constructor, basic constuctor will not be made

public class MemberDefault {
    String name;

    MemberDefault(String name){ //other constructor
        this.name = name;
    }
}

//now this is impossible
MemberDefault member = new MemberDefault(); //🔴compile error

//only this is possible
MemberDefault member = new MemberDefault("memberName"); //use other constructor

✅ constructor overloading

• not all fields in class has to be in constuctor

    MemberConstruct(String name, int age, int grade){
        this.name = name;
        this.age = age;
        this.grade = grade;
    }

    MemberConstruct(String name, int age){ //no grade, grade will be 0
        this.name = name;
        this.age = age;
    }

MemberConstruct member1 = new MemberConstruct("member1", 15, 90); //member1 15 90
MemberConstruct member2 = new MemberConstruct("member2", 16); //member2 16 0

• also can call constuctor in constuctor
• ⭐️ this() always has to come first line

    MemberConstruct(String name, int age, int grade){
        this.name = name;
        this.age = age;
        this.grade = grade;
    }

    MemberConstruct(String name, int age){ //call constructor in constructor
        this(name, age, 0); //⭐️first line
    }