Java-Class Instance Creation Part.2

About how to write an article

I don't refer to class slides, so I think it's explained from a different perspective. Maybe I'm saying it (should).

Please let me know if you may make a mistake because it is an ad lib!

It consists of 4 parts

Part.1 Try to instantiate it Part.2 Cleaner with constructor ← Imakoko Part.3 Let's control the access and limit the incoming value Part.4 Try using class variables / methods

Tether

In Part.1, I wrote a digital student ID card as a class and made Tanaka-kun and Noguchi-kun's student ID cards as instances. I will reuse the code this time as well.

DigitalStudentCard.java


class DigitalStudentCard{
  //Student number
  int num;

  //name
  String name;

  //Method for output (introduce yourself)
  String getSelfIntroduction(){
    return "Student number: " + num + "name: " + name);
  }
}

class UseClass{
  public static void main(String[] args){

    //Instance generation
    DigitalStudentCard tanaka = new DigitalStudentCard();

    //Assign to instance variable
    tanaka.num = 118114;
    tanaka.name = "Kakuei Tanaka";


    //Create another instance
    DigitalStudentCard noguchi = new DigitalStudentCard();

    //Assign to the variable of the newly created instance
    noguchi.num = 118514;
    noguchi.name = "Hideyo Noguchi";

    //output
    System.out.println(tanaka.getSelfIntroduction);
        //Student number:118114 Name:Kakuei Tanaka

    System.out.println(noguchi.getSelfIntroduction);
        //Student number:118514 Name:Hideyo Noguchi
  }
}

This time, let's use a constructor </ b> to write these code smarter.

What is a constructor?

A constructor is a method that is called only once when an instance is created </ b>. It's like the void setup () function of Processing was a function that is executed only once when the application starts.

It is used when you want to insert a value together when creating an instance.

I will write

Try adding a constructor to DigitalStudentCard. It's annoying to put it in later, so you can set num and name when instantiating

The constructor can be implemented by simply writing the class name as it is </ b>. If you want to receive arguments, you can write formal arguments just like a method.

DigitalStudentCard.java


class DigitalStudentCard{
  //Student number
  int num;

  //name
  String name;

  //constructor
  DigitalStudentCard(int num, String name){
    //Argument num to instance num(this.num)Substitute to
    this.num = num;

    //Also
    this.name = name;
  }

  //Method for output (introduce yourself)
  String getSelfIntroduction(){
    return "Student number: " + num + "name: " + name);
  }
}

When using this, give an argument at new DigitalStudentCard ().

class UseClass{
  public static void main(String[] args){

    //Instance generation
    DigitalStudentLicense tanaka = new DigitalStudentCard(118114, "Kakuei Tanaka");

    //Create another instance
    DigitalStudentLicense noguchi = new DigitalStudentCard(118514, "Hideyo Noguchi");

    //output
    System.out.println(tanaka.getSelfIntroduction);
        //Student number:118114 Name:Kakuei Tanaka

    System.out.println(noguchi.getSelfIntroduction);
        //Student number:118514 Name:Hideyo Noguchi
  }
}

By using the constructor, I was able to shorten the code even further.

By the way

Since the constructor can be overloaded like the method, you can write multiple constructors to branch the process.

DigitalStudentCard.java


class DigitalStudentCard{
  //Student number
  int num;

  //name
  String name;

  //constructor
  DigitalStudentCard(int num, String name){
    //Argument num to instance num(this.num)Substitute to
    this.num = num;

    //Also
    this.name = name;
  }

  DigitalStudentCard(){
    //Part.Make it possible to use it in the same way as 1.
  }

  //Method for output (introduce yourself)
  String getSelfIntroduction(){
    return "Student number: " + num + "name: " + name);
  }
}

next time

Next, let's do access modifiers! Next → Part.3 Control access to limit incoming values