[JAVA] Difference between class and instance

In this article, we aim to understand the nuances of classes, not the implementation or instantiation of them.

What is a class

Blueprint. Or something like a concept defined to create an entity.

What is an instance

An entity created based on a blueprint called a class. Take a car as an example We manufacture a ** instance ** (entity) called Benz from the ** class ** (design document) of the car. Each instance has different characteristics. If you make a black car, some of them will be Toyota cars, some will be Mercedes-Benz cars, they will be different in size and shape, and no instance will be the same.

Let's write the code and imagine

Here we use ** java **. We will create a car design document called a car class and create an instance.

1. Class design

First, let's create a car class design document. Please refer to the following page for the specific method of creating the design document. (Please wait as it is still under construction)


class Car {
    //Field declaration
    public String name = ""; //Initial value of the name ・ ・ ・ 1

    public String color = ""; //Initial color value ・ ・ ・ 2

    public int speed = 0; //Initial value of speed ・ ・ ・ 3

    //Method declaration
    public void setCar(String n, Sring c) { //・ ・ ・ 4
        this.name = n; //Set the instance name ... 4-1
        this.color = c; //Set the instance color ... 4-2

   public void setSpeed(int a) {// 5
        this.speed = a;//Initial value of speed ・ ・ ・ 5-1

Even if you don't know java, you can understand it intuitively. I made a car design document (class). The defined contents are as follows

  1. Set the car to have a name feature
  2. Set the car to have the characteristic of color
  3. Set the car to have the characteristic of speed
  4. Add specifications that allow you to set the name and color of the car
  5. Set a name for the car instance
  6. Set the color for the car instance
  7. Add specifications that can set the speed to the car
  8. Set the speed on the car instance

This is just a design document, so unless you create an instance I just created a design document.

2. Create an instance

This time, create a car instance based on the created design document.

--The name is Benz --The color is white --Speed is 100

will do.

In the Test.Java class, as shown below Add a method called makeCar and write it in it.


    public void makeCar() { 
       Car myCar = new Car(); //・ ・ ・ 1
       myCar.setCar("Benz","white"); //・ ・ ・ 2
       myCar.setSpeed(100); //・ ・ ・ 3

The contents defined in this makeCar are

  1. Create a car instance
  2. Set the name Benz and the color white to the created instance.
  3. Set speed 100 for the created instance


When processing object-oriented in languages such as Java and objective-c, the words ** class ** and ** instance ** will appear repeatedly.

If you don't know, it's something you usually have around you, and you may find it easier to create a class or instance and replace it.

I wrote an article below that explains the difference between instance methods and class methods. I hope you can refer to that as well. (making)

If you have any suggestions, please let me know! !!

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