[Java] Instance method, instance field, class method, class field, constructor summary

Summarize some of the confusing parts of learning Java. I'm referring to Progate.

1. Instance method

The instance method is the "behavior" that the instance has and the instance has.

How to define a method

public <Return type> <Method name>(){
 <The contents of the method>
}

Write like.

How to call a method

/*Create an instance of Person class, person1*/
Person person1 = new Person();

/*Call the hello method*/
person1.hello();

Call like.

2. Instance field

The instance field is a variable that stores the information that the instance has.

How to define instance fields

class Person {
  /*Declare the type because it is a variable*/
  public String name;
}

How to access instance fields

/*Create an instance*/
Person person1 = new Person();
/*Set a value for name*/
person1.name = "Suzuki";

/*Get the value of name*/
System.out.println(person1.name);

You can access it like this.

3. Class method

A method that belongs to a class.

How to define a class method

class Person {
  public static <Return type> <Method name>() {
    <The contents of the method>
  }
}

• Note that unlike the instance method, static is added.

How to call a class method

<name of the class>.<Method name>();

Call it like this. The public static void <method> () {} seen from the parent's face is actually a class method.

4. Class field

Fields that belong to the class. A variable that stores the information that the class has.

How to define class fields

class Person {
  public static <Data type> <Variable name>;
}

• Unlike the instance field, add static

How to access the class field

Main.java

class Main {
  public static void main(String[] args) {
    System.out.println("total" + Person.count + "Is a person");
    Person person1 = new Person( ... );
    System.out.println("total" + Person.count + "Is a person");
  }
}

Person.java

class Person {
  /*Store information about the number of people in a variable called count*/
  public static int count = 0;
  .........
  Person(String firstName, ...) {
    Person.count ++;
  }
}

python

>0 people in total
>1 person in total

• Every time an instance is created, 1 is added to count.

5. Constructor

A method that is automatically called after creating an instance with new. Note that the definition method is fixed

How to define a constructor

python

class Person {
  public String name;
  Person() {
    /*What you want to do when creating an instance*/
  }
}

The constructor is the part of Person () {}. It is similar in writing and declaring a method. However, unlike the method, there is no return value and no void is required. Because the return value of the constructor is always an instance of that class, The compiler will tell you even if you don't specify it. Below is an example of a compiler.

When creating an instance, pass the value you want to set in the instance field to the argument of the constructor

Person.java

class Person {
  public String name;
  Person(String name) {
    this.name = name;
  }
}

How to call the constructor

Main.java

Person person = new Person("Suzuki");
System.out.println(person.name);

6. Summary

It's easy to get confused! that's all!