Getting Started with Java_Chapter 8_About Instances and Classes

Introduction

These are ** terms ** and ** commentary collections ** that I, as a super-dreadnought beginner, have put together so that I can understand them. Getting Started with Java-Chapter 8-What do "instances" and "classes" mean? How is it working? However, it is summarized so that it can be understood.

How to create a virtual world

-** Procedure for creating an object **

① Define a class ② Create an object based on the class.

--Classes and objects are completely different.

--The active in the virtual world is " instance ".

• object ** Ambiguous ** that is sometimes used to refer to a class.

• instance It is not a mold (class), but a strict indication of the entity that is active in the virtual world created from that mold.

-** Instantiation ** Creating an instance from a class.

-** [Relationship between instance and class] ** It is ** "instance" ** that plays an active role in the virtual world, and the mold for creating that instance is ** "class" **.

Two classes in the program

"Character class" "God class"

• These two classes are necessary to create a program.

How to define a class

[Overview]

--"Class declaration" --"Declaration of attributes" --"Constant field"

• 「this」
• "member"

** Class declaration **

Hero.java

1   public class Hero {
2   }

** Attribute declaration (field) **

Hero.java

1   public class Hero {
2       String name;
3       int hp;
4   }

-** Field ** A ** variable ** declared ** inside a class block.

-** Field Declaration ** To declare an attribute, write ** variable declaration ** in class block.

*** "name" ** and ** "hp" ** are ** fields **.

** Constant field **

Prefix the field declaration with ** final **. By adding final, value cannot be rewritten.

teisuu.java

1   public class Matango {
2       int hp;
3       final int LEVEL = 10;

this It means a ** instance ** of "self ".

--The dot "." Has the same meaning as "no".

Matango.java

1   public class Matango {
2       int hp;
3       void sleep() {
4           this.hp = 100;
5           System.out.println( this.name + "Sleeped and recovered!");
6

Matango.java

 this.hp = 100;

-** Means that the value 100 is assigned to the hp field of your own instance **.

-** This is not omitted. ** ** Unexpected behavior may occur. When using a field, explicitly add this.

member

The elements in the class, " field " and " method ".

Class definition

① You will be able to ** create an instance ** based on that class. ② The type of variable that contains the ** instance born from that class ** becomes available. Example) Define Hero class → Hero type variables can be used.

-** Class type ** ** Type ** that can be used by defining a class.

• The method of preparing class type variables is the same as for int type and String type. Example) ** Hero (class type) h (variable) = 100 (value); **

-** Instance ** is usually used by putting it in ** class type variable **. Example) ** Hero h; **

-** Reasons for using class variables ** This is to programmatically identify one specific instance from among multiple instances with the same name that can exist in the virtual world.

How to make a god class (class with main method)

--God class is a class ** that has a ** main method. --The main method is the method used when ** executing the program **. (Programs created in other classes such as Hero class will work by using the main method)

Main.java

public static void main(String[] args) {

[Overview]

--"Instance creation" --"Assigning a value to a field" --"Call method of instance"

** Instantiation **

-** [Class name variable name = new class name ();] ** Example) ** Hero h = new (); **

--The instance is created in the part called "new Hero ()" ** on the right side, and the ** instance created by "=" is assigned to the type variable **.

** Assigning a value to a field **

-** [Variable name. Field name = Value;] **

Main.java

h.name = "Minato";

--Grammar for assigning values to the field of hero h See Listing 8-12 on page 319.

Hero.java

1   public class Hero {
2       String name;
3       int hp;
4       void attack();
5       void run();

Main.java

1   public class Main {
2       public static void main (String[] args) {
3           Hero h = new Hero () ;
4           h.name = "Minato"  ;  
5           h.hp = "100" ;
6           System.out.println("Brave" + h.name + "Was born!");
7       }
8   }

Description of ** "h.name =" Minato ";" ** of Main class

--h is the "h" of Hero h. It is a variable "h" of type Hero.

--name is a ** field ** defined in the Hero class.

-"Minato" is a ** value **.

** Invoking method of instance **

-** [Variable name. Method name ()] **

Method.java

h.sit(5);
h.slip();
h.sit(25);
h.run();

--h is ** h (variable) ** of Hero type h. --sit, slip, sit, run are ** method names **. --The "5" and "25" passed as arguments are ** values **.

To summarize briefly

--Use "new" to create an instance. --When using a field, "variable name.field name" --When calling a method, "variable .method name ()"

Summary

What do "instances" and "classes" mean in Getting Started with Java-Chapter 8-? Also, how is it working? I made it so that you can understand.

References

A refreshing introduction to Java-Second Edition-Impress Publishing, Inc. Kiyotaka Nakayama / Daigo Kunimoto