Java learning memo (basic)

Since I started studying Java Silver, I will upload it as my memorandum. Since it is a rough memo, details are not described.

Chapter 1 Java Basics

1. What can be included in the class declaration ・ Method ·field -Constructor (function executed at the same time as declaring a class with new)

2.3. package ・ Mechanism for grouping and separating classes and interfaces ・ Group by folder -Always belong to a package (unspecified items are treated as a group of "anonymous packages" → only the same anonymous packages can be accessed) -Package name "package 1st layer folder name. 2nd layer folder name. 3rd layer folder name;" -Describe to the first line of the source code

Example) 　Package aaa; 　Import java.io.*; 　Public class Sample{ 　}

import declaration Only classes in the same package as the Java.lang package can omit the import declaration. 　↓ Basic classes (String, System, etc.)

The "" asterisk refers to all classes that belong to that package. Example) 　Import java.util.;

Inheritance Example) 　Public class Sample{ 　protected int num = 10; 　}

If you want to inherit the above Sample class and use variables, public (modifier) class Derived class name extends Base class name {} to inherit. The class name that the base class name wants to use.

Same class, same package, other packages, different subclasses Public　　　 ○　　　　　　　○　　　　　　　○　　　　　　　○ private　　　 ○　　　　　　　☓　　　　　　　☓　　　　　　　☓ protected　　○　　　　　　　○　　　　　　　☓　　　　　　　○ None ○ ○ ○ ○ ○ ☓ ☓ ☓

6.7. Static import -Static = class variables and class declarations Description) Class name.Variable name Must be described every time as class name.method name. 　↓ Import static ~. Class name. Variable name Import static ~. Class name. Method name By importing above, you only need to describe the variable name and method name after that. Even if there are multiple overloaded methods with the same name, it is judged by the type and type of the argument, so only one import description is OK.

・ Class declaration Declare fields and methods. → Called a member.

·override Subclass methods work instead of superclass methods with the same name

Ststic If there is a variable with the same name in the imported class, the imported variable will be ignored.

Entry point When there are multiple methods in a class, the method to start processing is called an entry point. The description method in Java has been decided. Description) Public static void main (String [] argument name) { 　} ・ Being open to the public -Can be executed without creating an instance statically (call with new synonym for object) -The return value cannot be returned (void) -The method name must be main -The argument should receive one String array type 　↓ The String type with variable length arguments is also OK because it will be converted to an array type when compiling. Description) Public static void main (String ... argument name) { 　}