day 1 - day 4

Preface

I learned it intermittently for four months java Basics , Now I have almost forgotten , I want to spend another half month or so reviewing and sorting , This time, I only take notes of the key parts , Here is 1-4 Days of

One 、day 1 : JAVA Overview is environment related

1、java Commonly used DOS Commands and shortcuts

Refer to the previous article ：IDEA Common shortcut keys and setting methods

2、 install JDK And environment configuration

1、JDK、JRE、JVM The relationship between

JDK：JAVA Development kit
JRE：JAVA Running environment
JVM：java virtual machine
JDK = JRE+ java Development tools
JRE = JVM + java Core class library of （ class ）

2、JAVA Loading and execution process

1、 To write java Source code
2、javac compile , Get the corresponding .class Bytecode file
3、java function , Compile well class File loading to jvm Run in

3、 Why configure environment variables ？ How to configure ？

1、java In development , There needs to be one java Running environment （JDK）, Otherwise it won't work
2、 We put JDK Usually download to a directory （xx disc xx file ）. For example, you are in DOS Command , Unless you are installing JDK Of bin\ Execute under directory , otherwise java Orders cannot be executed .
Why? ？
1、 If the program we are currently executing （ Or order ） Does not exist under the current directory （ No, , Can't find ）,win10 The system will have an existing system named path Found in the directory specified by the environment variable of .

2、 If not found , There will be “javac perhaps java Not an internal or external command , Nor are they runnable programs or batch files ” A hint of .

3、 therefore , If we get into JDK The installation path \bin Under the table of contents , perform java or javac,
You will see javac Parameter prompt information （ That means it can be implemented java The command ）

4、 And we configure environment variables in path Add the directory we need to the environment variable of （ That is to say java Operating environment ）
So we can not only install JDK Of bin\ In the directory , It can be implemented in the whole system .（ Any directory can execute ）

5、 Because it will point to JAVA_HOME\bin, That is to say jdk Under the bin\ Catalog ）
Therefore, add JAVA_HOME Variable and its value is JDK The path of .
Configuring environment variables takes only two steps ：
① Increase in system variables JAVA_HOME, The path is where you are

② In environment variable path Editor in chief （ add to ）%JAVA_HOME%\bin This path

That is to say, we can make the system stay （ That's what it says Existing ） path Specified directory of environment variables （bin\） Look under （ because bin\ The following are executable java file ）

6、 We just configured the environment variables so that the system can find what we need to perform java or javac Instructions （ Because we added JAVA_HOME The variable of , And its path is JDK Where it is （ route ）.）

7、 hinder path Added to the variable %JAVA_HOME%\bin And in the JDK Under the directory foundation of that layer, go further jdk Under the bin\ Catalog
Since then , The system is in path The directory specified by the environment variable can be found ,

8、 Because we have added JAVA The catalog of ,
So in the future, we need to implement JAVA perhaps javac The command can run normally ~）

ps1： Why not path Write in bin\ What about the catalogue ？
If jdk The path of changes just need to be modified JAVA_HOME One place can , Don't change it all .
Later, if you need to install more jdk Version only needs to be modified in one place .

Put a link ： Why configure environment variables ？ How to configure ？

Two 、day 2 : Variable 、 Data types and operations 、 identifier

1、 Variable

1、 Three elements of variable ： type + name + The number
2、 Variables need to be applied before use
3、 Variables are scoped （ Cannot duplicate names in a scope ）
4、 A variable represents a storage area in memory

2、 data type

1、 Eight basic data types ： integer （byte、short、int 、long）、 floating-point （float、double）、 Character （char）、 Boolean type （boolean）
2、 Reference data type
class 、 Interface 、 Array
3、 Coercive transformation

3、 Operators and identifiers

Including arithmetic operators 、 Relational operator 、 Logical operators 、 Assignment operator 、 Ternary operator

Previous notes ：
Variable 、 data type
Operators and identifiers

3、 ... and 、day 3 : Hexadecimal conversion 、 Three code operation 、 Bitwise operators and nested structures

1、 Hexadecimal conversion
Hexadecimal conversion
2、 Three code operation
3、 An operator
Original code 、 Complement code 、 Inverse code 、 An operation
4、 Nested structure
switch Nested structure
while And for loop

Four 、day 4 : Array 、 Sort

1、 Knowledge review ：

Use of arrays and precautions
Expansion and reduction of arrays
Basic usage of two-dimensional array
Bubble sort

2、 Selection sort ：

The difference between selective sorting and bubble sorting ：

The selection sort is number i The elements are followed by i + 1 、i + 2… Until the last element , The first round determines the first element , The second round determines the second element , And so on ;（ The number of rounds is the number of elements, and the position of the ratio remains unchanged , Just compare with the following one in turn , Then exchange positions according to conditions ）
And bubble sort It's No i Elements Follow i + 1 Than , If the conditions are met, exchange to i + 1 The location of , then i + 1 Follow i+2 Than And so on **