Crazy God said Java -- java learning notes (there are some problems with the pictures, and upload the pictures from the computer when you have time)

Video address

!!! Video address

Markdown learning

title

First level Title: # + space + title

Secondary Title: ## + space + title

...

Level 6 Title: ######## + space + title

typeface

hello world

Bold: * * hello world**

hello world

Italics: * hello world

hello world

Bold Italic: * * * hello world***

hello world

Strikethrough: ~ ~ hello world~~

quote

Choose to read ying's blog and go to the peak of life

Quote: > choose to read ying's blog and go to the peak of life

Split line

Split line:---

Split line:***

picture

! [picture name] (picture address (can be local address or Internet address))

Hyperlinks

Click to jump to Baidu

list

Ordered list

1. + Space + content
2. + Space + content
3. + Space + content
4. + Space + content

Unordered list

- + Space + content
- + Space + content
- + Space + content

form

name	Sex	Age
Zhang San	male	19

name|Sex |Age

--|--|--

Zhang San|male|19

Enter the source code and delete the space

Or right-click to insert the table directly

code

print("hello world")

```python
print("hello world")
```

Post markdown grammar blog

Just copy the above markdown source code directly to csdn or other websites that support markdown methods

What is a computer

introduce

Computer: electronic computer, commonly known as computer
Modern electronic equipment that can run according to program, automatically and high-speed process massive data
It consists of hardware and software
Common forms: desktop computer, notebook computer, mainframe computer
It is widely used in various fields

computer hardware

Some physical devices form an organic whole according to the system requirements, which provides a material basis for the operation of computer software

Composition of computer hardware:

CPU
a main board
Memory
Power Supply
Main box
Hard disk
Graphics card
Keyboard, mouse
monitor
etc.

install equipment

The simplest computer can be made up of:

CPU
Memory
Motherboard
IO device

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Von Neumann architecture (father of computer)

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Computer software

Computer software can enable the computer to complete specific functions in a predetermined order
Computer software is divided into system software and application software according to its function
systems software:
- DOS,Windows,Linux,Unix
Application software:
- WPS,QQ, wechat
Software, development, software development
human-computer interaction

Computer shortcuts

Alt + F4 close window

Enter OK

Num turns on the use of the keypad

CapsLock enable case

Ctrl + s save

Ctrl + c copy

Ctrl + v paste

Ctrl + z undo

Ctrl + x cut

Dos command

Open Dos command

Command prompt under Windows system in start
Win key + R input cmd to open the console
Enter cmd under the path of the folder to open Dos commands in this directory

Open in administrator mode

Right click the command prompt under the Windows system in the start to open the administrator

Basic command

`D:`   Switch drive letter
`dir`  View all files in the current directory
`cd`   Switch directory
`cls`  Clear screen
`exit` Exit terminal
`calc` Open calculator
`mspaint`Drawing
`md Directory name`  create folder
`rd Directory name`   remove file 
`cd> file name`  create a file
`del file name`  Delete file

Getting started with Java

The birth of the Java Empire

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Java features and benefits

Simplicity
object-oriented
Portability
High performance
Distributed
Dynamics
Multithreading
Security
Robustness

Three versions of Java

JavaSE: Standard Edition (desktop program, console Development)
JavaME: embedded development (mobile phones, small appliances)
Java EE: E enterprise development (web side, server development)

JDK，JRE，JVM

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Build Java development environment

JDK download and installation

Uninstall JDK

Delete jdk package
JAVA_HOME delete
Delete the java variable in path
java -version view whether to delete

Install JDK8

Baidu search JDK8 and find the download address
Login account
Download the corresponding version of the computer
Double click Install JDK
Remember the installation path
Configure environment variables
- My computer – > properties – > advanced system settings – > environment variables
- System variable – > New
- Add key as Java_ The home value is the just jdk path
- Configure the path variable and add the path value as% JAVA_HOME%\bin
- Add the path variable with the value% JAVA_HOME%\jre\bin
Check installation
- Open cmd
- Enter java -version

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Hello World and simple syntax rules

Just create a new folder to store the code
Create a new java file
- The suffix is java
- Hello.java

Write hello Java file content

public class Hello{
	public static void main(String[] args){
		System.out.println("Hello World");
	}
}

Compile java files into bytecode

javac file name (with suffix)

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Run class file

java file name (without suffix)

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Possible problems

Case cannot be a problem. Java is case sensitive
Try to use English
The file name and class name must be consistent and capitalized
Symbols need to be in English

Compiled and interpreted

Compiled type

Compile the program directly

Operating system (fast)
Interpretive type

Explain what you do

Web pages (not required to be fast)

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IDEA installation

What is an IDE introduce

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Install IDEA

Go to the official website to search IDEA
download
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Just go on to the next step

Simple use

Open the software

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New project

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Create a Java class under src

Write Java code

// psvm generates public static void main(String[] args)
//Some generates system out. println();

Java basic syntax

Comment, identifier, keyword

notes

We usually write code and can understand it when the amount of code is relatively small, but once the project structure becomes complex, we need to use comments

Comments are not executed. They are for those of us who write code

At ordinary times, the code must pay attention to the specification

//Is a single line comment

/*For multiline comments
123*/

/**
 * Documentation Comments 
 */

identifier

All components of Java need names. Class names, variable names, and method names are all called identifiers

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keyword

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data type

Strongly typed language
- It is required that the use of variables should strictly comply with the regulations, and all variables must be defined before they can be used
Weakly typed language
Java data types fall into two categories
1. Basic type
  - plastic
    - byte
    - sort
    - int
    - long
  - character
    - char
  - Boolean
    - boole
2. reference type
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expand

Integer expansion:

Base system

decimal system
- 123456
Octal (0)
- 010
Hex (0x)
- 0x11

Floating point expansion:

float finite discrete rounding error is close to but not equal to

//It is best to use floating point numbers for comparison

The BigDecimal class can be used for precise comparison of mathematical tool classes

public class Demo03 {
    public static void main(String[] args) {
        float f = 0.1f;
        double d = 1.0/10;


        System.out.println(f);
        System.out.println(d);
        System.out.println(f==d);
        float d1 = 23232323232323f;
        float d2 = d1+1;
        System.out.println(d1==d2);
    }
}

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Character expansion

char c1 = 'a';
char c2 ='in';
System.out.println((int)c1);
System.out.println((int)c2);
//All characters are still numbers in nature
//Encoding Unicode 2 bytes
// \u0061 ==> a
// \t tab
// \n newline

Boolean value

boolean flag = true;
if(flag){}//an old hand
if(flag==true){}//Novice
// Less is More!  The code should be concise and easy to read

Type conversion

Because Java is a strongly typed language, type conversion is required for some operations

Cast = = = > High -- > low

Automatic conversion = = = > low -- > High

Range
 low----------------------------------------------->high
byte, short ,char -> int -> long -> float -> double

When you cast a type, you must pay attention to the scope

int i =128;
byte b =(byte)i;
System.out.println(i);
System.out.println(b);

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Automatic conversion

int i =128;
double b =i;
System.out.println(i);
System.out.println(b);

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char c = 'a';
int d = c+1;
System.out.println(d);
System.out.println((char)d);

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Note:

Boolean values cannot be converted
Object types cannot be converted to irrelevant types
Force conversion when converting high capacity to low capacity
There may be memory overflow or accuracy problems during conversion

be careful

Pay attention to overflow when operating large numbers

JDK7 new feature, numbers can be separated by underscores

int money = 10_0000_0000;
int years = 20;
int total = money*years;

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int money = 10_0000_0000;
int years = 20;
long total = (long)money*years;
System.out.println(total);

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Variable, constant

variable

Naming conventions
    All variables, methods and class names: see the meaning of the name
    Class member variables: initial lowercase and hump principle
    Local variables: initial lowercase and hump principle
    Constants: uppercase letters and underscores
    Class name: initial capitalization and hump principle
    Method name: initial lowercase and hump principle

Is the amount that can be changed

Java is a strongly typed language, and each variable must declare its type.

Java variable is the most basic storage unit for program summary. Its elements include variable name, variable type and scope

type varName [=value][{,varName[=value]}];
//Data type variable name = value can declare multiple variables of the same type separated by commas.

matters needing attention:
- Each variable has a type, which can be either a basic type or a reference type
- Variable name must be a legal identifier
- Variable declaration is a complete statement, so each declaration must end with a semicolon

Variable scope

Class variable

Add keywords static
static double salary = 2500;
Subordinate class
 The life cycle is from the beginning of the class to the end of the class

Instance variable

Outside the method inside the class
 Instance variable:It is subordinate to the object and does not need to be initialized. Is the default value 0 for the data type.0 false

local variable

In the method
 The lifecycle must be initialized from the beginning of the method to the end of the method

constant

Constant: a value that cannot be changed after initialization! Unchanged value

The so-called constant can be understood as a special variable. Its value cannot be changed after it is set, and it is not allowed to be changed during the operation of the program.

final Constant name = value
final double PI = 3.14

Constant names are usually in uppercase letters

operator

The Java language supports the following operators
- Arithmetic operators: +, -, *, /,%, + +, –
- Assignment operator:=
- Relational operators: >, <, > =, < =, = =,! =, instanceof
- Logical operators: & &, |, |,!
- Bitwise operators: &, |, ^, ~, > >, <, > > >
- Conditional operator:?:
  
  x ? y : z
  
  x is true and returns y
  
  x is false and returns z
- Extended assignment operators: + =, - =, * =/=
- Power operation 2 ^ 3 math pow(2,3)

Package mechanism

In order to better organize classes, java provides a package mechanism to distinguish the namespace of class names
The syntax format of the package statement is:

package pkg1[. pkg2[. pkg3...]];
Red silk domain name inversion is generally used as package name
In order to use the members of a package, we need to explicitly import the package in the Java program. This can be done using the import statement

import package1[.package2...].(classname|*)

JavaDoc

Javadoc commands are used to generate their own API documents

parameter information
```
/*
@author author
@version Version number
@since Indicates the earliest jdk version required
@param Parameter name
@return Return value
@throws Exception thrown
 */
```
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Generate your own API documentation

Find tool > > generate Javadoc in the toolbar

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Then the configuration window pops up

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Parameter description
\1. Whole project > > the whole project is generated
\2. Custom scope > > the custom scope is as follows: project files, project production files, project test files, test files of the project, unknown scope, and class hierarchy
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\3. include test source contains the test directory
\4. include JDK and... Include jdk and other third-party jar s
\5. link to JDK documentation... Link to JDK api
\6. output directy is the location where the generated documents are stored
\7. Levels (classes and methods) of documents generated by private, package, protected and public
\8. Generate... On the right is the content of the selected generated document, including hierarchical tree, navigation, index
\9. On the right is the content information, author version and other information contained in the generated document
\10. Locale language type, Zh cn
\11. Other command line arguments
\12. Maximum heap

Java process control

User interaction Scanner

In the basic syntax we learned before, we didn't realize the interaction between programs and people, but Java provides us with a tool class that we can get user input. Java.util.Scanner is a new feature of Java 5. We can get the user's input through the scanner class
Basic grammar

Scanner s = new Scanner(System.in);
Get the input string through the next and nextLine() methods of Scanner class. Before reading, we generally need to use hasNext() and hasNextLine() to judge whether there is still input data

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //Create a scanner to receive keyboard input
        Scanner s =  new Scanner(System.in);
        //Judge whether the user has entered a string
        if (s.hasNext()){
            //Receive in next mode
            String str = s.next();
            System.out.println("What you entered is:"+str);
        }
    	s.close()
    }
}

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //Create a scanner to receive keyboard input
        Scanner s =  new Scanner(System.in);
        //Judge whether the user has entered a string
        if (s.hasNextLine()){
            //Receive in next mode
            String str = s.nextLine();
            System.out.println("What you entered is:"+str);
        }
        s.close();
    }
}

nest()
- Be sure to read valid characters before you can end the input
- The next() method will automatically remove the whitespace encountered before entering valid characters
- Only after entering a valid character will the blank space entered after it be used as a separator or terminator
- next() cannot get a string with spaces
nextLine()
- Take Enter as the ending character, that is, the nextLine() method returns all characters before entering carriage return
- Can get blank

It is unnecessary to judge whether there is input

Judge whether it is xxx type

Example: judging integers

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //Create a scanner to receive keyboard input
        Scanner scanner =  new Scanner(System.in);
        //Receive data from keyboard
        int i = 0;
        float f =0.0f;
        System.out.println("Please enter an integer:");
        if(scanner.hasNextInt()){
            i = scanner.nextInt();
            System.out.println("Integer data:" + i);
        }else{
            System.out.println("The input is not an integer");
        }
        System.out.println("Please enter decimal:");
        if(scanner.hasNextFloat()){
            f = scanner.nextFloat();
            System.out.println("The decimal entered is:" + f);
        }else{
            System.out.println("Entered is not a decimal");
        }
        scanner.close();
    }
}

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //Enter multiple numbers for summation, enter numbers, press enter to confirm, and enter non numbers to end
        //Create a scanner to receive keyboard input
        Scanner scanner =  new Scanner(System.in);
        //f is the number of received inputs, sum is the sum, and num is the number of inputs
        double f = 0;
        double sum = 0;
        int num = 0;
        while (scanner.hasNextDouble()){
            f = scanner.nextDouble();
            sum += f;
            num ++;
        }
        System.out.println("The average number entered is:"+sum/num);
        scanner.close();
    }
}

Sequential structure

The basic structure of Java is a sequential structure. Unless otherwise specified, it will be executed sentence by sentence.
Sequential structure is the simplest algorithm structure

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Between statements and between boxes, it is carried out in the order from top to bottom. It is composed of several processing steps executed in turn. It is a basic algorithm structure that any algorithm is inseparable from.

Select structure

if single selection structure

We often have to judge whether something is feasible before we execute it. Such a process is used in the program with if
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grammar

if(Boolean expression){
	//The statement to be executed if the Boolean expression is true.
}

//Whether the input string is equal to hello. If so, the output of Hello does not mean nothing.
//equals is required for string comparison. Try not to use it==
import java.util.Scanner;
public class Demo01 {
    public static void main(String[] args) {
        //Create a scanner to receive keyboard input
        Scanner scanner =  new Scanner(System.in);
        System.out.println("Please enter:");
        String s = scanner.nextLine();
        //equals: determines whether the strings are equal
        if(s.equals("Hello")){
            System.out.println(s);
        }
        System.out.println("End");
        scanner.close();
    }
}

if double selection structure

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grammar

if(Boolean expression){
	//Statement executed if Boolean expression is true
}else{
    //Statement executed if Boolean expression is false
}

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //Create a scanner to receive keyboard input
        Scanner scanner =  new Scanner(System.in);
        System.out.println("Please enter your grade:");
        int s = scanner.nextInt();
        //equals: determines whether the strings are equal
        if(s>=60){
            System.out.println("pass");
        }else{
            System.out.println("Fail in grade");
        }
        scanner.close();
    }
}

if multiple selection structure

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grammar

if(Boolean expression 1){
    //Boolean expression 1 sets up the code to execute
}else if(Boolean expression 2){
    //Boolean expression 2 sets up the code to execute
}else if(Boolean expression 3){
    //Boolean expression 3 sets up the code to execute
}else{
    //Boolean expressions are not valid for executing code
}

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //Create a scanner to receive keyboard input
        Scanner scanner =  new Scanner(System.in);
        System.out.println("Please enter your grade:");
        int s = scanner.nextInt();
        //equals: determines whether the strings are equal
        if(s==100){
            System.out.println("Congratulations, full marks");
        }else if(s>=80 && s<100){
            System.out.println("Excellent results");
        }else if(s>=60 && s<80){
            System.out.println("Good results");
        }else if(s>=0&&s<60){
            System.out.println("fail,");
        }else{
            System.out.println("The grade entered is incorrect");
        }
        scanner.close();
    }
}

Nested if structure
- It is legal to use nested if... Else statements. That is, you can use an if or else if statement in another if or else if statement. Else if... Else can be nested like an IF statement
- grammar
```
if(Boolean expression 1){
    //If Boolean expression 1 holds, execute the statement
    if(Boolean expression 2){
        //If Boolean expression 2 holds, execute the statement
    }
}
```
switch multiple selection structure
- switch intelligence matches an exact value, not a range
- Another way to implement the multi selection structure is the switch case statement
- The switch case statement determines whether a variable is equal to a value in a series of values. Each value is called a branch
- The variable types in the switch statement can be:
  - byte, short, int or char
  - Starting from Java SE 7, switch supports String type, and the case label must be a String constant or literal
- grammar
```
switch(expression){
    case value:
        //sentence
        break;
    case value:
        //sentence
        break;
    default:
        //sentence
}
```

Cyclic structure

while loop
- As long as the Boolean expression is true, it is always executed
- In most cases, we stop the loop. We need to end the loop by invalidating the expression
- In a few cases, it needs to be looped all the time, such as the monitoring of the server
- If the loop is always true, it will cause wireless loop [dead loop]. We should try our best to avoid dead loop in normal business programming, which will affect the performance of the program or cause the program to get stuck and crash
```
while(Boolean expression){
    //Code executed in a loop
}
```
```
import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int i = scanner.nextInt();
        int sum = 0;
        while (sum<=i){
            System.out.println(sum);
            sum++;
        }
        scanner.close();
    }
}
```
do... while loop
- Execute before Judge
- It's the same as a while loop, but execute the code at least once

for loop

Although all loop structures can be represented by while or do... While, Java provides another statement - for loop, which makes some loop structures simpler
for loop statement is a general structure that supports iteration. It is the most effective and flexible loop structure
The number of times the for loop is executed is determined before execution. The syntax is as follows
```
for(initialization;Boolean expression;to update){
    //Code statement
}
```

Find odd and even numbers within 100

public class Demo01 {
    public static void main(String[] args) {
        int jishu=0;
        int oushu=0;
        for(int i=1;i<=100;i++){
            if(i%2==0){
                oushu += i;
            }else{
                jishu += i;
            }

        }
        System.out.println("Odd number is" + jishu);
        System.out.println("Even number is" + oushu);
    }
}

Find 99 multiplication table

public class Demo01 {
    public static void main(String[] args) {
        for(int i=1;i<=9;i++){
            for(int j=1;j<=i;j++){
                System.out.print(i+"*" + j +"="+i*j+" ");
            }
            System.out.println();
        }
    }
}

An enhanced for loop is introduced in Java 5, which is mainly used for arrays

grammar

for(Declaration statement : expression){
    //Executed code
}

example

int[] num = {10,20,30,40,50};
for(int x:num){
    System.out.println(x);
}

break&continue

Break in the body of any loop statement, you can use break to control the flow of the loop. Break is used to forcibly exit the loop without executing the remaining statements in the loop (break statements are also used in switch)
In the body of a loop statement, continue statement is used to terminate a loop process, that is, skip the statement that has not been executed in the loop, and then determine whether to execute the loop next time
goto keyword
- goto is prone to problems
- The goto keyword has long appeared in programming languages. Although goto is still a reserved word in Java, it is not officially used in the language. There is no goto in Java. However, we can see some shadows of goto in the two keywords break and continue - labeled break and continue
- "label" refers to the identifier followed by a colon, for example: label:
- For Java, the only place where the label is used is before the loop statement. Second, the only reason for setting the label before the loop is that we want to nest another loop in it. Because the break and continue keywords usually only end the current loop and are weakly used with the label, they can end to the place where the label exists

practice

Print triangles

public class Demo01 {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            for (int j = 5; j >= i; j--) {
                System.out.print(" ");
            }
            for (int j = 1; j <= 2*i+1; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

Java Procedure

System.out.println(), what is it
A Java method is a collection of statements that together perform a function
- Method is an ordered combination of steps to solve a class of problems
- Method is contained in a class or object
- Methods are created in the program and referenced elsewhere
Principle of design method: the original intention of method is function block, which is the collection of statement blocks to realize a certain function. When designing methods, we'd better keep the atomicity of methods, that is, a method can only complete one function, which is conducive to our later expansion.
- Addition method
- ```
public class Demo01 {
    public static int add(int a,int b){
        return a+b;
    }

    public static void main(String[] args) {
        int a = 1;
        int b = 5;
        int sum = add(1,2);
        System.out.println(sum);
    }
}
```
Java methods are similar to functions in other languages. They are a piece of code used to complete specific functions. Generally, defining a method includes the following syntax:
- The method contains a method header and a method body. The following are all parts of a method:
  - Modifier: modifier, which is optional and tells the compiler how to call the method. Defines the access type of the method
  - Return value type: the method may return a value. returnValueType is the data type of the return value of the method. Some methods perform the required operation but do not return a value. In this case, returnValueType is the keyword void.
  - Method name: is the actual name of the method. The method name and parameter table together constitute the method signature.
  - Parameter type: the parameter is like a placeholder. When a method is called, it is passed to a parameter, which is called an argument or variable. Parameter list refers to the parameter type, order and number of parameters of the method. Parameters are optional, and methods can contain no parameters.
    - Formal parameter: used to receive external input data when the method is called
    - Argument: the data actually passed to the method when the method is called
  - Method body: the method contains specific statements that define the function of changing the method

Syntax:

Modifier return value type method name(Parameter type parameter name){
    ...
    Method body;
    ...
    return Return value;
}

Method call:
- Calling method: object name Method name (argument list)
- Java supports two methods of calling methods, which are selected according to whether the method returns a value
- When a method returns a value, the method call is usually treated as a value. For example:
  
  int larger = max(30,40)
- The return value of the method is void, and the method call must be a statement.
  
  System.out.println("Hello World")

Method overloading

We can set multiple methods with duplicate names, but we should ensure that the parameters of each method are different.

Overloading is a function with the same function name but different formal parameters in a class
Rules for overloading methods
- Method names must be the same
- The parameter list must be different (different number, different types, different order of parameters, etc.)
- The return types of methods can be the same or different
- Just different return types are not enough to overload methods
Realization theory
- If the method names are the same, the compiler will compare the matching one by one according to the number of parameters and parameter types of the calling method to select the corresponding method. If the matching fails, the compiler will report an error

Command line parameters

Sometimes you want to pass a message to a program when it runs. This is achieved by passing command line arguments to the main() function

example
```
public class Demo01 {
    public static void main(String[] args) {
        for (int i = 0; i < args.length; i++) {
            System.out.println("args[" + i +"]:" + args[i]);
        }
    }
}
```
It should be noted here that you cannot run under this file path, otherwise you will not find the main class. You need to run under the full path file address in the first path of idea before you can run

Variable parameters

There may be uncertain transmission parameters. We can use the main method to pass a variable parameter.
JDK1. Starting from 5, Java supports passing variable parameters of the same type to a method.
In the method declaration, add an ellipsis (...) after specifying the parameter type
Only one variable parameter can be specified in a method. It must be the last parameter of the method, and any ordinary parameter must be declared before it

public class Demo01 {
    public  void test(int... i){
        for (int j:i) {
            System.out.println(j);
        }
    }
    public static void main(String[] args) {
        Demo01 demo01 = new  Demo01();
        demo01.test(1,2);
    }
}

recursion

Method A calls method B, which is easy for us to understand
Recursion is calling itself
Using recursion, we can solve some complex problems with simple programs. It usually transforms a large problem layer by layer into a smaller problem similar to the original problem. The recursive strategy only needs a small number of programs to describe the multiple repeated calculations required in the problem-solving process, which greatly reduces the amount of code of the program. The ability of recursion is to define an infinite set of objects with limited statements
The recursive structure is divided into two parts
- Recursive header: when not to call its own method. If there is no head, it will fall into a dead cycle
- Recursive body: when do I need to call my own method

//Factorization of input number
import java.util.Scanner;

public class Demo01 {
    public static  int f(int n){
        if (n==1){
            return 1;
        }else{
            return n*f(n-1);
        }
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt();
        System.out.println(f(num));
    }
}

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Pre stage
boundary condition
Return phase

array

Array overview

An array is an ordered collection of data of the same type
Array describes several data of the same type, which are arranged and combined in a certain order
Among them, each data takes an array element, and each array element can access them through a subscript

Array declaration creation

Array variables must be declared before they can be used in the program. The following is the method of declaring array variables:

dataTyye[] arrayRefVar	//Preferred method 					 int[] nums;
dataType arrayRefvar[]	//The effect is the same, but it is not the preferred method 	     int nums[];

The Java language uses the new operator to create arrays. The syntax is as follows:
```
dataType[] arrayRefVar = new dataType[arraySize];
int[] nums = new int[5]
```
The elements of the array are accessed through the index. The array index starts from 0

Get array length

arrays.length			nums.length

public class Demo01 {
    public static void main(String[] args) {
        int[] nums;             //Declaration array
        nums = new int[5];      //Create an array to open up space, where you can store 5 int type data
        nums[0] = 1;
        nums[1] = 2;
        nums[2] = 3;
        nums[3] = 4;
        nums[4] = 5;
    }
}

Array initialization

initiate static

int[] a = {1,2,3};
Man[] mans = {new Man(1,1),new Man(2,2)}

dynamic initialization

int[] a = new int[2];
a[0] = 1;
a[1] = 2;

Default initialization of arrays

//Array is a reference type. Its elements are equivalent to the instance variables of the class. Therefore, once the array is allocated space, each element is implicitly initialized in the same way as the instance variables

Memory analysis

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Characteristics of array

The length is determined. Once an array is created, its size cannot be changed
Its elements must be of the same type, and mixed types are not allowed
The elements in the array can be any element type, including its basic type and reference type
Array variables belong to the reference type, and arrays can also be regarded as objects. Each element in the array is equivalent to the member variables of the object. The array itself is an object. In Java, the object is in the heap. Therefore, whether the array saves the original type or other object types, the array object itself is in the heap.
If the legal range [0,length-1] of the subscript exceeds the boundary, an error will be reported

Array usage

For each loop

//For each traversal array
public class Demo01 {
    public static void main(String[] args) {
        //No subscript
        int[] nums = {1,2,3,4,5};
        for (int i: nums) {
            System.out.println(i);
        }
    }
}

Array as method parameter

//Reverse string
public class Demo01 {
    public static void main(String[] args) {
        int[] nums = {1,2,3,4,5};
        int[] result = reverse(nums);
        for (int i = 0; i < result.length; i++) {
            System.out.println(result[i]);
        }
    }
    public static int[] reverse(int[] arrays){
        int[] result = new int[arrays.length];
        for (int i = 0,j = arrays.length-1; i < arrays.length; i++,j--) {
            result[j] = arrays[i];
        }
        return result;
    }
}

Multidimensional array

Multidimensional arrays can be regarded as arrays. For example, a binary array is a special one bit array, and each element is a one-dimensional array.
Two dimensional array
```
int a[][] = new int[2][5]
```
Analysis: the above two-dimensional array a can be regarded as an array with two rows and five columns

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```
public class Demo01 {
    public static void main(String[] args) {
        int[][] a = {{1,2},{3,4},{5,6},{7,8}};
        
    }
}
```

Arrays class

Array tool class java util. Arrays
There are no methods for array objects, but the API provides a tool class Arrays for us to use, so we can perform some basic operations on data objects.
View JDK help documentation
All the methods in the Arrays class are static methods decorated with static. When using them, you can call them directly with the class name. Second, you don't need to call them with objects (Note: it's "don't use" instead of "can't")
It has the following functions
- Assign value to array: through fill method
- Sort the array: sort in ascending order through the sort method
- Compare arrays: use the equals method to compare whether the element values in the array are equal
- Find array elements: binary search can be performed on the sorted array through the binarySearch method
```
public class Demo01 {
    public static void main(String[] args) {
        int[] a={5,3,1,4,2};
        System.out.println(a); //[I@1b6d3586
        System.out.println(Arrays.toString(a)); //[5, 3, 1, 4, 2]
        Arrays.sort(a);
        System.out.println(Arrays.toString(a)); //[1, 2, 3, 4, 5]

    }
}
```

Bubble sorting

import java.util.Arrays;
public class Demo01 {
    public static void main(String[] args) {
        int[] a = {5,1,3,7,4};
        int[] b = buSort(a);
        System.out.println(Arrays.toString(b));
    }
    public static int[] buSort(int[] arrays){
        for (int i = 0; i < arrays.length-1; i++) {
            for (int j = 0; j < arrays.length-i-1; j++) {
                if(arrays[j]<arrays[j+1]){
                    int a = arrays[j];
                    arrays[j] = arrays[j+1];
                    arrays[j+1] = a;
                }
            }
        }
        return arrays;
    }
}

Sparse array

Gobang has the operations of saving, exiting and continuing to play

I think a lot of zeros are stored, so I record a lot of meaningless data
Solution: sparse array
When most elements in an array are 0 or an array with the same value, you can use a sparse array to save the array.
Sparse arrays are handled as follows:
- A record array has several rows and columns, and how many different values
- Record the elements, rows, columns and values with non-stop values in a small-scale array, so as to reduce the size of the program
As shown in the following figure: the original array is on the left and the sparse array is on the right

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import java.util.Arrays;
public class Demo01 {
    public static void main(String[] args) {
      int[][] array1 = new int[11][11];
      array1[1][2] = 1 ;
      array1[2][3] = 2 ;
      for(int[] anInt:array1){
          for(int i:anInt){
              System.out.print(i+" ");
          }
          System.out.println();
      }
      int[][] result = xiShu(array1);
        for (int i = 0; i < result.length; i++) {
            System.out.println(Arrays.toString(result[i]));
        }
    }
 	//Convert 2D array to sparse array
    public static int[][] xiShu(int[][] arrays){
        int sum = 0;

        for (int i = 0; i < 11; i++) {
            for (int j = 0; j < 11; j++) {
                if(arrays[i][j] != 0){
                    sum++;
                }
            }
        }
        int[][] result = new int[sum+1][3];
        result[0][0]= 11;
        result[0][1]= 11;
        result[0][2]= sum;
        int resnum = 1;
        for (int i = 0; i < 11; i++) {
            for (int j = 0; j < 11; j++) {
                if(arrays[i][j] != 0){
                    result[resnum][0] = i;
                    result[resnum][1] = j;
                    result[resnum][2] = arrays[i][j];
                    resnum++;
                }
            }
        }
        return result;
    }
}

Keywords: Java

Added by cyrixware on Sun, 26 Dec 2021 08:23:31 +0200

Programming VIP

Crazy God said Java -- java learning notes (there are some problems with the pictures, and upload the pictures from the computer when you have time)

Video address

Markdown learning

title

typeface

quote

Split line

picture

Hyperlinks

list

Ordered list

Unordered list

form

code

Post markdown grammar blog

What is a computer

introduce

computer hardware

install equipment

Von Neumann architecture (father of computer)

Computer software

Computer shortcuts

Dos command

Open Dos command

Open in administrator mode

Basic command

Getting started with Java

The birth of the Java Empire

Java features and benefits

Three versions of Java

JDK，JRE，JVM

Build Java development environment

JDK download and installation

Uninstall JDK

Install JDK8

Hello World and simple syntax rules

Possible problems

Compiled and interpreted

IDEA installation

Install IDEA

Simple use

Java basic syntax

Comment, identifier, keyword

notes

identifier

keyword

data type

expand

Integer expansion:

Floating point expansion:

Character expansion

Boolean value

Type conversion

be careful

Variable, constant

variable

constant

operator

Package mechanism

JavaDoc

Java process control

User interaction Scanner

Judge whether it is xxx type

Example: judging integers

Sequential structure

Select structure

Cyclic structure

break&continue

practice

Java Procedure

Method overloading

Command line parameters

Variable parameters

recursion

array

Array overview

Array declaration creation

Memory analysis

Characteristics of array