JAVA

Setup

JDK

Windows 配置 JDK 可以使用 PowerShell 执行以下命令。

winget search openjdk
winget install AdoptOpenJDK.OpenJDK.8

如果需要安装多个JDK版本，可以使用 jvms

IDE可以选择 Eclipse 或 VSCode

`Eclipse` 插件

Eclipse EGit: Git Integration for Eclipse

Eclipse Linux Tools

Eclipse Maven Integration - m2eclipse

`VSCode`

winget search vscode
winget install Microsoft.VisualStudioCode

Plz refer to Java in Visual Studio Code

Check VSCodium if you need a freely-licensed binary distribution

winget search vscodium
winget install VSCodium.VSCodium

Web Backend Sample

spring-petclinic

quarkus-super-heroes

Hello World

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

javac -d <classpath_dir> App.java
java -classpath <classpath_dir> App

Strings

char

char is a Java primitive data type.
Primitive means that the variable holds the actual value, not a reference.
Others: boolean, byte, short, int, long. float and double.
char not Char
The char data type can store: 1.Any letter, 2.Numbers between 0 to 65535 Inclusive, 3.16-bit Unicode characters including special characters.
A value for char is enclosed in single quotes "
There is also an object wrapper called character for a char data type.

Examples:

char ch = 'j';
char uniChar = '\0x004A'; // j in unicode
char[] charArray = { '1', '2', '3', '4', '@'};

Methods:

isLetter()
isDigit()
isWhitespace()
isUpperCase()
isLowerCase()
toUpperCase()
toLowerCase()
toString() // returns a String object

String

String is a provided class in Java
String not string
String (and any other class) is a reference to an object
A value for String is enclosed in double quotes ""
String can also be stored in arrays, which are also objects
String is made up of an array of chars
Because strings are objects, they are immutable (cannot be changed once created)
String Buffer & String Builder Classes provide mutable string functionality.

Examples:

String s = "j";
String[] strarray = {"One", "Two", "Three", "Four", "Five"};

Java 8+ methods

char charAt(int index)
int compareTo(String anotherString)
String concat(String str)
int hashCode()
int indexOf
int length()
String replace and String replaceAll
String[] split(String regex)
boolean startsWith(String prefix)
String substring(int beginIndex)
String toLowerCase()
String toUpperCase()
String trim()

String Arrays

Every main method in Java has a string array declaration as part of it's standard formatting:

public static void main(String[] args) {
}

String arrays can contain multiple string elements, also called tokens
elements and tokens are referred to via their corresponding index number(int)
Java array indexes start at 0

ArrayLists

You need to know about java.util.ArrayList
The size of an ArrayList can be modified, but the size of an array cannot.
Elements can be added and removed from an ArrayList at any time

Example

ArrayList<String> flexibleList = new ArrayList<String>();

Formatting Strings

char to String

String.valueOf(char)

To format char as a String, use the String.valueOf(char) method

public static String charToString(char c) {
    return Character.toString(c);
}

String to char

charAt(int)

to format a character in a String as a char, use the charAt(int) method

public static char StringToChar(String str) {
    return str.charAt(0);
}

String to char array

toCharArray() Method

creates an accessible char[] array
Java stores strings as primitive char[] arrays internally but they are not accessible
Useful when working with non delimited string.

String str = "onetwothreefourfive";

char chararray[] = str.toCharArray();

getChars()

Copy characters from a string or a part of a string into a char[] array
Arguments are provided for string start, string end, array start, and Destination

mystr.getChars(0, 16, chararr, 0);

Streaming - chars() method

Creates a Stream from a String object
Use the mapToObj() and toArray() with chars() to convert a string to array of characters
helpful when selecting characters in a string based on conditions of each character

Character[] arraychars = str.chars().mapToObj(c -> (char) c).toArray(Character::new);

String Array to String

Arrays.toString()

Simple way to convert an array to a string

String str = Arrays.toString(strarray);

StringBuilder() and append()

Provides the option to conditionally add array elements to a string

StringBuilder stringb = new StringBuilder();
for (int i = 0; i < strarray.length; ++i) {
    stringb.append(strarray[i] + ' ');
}

String Joiner

Options for adding strings at the beginning and end of the constructed string
Also defined characters to be used between array elements
java.util.StringJoiner must be imported into your class

StringJoiner stringj = new StringJoiner(";;;", "[", "]");

Stream and Collector

Useful if you are provided with a string to consume
Also can be used to produce an ArrayList
java.util.stream.* and java.util.Arrays must be imported into your class

return Stream.of(strarray).collect(Collectors.joining());

String to String Array

split() method

Splits a delimited string into string[] array using a specified character as a delimiter

String[] strarray = str.split(",");

pattern.split()

Splits a delimited string into string[] array using a specified pattern as a delimiter
java.util.regex Pattern must be imported into your class

String splitpattern = "\\s\\s";  // 2 spaces
Pattern pattern = Pattern.compile(splitpattern);

StringTokenizer Class

Splits a string object into tokens
Used to split a non-delimited string into an array
Delimiters can be specified, default = space
java.util.StringTokenizer must be import into your class

import java.util.StringTokenizer;
// ...
StringTokenizer = tokenizer = new StringTokenizer(str);
String[] strarray = new String[tokenizer.countTokens()];
// add tokens to an array
int i = 0;
while (tokenizer.hasMoreTokens()) {
    strarray[i] = tokenizer.nextToken();
    ++i;
}

Numeric

Records

Conditional Logic

switch(Java 7+)

Multiple possible execution paths
Chooses one possibility out of multiple options
break statement is used to terminate statement flow
default statement is optional

Syntax:

switch (expression) {
    case value1 :
        statements..
        break;
    case value2 :
        statements..
    default :
        statements..
}

switch(Java 14+)

Syntax

switch (expression) {
    case value1 -> boolean;
    case values -> boolean;
    case value3 -> boolean;
    default :-> {
        statements..
    }
}

Arrays

Collections

Maps

HashMap 非线程安全，可使用线程安全的 ConcurrentHashMap 替代。

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