1045 lines
24 KiB
Markdown
1045 lines
24 KiB
Markdown
Java 备忘清单
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===
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该备忘单是针对 Java 初学者的速成课程，有助于复习 Java 语言的基本语法。
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入门
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--------
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### Hello.java
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<!--rehype:wrap-class=row-span-2-->
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```java
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public class Hello {
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// 主要方法
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public static void main(String[] args)
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{
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// 输出: Hello, world!
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System.out.println("Hello, world!");
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}
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}
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```
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编译和运行
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```shell
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$ javac Hello.java
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$ java Hello
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Hello, world!
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```
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### 变量 Variables
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```java
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int num = 5;
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float floatNum = 5.99f;
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char letter = 'D';
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boolean bool = true;
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String site = "jaywcjlove.github.io";
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```
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### 原始数据类型
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<!--rehype:wrap-class=row-span-2-->
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| 数据类型 | 大小 | 默认 | 范围 |
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|-----------|--------|---------|---------------------|
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| `byte` | 1 byte | 0 | -128 ^to^ 127 |
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| `short` | 2 byte | 0 | -2^15^ ^to^ 2^15^-1 |
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| `int` | 4 byte | 0 | -2^31^ ^to^ 2^31^-1 |
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| `long` | 8 byte | 0 | -2^63^ ^to^ 2^63^-1 |
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| `float` | 4 byte | 0.0f | _N/A_ |
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| `double` | 8 byte | 0.0d | _N/A_ |
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| `char` | 2 byte | \\u0000 | 0 ^to^ 65535 |
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| `boolean` | _N/A_ | false | true / false |
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<!--rehype:className=show-header-->
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### 字符串 Strings
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```java
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String first = "John";
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String last = "Doe";
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String name = first + " " + last;
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System.out.println(name);
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```
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查看: [Strings](#java-字符串)
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### 循环 Loops
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```java
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String word = "QuickRef";
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for (char c: word.toCharArray()) {
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System.out.print(c + "-");
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}
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// 输出: Q-u-i-c-k-R-e-f-
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```
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查看: [Loops](#java-循环)
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### 数组 Arrays
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```java
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char[] chars = new char[10];
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chars[0] = 'a'
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chars[1] = 'b'
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String[] letters = {"A", "B", "C"};
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int[] mylist = {100, 200};
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boolean[] answers = {true, false};
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```
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查看: [Arrays](#java-数组)
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### 交换变量 Swap
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```java
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int a = 1;
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int b = 2;
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System.out.println(a + " " + b); // 1 2
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int temp = a;
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a = b;
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b = temp;
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System.out.println(a + " " + b); // 2 1
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```
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### 类型转换 Type Casting
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```java
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// Widening
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// byte<short<int<long<float<double
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int i = 10;
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long l = i; // 10
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// Narrowing
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double d = 10.02;
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long l = (long)d; // 10
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String.valueOf(10); // "10"
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Integer.parseInt("10"); // 10
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Double.parseDouble("10"); // 10.0
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```
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### 条件语句 Conditionals
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```java
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int j = 10;
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if (j == 10) {
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System.out.println("I get printed");
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} else if (j > 10) {
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System.out.println("I don't");
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} else {
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System.out.println("I also don't");
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}
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```
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查看: [Conditionals](#条件语句 Conditionals)
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### 用户输入
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```java
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Scanner in = new Scanner(System.in);
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String str = in.nextLine();
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System.out.println(str);
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int num = in.nextInt();
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System.out.println(num);
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```
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Java 字符串
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-------
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### 基本的
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```java
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String str1 = "value";
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String str2 = new String("value");
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String str3 = String.valueOf(123);
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```
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### 字符串连接
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```java
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String s = 3 + "str" + 3; // 3str3
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String s = 3 + 3 + "str"; // 6str
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String s = "3" + 3 + "str"; // 33str
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String s = "3" + "3" + "23"; // 3323
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String s = "" + 3 + 3 + "23"; // 3323
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String s = 3 + 3 + 23; // 29
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```
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### 字符串生成器
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<!--rehype:wrap-class=row-span-3-->
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```java
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StringBuilder sb = new StringBuilder(10);
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```
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---
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```java
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┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
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| | | | | | | | | |
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└───┴───┴───┴───┴───┴───┴───┴───┴───┘
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0 1 2 3 4 5 6 7 8 9
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```
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---
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```java
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sb.append("Reference");
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```
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---
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```java
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┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
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| R | e | f | e | r | e | n | c | e |
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└───┴───┴───┴───┴───┴───┴───┴───┴───┘
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0 1 2 3 4 5 6 7 8 9
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```
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---
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```java
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sb.delete(3, 9);
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```
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---
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```java
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┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
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| R | e | f | | | | | | |
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└───┴───┴───┴───┴───┴───┴───┴───┴───┘
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0 1 2 3 4 5 6 7 8 9
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```
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---
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```java
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sb.insert(0, "My ");
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```
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---
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```java
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┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
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| M | y | | R | e | f | | | |
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└───┴───┴───┴───┴───┴───┴───┴───┴───┘
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0 1 2 3 4 5 6 7 8 9
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```
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---
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```java
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sb.append("!");
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```
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---
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```java
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┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
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| M | y | | R | e | f | ! | | |
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└───┴───┴───┴───┴───┴───┴───┴───┴───┘
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0 1 2 3 4 5 6 7 8 9
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```
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### 比较
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```java
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String s1 = new String("QuickRef");
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String s2 = new String("QuickRef");
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s1 == s2 // false
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s1.equals(s2) // true
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"AB".equalsIgnoreCase("ab") // true
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```
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### 操纵
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```java
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String str = "Abcd";
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str.toUpperCase(); // ABCD
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str.toLowerCase(); // abcd
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str.concat("#"); // Abcd#
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str.replace("b", "-"); // A-cd
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" abc ".trim(); // abc
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"ab".toCharArray(); // {'a', 'b'}
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```
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### 信息
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```java
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String str = "abcd";
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str.charAt(2); // c
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str.indexOf("a") // 0
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str.indexOf("z") // -1
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str.length(); // 4
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str.toString(); // abcd
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str.substring(2); // cd
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str.substring(2,3); // c
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str.contains("c"); // true
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str.endsWith("d"); // true
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str.startsWith("a"); // true
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str.isEmpty(); // false
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```
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### 不可变
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```java
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String str = "hello";
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str.concat("world");
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// 输出: hello
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System.out.println(str);
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```
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---
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```java
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String str = "hello";
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String concat = str.concat("world");
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// 输出: helloworld
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System.out.println(concat);
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```
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一旦创建就不能修改，任何修改都会创建一个新的String
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Java 数组
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-------
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### 声明 Declare
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```java
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int[] a1;
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int[] a2 = {1, 2, 3};
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int[] a3 = new int[]{1, 2, 3};
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int[] a4 = new int[3];
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a4[0] = 1;
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a4[2] = 2;
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a4[3] = 3;
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```
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### 修改 Modify
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```java
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int[] a = {1, 2, 3};
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System.out.println(a[0]); // 1
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a[0] = 9;
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System.out.println(a[0]); // 9
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System.out.println(a.length); // 3
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```
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### 循环 (读 & 写)
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```java
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int[] arr = {1, 2, 3};
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for (int i=0; i < arr.length; i++) {
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arr[i] = arr[i] * 2;
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System.out.print(arr[i] + " ");
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}
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// 输出: 2 4 6
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```
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### Loop (Read)
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```java
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String[] arr = {"a", "b", "c"};
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for (int a: arr) {
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System.out.print(a + " ");
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}
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// 输出: a b c
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```
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### 二维数组 Multidimensional Arrays
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```java
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int[][] matrix = { {1, 2, 3}, {4, 5} };
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int x = matrix[1][0]; // 4
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// [[1, 2, 3], [4, 5]]
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Arrays.deepToString(matrix)
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for (int i = 0; i < a.length; ++i) {
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for(int j = 0; j < a[i].length; ++j) {
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System.out.println(a[i][j]);
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}
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}
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// 输出: 1 2 3 4 5 6 7
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```
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### 排序 Sort
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```java
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char[] chars = {'b', 'a', 'c'};
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Arrays.sort(chars);
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// [a, b, c]
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Arrays.toString(chars);
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```
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Java 条件语句
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-----------
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### 运算符
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<!--rehype:wrap-class=row-span-3-->
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- `+` _(加法运算符(也用于字符串连接))_
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- `-` _(减法运算符)_
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- `*` _(乘法运算符)_
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- `/` _(分区运算符)_
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- `%` _(余数运算符)_
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- `=` _(简单赋值运算符)_
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- `++` _(增量运算符；将值增加 1)_
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- `--` _(递减运算符；将值减 1)_
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- `!` _(逻辑补码运算符；反转布尔值)_
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<!--rehype:className=style-round-->
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---
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- `==` _(等于)_
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- `!=` _(不等于)_
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- `>` _(比...更棒)_
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- `>=` _(大于或等于)_
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- `<` _(少于)_
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- `<=` _(小于或等于)_
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<!--rehype:className=cols-2 style-round-->
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---
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- `&&` _条件与_
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- `||` _条件或_
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- [?:](#三元运算符) _三元(if-then-else 语句的简写)_
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<!--rehype:className=style-round-->
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---
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- `instanceof` _(将对象与指定类型进行比较)_
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<!--rehype:className=style-round-->
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---
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- `~` _(一元按位补码)_
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- `<<` _(签名左移)_
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- `>>` _(有符号右移)_
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- `>>>` _(无符号右移)_
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- `&` _(按位与)_
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- `^` _(按位异或)_
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- `|` _(按位包含 OR)_
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<!--rehype:className=cols-2 style-round-->
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### If else
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```java
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int k = 15;
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if (k > 20) {
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System.out.println(1);
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} else if (k > 10) {
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System.out.println(2);
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} else {
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System.out.println(3);
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}
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```
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### Switch
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<!--rehype:wrap-class=row-span-2-->
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```java
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int month = 3;
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String str;
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switch (month) {
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case 1:
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str = "January";
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break;
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case 2:
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str = "February";
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break;
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case 3:
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str = "March";
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break;
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default:
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str = "Some other month";
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break;
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}
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// 输出: Result March
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System.out.println("Result " + str);
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```
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### 三元运算符
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```java
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int a = 10;
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int b = 20;
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int max = (a > b) ? a : b;
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// 输出: 20
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System.out.println(max);
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```
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Java 循环
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----
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### For 循环
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```java
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for (int i = 0; i < 10; i++) {
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System.out.print(i);
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}
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// 输出: 0123456789
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```
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---
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```java
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for (int i = 0,j = 0; i < 3; i++,j--) {
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System.out.print(j + "|" + i + " ");
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}
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// 输出: 0|0 -1|1 -2|2
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```
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### 增强的 For 循环
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```java
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int[] numbers = {1,2,3,4,5};
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for (int number: numbers) {
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System.out.print(number);
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}
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// 输出: 12345
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```
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用于循环数组或列表
|
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### While 循环
|
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```java
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int count = 0;
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while (count < 5) {
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System.out.print(count);
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count++;
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}
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// 输出: 01234
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```
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### Do While 循环
|
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|
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```java
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int count = 0;
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do {
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System.out.print(count);
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count++;
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} while (count < 5);
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// 输出: 01234
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```
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### 继续声明
|
||
|
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```java
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for (int i = 0; i < 5; i++) {
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if (i == 3) {
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continue;
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}
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System.out.print(i);
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}
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// 输出: 01245
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```
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### 中断语句
|
||
|
||
```java
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for (int i = 0; i < 5; i++) {
|
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System.out.print(i);
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if (i == 3) {
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break;
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}
|
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}
|
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// 输出: 0123
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```
|
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|
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Java 多线程
|
||
--------------------
|
||
|
||
### 创建线程
|
||
|
||
```java
|
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// 实现Runnable接口
|
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public class RunnableThread implements Runnable {
|
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@Override
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public void run() {
|
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// todo something
|
||
}
|
||
}
|
||
|
||
// 实现Callable接口,T 替换成实际类型
|
||
public class CallableTask implements Callable<T> {
|
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@Override
|
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public T call() throws Exception {
|
||
// todo something
|
||
return null;
|
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}
|
||
}
|
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|
||
// 继承Thrad类
|
||
public class ExtendsThread extends Thread {
|
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@Override
|
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public void run() {
|
||
// todo something
|
||
}
|
||
}
|
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|
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// 运行线程
|
||
public static void main(String[] args) throws ExecutionException, InterruptedException {
|
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new Thread(new RunnableThread()).start();
|
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new ExtendsThread2().start();
|
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FutureTask<Integer> integerFutureTask = new FutureTask<>(new CallableTask());
|
||
integerFutureTask.run();
|
||
}
|
||
|
||
```
|
||
|
||
### 线程池
|
||
|
||
```java
|
||
/**
|
||
* corePoolSize: 核心线程数
|
||
* maximumPoolSize: 最大线程数
|
||
* keepAliveTime: 线程空闲时间
|
||
* timeUni: 线程空闲时间单位
|
||
* workQueue: 线程等待队列
|
||
* threadFactory: 线程创建工厂
|
||
* handler: 拒绝策略
|
||
*/
|
||
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
|
||
2, 5,
|
||
5, TimeUnit.SECONDS,
|
||
new ArrayBlockingQueue<>(10),
|
||
new DefaultThreadFactory("pollName"),
|
||
new ThreadPoolExecutor.CallerRunsPolicy());
|
||
|
||
// 内置的线程池, 不推荐生产使用
|
||
Executors.newCachedThreadPool();
|
||
Executors.newFixedThreadPool(10);
|
||
Executors.newScheduledThreadPool(10);
|
||
Executors.newSingleThreadExecutor();
|
||
```
|
||
|
||
### synchronized
|
||
|
||
<!--rehype:wrap-class=row-span-1-->
|
||
|
||
```java
|
||
// 代码块
|
||
synchronized(obj) {
|
||
...
|
||
}
|
||
|
||
// (静态)方法
|
||
public synchronized (static) void methodName() {
|
||
...
|
||
}
|
||
```
|
||
|
||
### ThreadLocal
|
||
|
||
```java
|
||
// 使用完之后一定要记得remove, 否则会内存泄露
|
||
ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
|
||
threadLocal.set(1);
|
||
threadLocal.get();
|
||
threadLocal.remove();
|
||
```
|
||
|
||
### 线程等待与唤醒
|
||
|
||
```java
|
||
// 需要synchronized修饰的代码块才能使用
|
||
wait();
|
||
notify();
|
||
notifyAll();
|
||
|
||
// 使用lock的条件唤醒
|
||
ReentrantLock lock = new ReentrantLock();
|
||
Condition condition= lock.newCondition();
|
||
lock.lock();
|
||
try{
|
||
// 当前线程唤醒或等待
|
||
condition.await();
|
||
condition.signal();
|
||
condition.signalAll();
|
||
} finally {
|
||
lock.unlock
|
||
}
|
||
|
||
// LockSupport,可以先unpark,后续park不会阻塞线程
|
||
LockSupport.park(obj);
|
||
LockSupport.unpark(thread);
|
||
```
|
||
|
||
### 线程编排
|
||
|
||
```java
|
||
// CountDownLatch
|
||
CountDownLatch countDownLatch = new CountDownLatch(2);
|
||
new Thread(() -> {
|
||
try {
|
||
...
|
||
}finally {
|
||
countDownLatch.countDown();
|
||
}
|
||
}).start();
|
||
countDownLatch.await();
|
||
|
||
// CompletableFuture
|
||
CompletableFuture<Void> task1 = CompletableFuture.runAsync(() -> {});
|
||
CompletableFuture<Void> task2 = CompletableFuture.runAsync(() -> {});
|
||
CompletableFuture<Void> task3 = CompletableFuture.runAsync(() -> {});
|
||
CompletableFuture.allOf(task1, task2, task3).get();
|
||
|
||
// Semaphore
|
||
Semaphore semaphore = new Semaphore(5);
|
||
try {
|
||
semaphore.acquire();
|
||
} finally {
|
||
semaphore.release();
|
||
}
|
||
```
|
||
|
||
Java 框架搜集
|
||
--------------------
|
||
|
||
### Java 集合
|
||
<!--rehype:wrap-class=col-span-2 row-span-2-->
|
||
|
||
集合 | Interface | 有序 | 已排序 | 线程安全 | 复制 | Nullable
|
||
:-|:-|:-|:-|:-|:-|:-
|
||
[ArrayList](https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html) | List | Y | _N_ | _N_ | Y | Y
|
||
[Vector](https://docs.oracle.com/javase/8/docs/api/java/util/Vector.html) | List | Y | _N_ | Y | Y | Y
|
||
[LinkedList](https://docs.oracle.com/javase/8/docs/api/java/util/LinkedList.html) | List, Deque | Y | _N_ | _N_ | Y | Y
|
||
[CopyOnWriteArrayList](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CopyOnWriteArrayList.html) | List | Y | _N_ | Y | Y | Y
|
||
[HashSet](https://docs.oracle.com/javase/8/docs/api/java/util/HashSet.html) | Set | _N_ | _N_ | _N_ | _N_ | One `null`
|
||
[LinkedHashSet](https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashSet.html) | Set | Y | _N_ | _N_ | _N_ | One `null`
|
||
[TreeSet](https://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html) | Set | Y | Y | _N_ | _N_ | _N_
|
||
[CopyOnWriteArraySet](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CopyOnWriteArraySet.html) | Set | Y | _N_ | Y | _N_ | One `null`
|
||
[ConcurrentSkipListSet](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentSkipListSet.html) | Set | Y | Y | Y | _N_ | _N_
|
||
[HashMap](https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html) | Map | _N_ | _N_ | _N_ | _N (key)_ | One `null` _(key)_
|
||
[HashTable](https://docs.oracle.com/javase/8/docs/api/java/util/Hashtable.html) | Map | _N_ | _N_ | Y | _N (key)_ | _N (key)_
|
||
[LinkedHashMap](https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html) | Map | Y | _N_ | _N_ | _N (key)_ | One `null` _(key)_
|
||
[TreeMap](https://docs.oracle.com/javase/8/docs/api/java/util/TreeMap.html) | Map | Y | Y | _N_ | _N (key)_ | _N (key)_
|
||
[ConcurrentHashMap](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html) | Map | _N_ | _N_ | Y | _N (key)_ | _N_
|
||
[ConcurrentSkipListMap](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentSkipListMap.html) | Map | Y | Y | Y | _N (key)_ | _N_
|
||
[ArrayDeque](https://docs.oracle.com/javase/8/docs/api/java/util/ArrayDeque.html) | Deque | Y | _N_ | _N_ | Y | _N_
|
||
[PriorityQueue](https://docs.oracle.com/javase/8/docs/api/java/util/PriorityQueue.html) | Queue | Y | _N_ | _N_ | Y | _N_
|
||
[ConcurrentLinkedQueue](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentLinkedQueue.html) | Queue | Y | _N_ | Y | Y | _N_
|
||
[ConcurrentLinkedDeque](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentLinkedDeque.html) | Deque | Y | _N_ | Y | Y | _N_
|
||
[ArrayBlockingQueue](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ArrayBlockingQueue.html) | Queue | Y | _N_ | Y | Y | _N_
|
||
[LinkedBlockingDeque](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/LinkedBlockingDeque.html) | Deque | Y | _N_ | Y | Y | _N_
|
||
[PriorityBlockingQueue](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/PriorityBlockingQueue.html) | Queue | Y | _N_ | Y | Y | _N_
|
||
<!--rehype:className=show-header-->
|
||
|
||
### ArrayList
|
||
|
||
```java
|
||
List<Integer> nums = new ArrayList<>();
|
||
// 添加
|
||
nums.add(2);
|
||
nums.add(5);
|
||
nums.add(8);
|
||
// 检索
|
||
System.out.println(nums.get(0));
|
||
// 为循环迭代编制索引
|
||
for (int i = 0; i < nums.size(); i++) {
|
||
System.out.println(nums.get(i));
|
||
}
|
||
nums.remove(nums.size() - 1);
|
||
nums.remove(0); // 非常慢
|
||
for (Integer value : nums) {
|
||
System.out.println(value);
|
||
}
|
||
// lambda 打印元素
|
||
nums.forEach(e -> System.out.println(e.toString()));
|
||
```
|
||
|
||
### HashMap
|
||
|
||
```java
|
||
Map<Integer, String> m = new HashMap<>();
|
||
m.put(5, "Five");
|
||
m.put(8, "Eight");
|
||
m.put(6, "Six");
|
||
m.put(4, "Four");
|
||
m.put(2, "Two");
|
||
// 检索
|
||
System.out.println(m.get(6));
|
||
// Lambda forEach
|
||
m.forEach((key, value) -> {
|
||
String msg = key + ": " + value;
|
||
System.out.println(msg);
|
||
});
|
||
```
|
||
|
||
### ConcurrentHashMap
|
||
|
||
```java
|
||
ConcurrentHashMap<Integer, String> m
|
||
= new ConcurrentHashMap<>();
|
||
m.put(100, "Hello");
|
||
m.put(101, "Geeks");
|
||
m.put(102, "Geeks");
|
||
// 移除
|
||
m.remove(101, "Geeks");
|
||
|
||
// 如果不存在，就添加，存在就不变更
|
||
m.putIfAbsent(103, "Hello");
|
||
|
||
// 替换
|
||
m.replace(101, "Hello", "For");
|
||
System.out.println(m);
|
||
```
|
||
|
||
### HashSet
|
||
|
||
```java
|
||
Set<String> set = new HashSet<>();
|
||
if (set.isEmpty()) {
|
||
System.out.println("Empty!");
|
||
}
|
||
set.add("dog");
|
||
set.add("cat");
|
||
set.add("mouse");
|
||
set.add("snake");
|
||
set.add("bear");
|
||
if (set.contains("cat")) {
|
||
System.out.println("Contains cat");
|
||
}
|
||
set.remove("cat");
|
||
for (String element : set) {
|
||
System.out.println(element);
|
||
}
|
||
set.forEach(e -> System.out.println(e.toString()));
|
||
```
|
||
|
||
### ArrayDeque
|
||
|
||
```java
|
||
Deque<String> a = new ArrayDeque<>();
|
||
// 使用 add()
|
||
a.add("Dog");
|
||
// 使用 addFirst()
|
||
a.addFirst("Cat");
|
||
// 使用 addLast()
|
||
a.addLast("Horse");
|
||
// [Cat, Dog, Horse]
|
||
System.out.println(a);
|
||
// 访问元素
|
||
System.out.println(a.peek());
|
||
// 移除元素
|
||
System.out.println(a.pop());
|
||
```
|
||
|
||
杂项 Misc
|
||
----
|
||
|
||
### 访问修饰符
|
||
<!--rehype:wrap-class=col-span-2-->
|
||
|
||
| 修饰符 | Class | Package | Subclass | World |
|
||
|-------------|-------|---------|----------|-------|
|
||
| public | Y | Y | Y | Y |
|
||
| protected | Y | Y | Y | _N_ |
|
||
| no modifier | Y | Y | _N_ | _N_ |
|
||
| private | Y | _N_ | _N_ | _N_ |
|
||
<!--rehype:className=show-header-->
|
||
|
||
### 常用表达
|
||
|
||
```java
|
||
String text = "I am learning Java";
|
||
// 删除所有空格
|
||
text.replaceAll("\\s+", "");
|
||
// 拆分字符串
|
||
text.split("\\|");
|
||
text.split(Pattern.quote("|"));
|
||
```
|
||
|
||
查看: [Regex in java](./regex.md#java-中的正则表达式)
|
||
|
||
### 注释 Comment
|
||
|
||
```java
|
||
// 我是单行注释！
|
||
|
||
/*
|
||
而我是一个
|
||
多行注释！
|
||
*/
|
||
/**
|
||
* 这个
|
||
* 是
|
||
* 文档
|
||
* 注释
|
||
*/
|
||
```
|
||
|
||
### 关键字
|
||
<!--rehype:wrap-class=col-span-2-->
|
||
|
||
- abstract
|
||
- continue
|
||
- for
|
||
- new
|
||
- switch
|
||
- assert
|
||
- default
|
||
- goto
|
||
- package
|
||
- synchronized
|
||
- boolean
|
||
- do
|
||
- if
|
||
- private
|
||
- this
|
||
- break
|
||
- double
|
||
- implements
|
||
- protected
|
||
- throw
|
||
- byte
|
||
- else
|
||
- import
|
||
- public
|
||
- throws
|
||
- case
|
||
- enum
|
||
- instanceof
|
||
- return
|
||
- transient
|
||
- catch
|
||
- extends
|
||
- int
|
||
- short
|
||
- try
|
||
- char
|
||
- final
|
||
- interface
|
||
- static
|
||
- void
|
||
- class
|
||
- finally
|
||
- long
|
||
- strictfp
|
||
- volatile
|
||
- const
|
||
- float
|
||
- native
|
||
- super
|
||
- while
|
||
<!--rehype:className=cols-7 style-none-->
|
||
|
||
### 数学方法
|
||
|
||
方法 | 说明
|
||
:-|:-
|
||
`Math.max(a,b)` | `a` 和 `b` 的最大值
|
||
`Math.min(a,b)` | `a` 和 `b` 的最小值
|
||
`Math.abs(a)` | 绝对值
|
||
`Math.sqrt(a)` | `a` 的平方根
|
||
`Math.pow(a,b)` | `b` 的幂
|
||
`Math.round(a)` | 最接近的整数
|
||
`Math.sin(ang)` | 正弦
|
||
`Math.cos(ang)` | `ang` 的余弦
|
||
`Math.tan(ang)` | `ang` 的切线
|
||
`Math.asin(ang)` | `ang` 的反正弦
|
||
`Math.log(a)` | `a` 的自然对数
|
||
`Math.toDegrees(rad)` | 以度为单位的角度弧度
|
||
`Math.toRadians(deg)` | 以弧度为单位的角度度
|
||
|
||
### 异常 Try/Catch/Finally
|
||
|
||
```java
|
||
try {
|
||
// something
|
||
} catch (Exception e) {
|
||
e.printStackTrace();
|
||
} finally {
|
||
System.out.println("always printed");
|
||
}
|
||
```
|
||
|
||
### 反射
|
||
|
||
```java
|
||
/**
|
||
* 利用反射动态加载依赖库
|
||
* java9及以上版本可用
|
||
* @param jar jar文件
|
||
*/
|
||
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
|
||
method.setAccessible(true);
|
||
MethodHandle addURL = lookup.unreflect(method);
|
||
URL url = jar.toURI().toURL();
|
||
URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {url});
|
||
addURL.invoke(urlClassLoader, url);
|
||
//java8
|
||
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
|
||
method.setAccessible(true);
|
||
method.invoke(classLoader, url);
|
||
```
|
||
|
||
### util工具类
|
||
|
||
- `ArrayDeque` 提供 resizable-array 并实现 Deque 接
|
||
- `Arrays` 包含一个静态工厂，允许将数组视为列表
|
||
- `Collections` 包含对集合进行操作或返回集合的静态方法
|
||
- `Date` 表示特定的时间瞬间，精度为毫秒
|
||
- `Dictionary` 是任何类的抽象父类，例如 Hashtable，它将键映射到值
|
||
- `EnumMap` 一个专门用于枚举键的 Map 实现
|
||
- `EnumSet` 一个专门用于枚举键的 Set 实现
|
||
- `Formatter` 提供对布局对齐和对齐、数字、字符串和日期/时间数据的常用格式以及特定于语言环境的输出的支持
|
||
- `SecureRandom` 实例用于生成安全的伪随机数流
|
||
- `UUID` 表示一个不可变的通用唯一标识符
|
||
- `Vector` 实现了一个可增长的对象数组
|
||
- `LocalDate` 表示没有时区的日期，只包含年月日，不可变并且线程安全的，java8 及以上版本可用
|
||
- `LocalTime` 表示没有时区的时间，只包含时分秒，不可变并且线程安全的，java8 及以上版本可用
|
||
- `LocalDateTime` 表示没有时区的日期时间，同时包含年月日时分秒，不可变并且线程安全的，java8 及以上版本可用
|
||
|
||
### IO流
|
||
|
||
```java
|
||
// 输入流转输出流
|
||
byte[] inputContent = "test".getBytes();
|
||
try (InputStream inputStream = new ByteArrayInputStream(inputContent);
|
||
OutputStream outputStream = new ByteArrayOutputStream()) {
|
||
byte[] buffer = new byte[1024];
|
||
int len;
|
||
while ((len = inputStream.read(buffer)) != -1) {
|
||
outputStream.write(buffer, 0, len);
|
||
}
|
||
} catch (IOException e) {
|
||
throw new RuntimeException(e);
|
||
}
|
||
```
|
||
|
||
### Collections 工具类
|
||
|
||
```java
|
||
// 计算出现频率
|
||
List<Integer> list = new ArrayList<>();
|
||
list.add(1);
|
||
list.add(2);
|
||
list.add(2);
|
||
list.add(3);
|
||
list.add(3);
|
||
list.add(3);
|
||
int frequency = Collections.frequency(list, 2); // frequency = 2
|
||
```
|
||
|
||
### Stream 流
|
||
|
||
```java
|
||
// 统计词频
|
||
List<Integer> list = new ArrayList<>();
|
||
list.add(1);
|
||
list.add(1);
|
||
list.add(3);
|
||
list.add(2);
|
||
list.add(2);
|
||
list.add(2);
|
||
Map<Integer, Long> frequencyMap = list.stream().collect(Collectors.groupingBy(x -> x, Collectors.counting()));
|
||
// 1: 2
|
||
// 2: 3
|
||
// 3: 1
|
||
```
|
||
|
||
另见
|
||
---
|
||
|
||
- [Java 官网](https://www.oracle.com/cn/java/) _(oracle.com/cn/java)_
|