Java 备忘清单
===
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。
入门
--------
### Hello.java
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```java
public class Hello {
// 主要方法
public static void main(String[] args)
{
// 输出: Hello, world!
System.out.println("Hello, world!");
}
}
```
编译和运行
```shell
$ javac Hello.java
$ java Hello
Hello, world!
```
### 变量 Variables
```java
int num = 5;
float floatNum = 5.99f;
char letter = 'D';
boolean bool = true;
String site = "quickref.me";
```
### 原始数据类型
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| 数据类型 | 大小 | 默认 | 范围 |
|-----------|--------|---------|---------------------|
| `byte` | 1 byte | 0 | -128 ^to^ 127 |
| `short` | 2 byte | 0 | -2^15^ ^to^ 2^15^-1 |
| `int` | 4 byte | 0 | -2^31^ ^to^ 2^31^-1 |
| `long` | 8 byte | 0 | -2^63^ ^to^ 2^63^-1 |
| `float` | 4 byte | 0.0f | _N/A_ |
| `double` | 8 byte | 0.0d | _N/A_ |
| `char` | 2 byte | \\u0000 | 0 ^to^ 65535 |
| `boolean` | _N/A_ | false | true / false |
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### 字符串 Strings
```java
String first = "John";
String last = "Doe";
String name = first + " " + last;
System.out.println(name);
```
查看: [Strings](#java-字符串)
### 循环 Loops
```java
String word = "QuickRef";
for (char c: word.toCharArray()) {
System.out.print(c + "-");
}
// 输出: Q-u-i-c-k-R-e-f-
```
查看: [Loops](#java-循环)
### 数组 Arrays
```java
char[] chars = new char[10];
chars[0] = 'a'
chars[1] = 'b'
String[] letters = {"A", "B", "C"};
int[] mylist = {100, 200};
boolean[] answers = {true, false};
```
查看: [Arrays](#java-数组)
### Swap
```java
int a = 1;
int b = 2;
System.out.println(a + " " + b); // 1 2
int temp = a;
a = b;
b = temp;
System.out.println(a + " " + b); // 2 1
```
### Type Casting
```java
// Widening
// byte<short<int<long<float<double
int i = 10;
long l = i; // 10
// Narrowing
double d = 10.02;
long l = (long)d; // 10
String.valueOf(10); // "10"
Integer.parseInt("10"); // 10
Double.parseDouble("10"); // 10.0
```
### 条件语句 Conditionals
```java
int j = 10;
if (j == 10) {
System.out.println("I get printed");
} else if (j > 10) {
System.out.println("I don't");
} else {
System.out.println("I also don't");
}
```
查看: [Conditionals](#条件语句 Conditionals)
### 用户输入
```java
Scanner in = new Scanner(System.in);
String str = in.nextLine();
System.out.println(str);
int num = in.nextInt();
System.out.println(num);
```
Java 字符串
-------
### 基本的
```java
String str1 = "value";
String str2 = new String("value");
String str3 = String.valueOf(123);
```
### 字符串连接
```java
String s = 3 + "str" + 3; // 3str3
String s = 3 + 3 + "str"; // 6str
String s = "3" + 3 + "str"; // 33str
String s = "3" + "3" + "23"; // 3323
String s = "" + 3 + 3 + "23"; // 3323
String s = 3 + 3 + 23; // 29
```
### 字符串生成器
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```java
StringBuilder sb = new StringBuilder(10);
```
---
```java
┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| | | | | | | | | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0 1 2 3 4 5 6 7 8 9
```
---
```java
sb.append("Reference");
```
---
```java
┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| R | e | f | e | r | e | n | c | e |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0 1 2 3 4 5 6 7 8 9
```
---
```java
sb.delete(3, 9);
```
---
```java
┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| R | e | f | | | | | | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0 1 2 3 4 5 6 7 8 9
```
---
```java
sb.insert(0, "My ");
```
---
```java
┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y | | R | e | f | | | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0 1 2 3 4 5 6 7 8 9
```
---
```java
sb.append("!");
```
---
```java
┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y | | R | e | f | ! | | |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0 1 2 3 4 5 6 7 8 9
```
### 比较
```java
String s1 = new String("QuickRef");
String s2 = new String("QuickRef");
s1 == s2 // false
s1.equals(s2) // true
"AB".equalsIgnoreCase("ab") // true
```
### 操纵
```java
String str = "Abcd";
str.toUpperCase(); // ABCD
str.toLowerCase(); // abcd
str.concat("#"); // Abcd#
str.replace("b", "-"); // A-cd
" abc ".trim(); // abc
"ab".toCharArray(); // {'a', 'b'}
```
### 信息
```java
String str = "abcd";
str.charAt(2); // c
str.indexOf("a") // 0
str.indexOf("z") // -1
str.length(); // 4
str.toString(); // abcd
str.substring(2); // cd
str.substring(2,3); // c
str.contains("c"); // true
str.endsWith("d"); // true
str.startsWith("a"); // true
str.isEmpty(); // false
```
### 不可变
```java
String str = "hello";
str.concat("world");
// 输出: hello
System.out.println(str);
```
---
```java
String str = "hello";
String concat = str.concat("world");
// 输出: helloworld
System.out.println(concat);
```
一旦创建就不能修改，任何修改都会创建一个新的String
Java 数组
-------
### 申明 Declare
```java
int[] a1;
int[] a2 = {1, 2, 3};
int[] a3 = new int[]{1, 2, 3};
int[] a4 = new int[3];
a4[0] = 1;
a4[2] = 2;
a4[3] = 3;
```
### 修改 Modify
```java
int[] a = {1, 2, 3};
System.out.println(a[0]); // 1
a[0] = 9;
System.out.println(a[0]); // 9
System.out.println(a.length); // 3
```
### 循环 (读 & 写)
```java
int[] arr = {1, 2, 3};
for (int i=0; i < arr.length; i++) {
arr[i] = arr[i] * 2;
System.out.print(arr[i] + " ");
}
// 输出: 2 4 6
```
### Loop (Read)
```java
String[] arr = {"a", "b", "c"};
for (int a: arr) {
System.out.print(a + " ");
}
// 输出: a b c
```
### Multidimensional Arrays
```java
int[][] matrix = { {1, 2, 3}, {4, 5} };
int x = matrix[1][0]; // 4
// [[1, 2, 3], [4, 5]]
Arrays.deepToString(matrix)
for (int i = 0; i < a.length; ++i) {
for(int j = 0; j < a[i].length; ++j) {
System.out.println(a[i][j]);
}
}
// 输出: 1 2 3 4 5 6 7
```
### Sort
```java
char[] chars = {'b', 'a', 'c'};
Arrays.sort(chars);
// [a, b, c]
Arrays.toString(chars);
```
Java 条件语句
-----------
### 运算符
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- `+` _(加法运算符(也用于字符串连接))_
- `-` _(减法运算符)_
- `*` _(乘法运算符)_
- `/` _(分区运算符)_
- `%` _(余数运算符)_
- `=` _(简单赋值运算符)_
- `++` _(增量运算符；将值增加 1)_
- `--` _(递减运算符；将值减 1)_
- `!` _(逻辑补码运算符；反转布尔值)_
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---
- `==` _(等于)_
- `!=` _(不等于)_
- `>` _(比...更棒)_
- `>=` _(大于或等于)_
- `<` _(少于)_
- `<=` _(小于或等于)_
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---
- `&&` _条件与_
- `||` _条件或_
- [?:](#三元运算符) _三元(if-then-else 语句的简写)_
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---
- `instanceof` _(将对象与指定类型进行比较)_
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---
- `~` _(一元按位补码)_
- `<<` _(签名左移)_
- `>>` _(有符号右移)_
- `>>>` _(无符号右移)_
- `&` _(按位与)_
- `^` _(按位异或)_
- `|` _(按位包含 OR)_
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### If else
```java
int k = 15;
if (k > 20) {
System.out.println(1);
} else if (k > 10) {
System.out.println(2);
} else {
System.out.println(3);
}
```
### Switch
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```java
int month = 3;
String str;
switch (month) {
case 1:
str = "January";
break;
case 2:
str = "February";
break;
case 3:
str = "March";
break;
default:
str = "Some other month";
break;
}
// 输出: Result March
System.out.println("Result " + str);
```
### 三元运算符
```java
int a = 10;
int b = 20;
int max = (a > b) ? a : b;
// 输出: 20
System.out.println(max);
```
Java 循环
----
### For 循环
```java
for (int i = 0; i < 10; i++) {
System.out.print(i);
}
// 输出: 0123456789
```
---
```java
for (int i = 0,j = 0; i < 3; i++,j--) {
System.out.print(j + "|" + i + " ");
}
// 输出: 0|0 -1|1 -2|2
```
### 增强的 For 循环
```java
int[] numbers = {1,2,3,4,5};
for (int number: numbers) {
System.out.print(number);
}
// 输出: 12345
```
用于循环数组或列表
### While 循环
```java
int count = 0;
while (count < 5) {
System.out.print(count);
count++;
}
// 输出: 01234
```
### Do While 循环
```java
int count = 0;
do {
System.out.print(count);
count++;
} while (count < 5);
// 输出: 01234
```
### 继续声明
```java
for (int i = 0; i < 5; i++) {
if (i == 3) {
continue;
}
System.out.print(i);
}
// 输出: 01245
```
### 中断语句
```java
for (int i = 0; i < 5; i++) {
System.out.print(i);
if (i == 3) {
break;
}
}
// 输出: 0123
```
Java 框架搜集
--------------------
### Java 搜集
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搜集 | 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_
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### 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);
}
```
### 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);
});
```
### 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);
}
```
### 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
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| 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_ |
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### 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
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- 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
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### 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
### Try/Catch/Finally
```java
try {
// something
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("always printed");
}
```
另见
---
- [Java 官网](https://www.java.com/zh-CN/) _(java.com)_