Hello. 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

 $  javac  Hello.java
$  java  Hello
Hello, world!
  

变量 Variables

 int num = 5;
float floatNum = 5.99f;
char letter = 'D';
 boolean   bool  = true;
String site = "quickref.me";
  

原始数据类型

字符串 Strings

  String  first = "John";
String last = "Doe";
String name = first + " " + last;
System.out.println(name);
  

查看: Strings

循环 Loops

 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

数组 Arrays

 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

Swap

 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

 // 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

 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

 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

 String str1 = "value"; 
String str2 = new String("value");
String str3 = String.valueOf(123);
  

字符串连接

 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

  StringBuilder  sb = new StringBuilder(10);
  

 ┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
|   |   |   |   |   |   |   |   |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9
  

 sb.append("Reference");
  

 ┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| R | e | f | e | r | e | n | c | e |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9
  

 sb.delete(3, 9);
  

 ┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| R | e | f |   |   |   |   |   |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9
  

 sb.insert(0, "My ");
  

 ┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y |   | R | e | f |   |   |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9
  

 sb.append("!");
  

 ┌───┬───┬───┬───┬───┬───┬───┬───┬───┐
| M | y |   | R | e | f | ! |   |   |
└───┴───┴───┴───┴───┴───┴───┴───┴───┘
0   1   2   3   4   5   6   7   8   9
  

比较

 String s1 = new String("QuickRef"); 
String s2 = new String("QuickRef"); 
s1 == s2          // false
s1.equals(s2)     // true
"AB".equalsIgnoreCase("ab")  // true
  

操纵

 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

 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

 String str = "hello";
str.concat("world");
// 输出: hello
System.out.println(str);
  

 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

 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

 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)

 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)

 String[] arr = {"a", "b", "c"};
for (int a: arr) {
    System.out.print(a + " ");
}
// 输出: a b c 
  

Multidimensional Arrays

 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

 char[] chars = {'b', 'a', 'c'};
Arrays.sort(chars);
// [a, b, c]
Arrays.toString(chars);
  

Java 条件语句

运算符

• + (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)

• == (等于)
• != (不等于)
• > (比…更棒)
• >= (大于或等于)
• < (少于)
• <= (小于或等于)

• && 条件与
• || 条件或
• ?: 三元(if-then-else 语句的简写)

• instanceof (将对象与指定类型进行比较)

• ~ (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)

If else

 int k = 15;
if (k > 20) {
  System.out.println(1);
} else if (k > 10) {
  System.out.println(2);
} else {
  System.out.println(3);
}
  

Switch

 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

 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

 for (int i = 0; i < 10; i++) {
  System.out.print(i);
}
// 输出: 0123456789
  

 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

 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

 int count = 0;
while (count < 5) {
  System.out.print(count);
  count++;
}
// 输出: 01234
  

Do While 循环

 int count = 0;
do {
  System.out.print(count);
  count++;
} while (count < 5);
// 输出: 01234
  

继续声明

 for (int i = 0; i < 5; i++) {
  if (i == 3) {
    continue;
  }
  System.out.print(i);
}
// 输出: 01245
  

中断语句

 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

ArrayList

 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

 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

 Set<String> set = new  Hash Set<>();
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

 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

常用表达

 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

注释 Comment

 // 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

• 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

数学方法

Try/Catch/Finally

 try {
  // something
} catch (Exception e) {
  e.printStackTrace();
} finally {
  System.out.println("always printed");
}
  

另见

• Java 官网 (java.com)