Java 最常用函数

你的名字 2024-03-30 09:19 66阅读 0赞

**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**

<table> 
 <tbody> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 数据类型 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 大小 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 默认 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 范围 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> byte </td> 
 <td style="vertical-align:top;"> 1 byte </td> 
 <td style="vertical-align:top;"> 0 </td> 
 <td style="vertical-align:top;"> -128 ^to^ 127 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> short </td> 
 <td style="vertical-align:top;"> 2 byte </td> 
 <td style="vertical-align:top;"> 0 </td> 
 <td style="vertical-align:top;"> -2^15^ ^to^ 2^15^-1 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> int </td> 
 <td style="vertical-align:top;"> 4 byte </td> 
 <td style="vertical-align:top;"> 0 </td> 
 <td style="vertical-align:top;"> -2^31^ ^to^ 2^31^-1 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> long </td> 
 <td style="vertical-align:top;"> 8 byte </td> 
 <td style="vertical-align:top;"> 0 </td> 
 <td style="vertical-align:top;"> -2^63^ ^to^ 2^63^-1 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> float </td> 
 <td style="vertical-align:top;"> 4 byte </td> 
 <td style="vertical-align:top;"> 0.0f </td> 
 <td style="vertical-align:top;"> N/A </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> double </td> 
 <td style="vertical-align:top;"> 8 byte </td> 
 <td style="vertical-align:top;"> 0.0d </td> 
 <td style="vertical-align:top;"> N/A </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> char </td> 
 <td style="vertical-align:top;"> 2 byte </td> 
 <td style="vertical-align:top;"> \u0000 </td> 
 <td style="vertical-align:top;"> 0 ^to^ 65535 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> boolean </td> 
 <td style="vertical-align:top;"> N/A </td> 
 <td style="vertical-align:top;"> false </td> 
 <td style="vertical-align:top;"> true / false </td> 
 </tr> 
 </tbody> 
</table>

**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**

<table> 
 <tbody> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 搜集 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Interface </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 有序 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 已排序 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 线程安全 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 复制 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Nullable </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ArrayList </td> 
 <td style="vertical-align:top;"> List </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Vector </td> 
 <td style="vertical-align:top;"> List </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> LinkedList </td> 
 <td style="vertical-align:top;"> List, Deque </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> CopyOnWriteArrayList </td> 
 <td style="vertical-align:top;"> List </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> HashSet </td> 
 <td style="vertical-align:top;"> Set </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> One null </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> LinkedHashSet </td> 
 <td style="vertical-align:top;"> Set </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> One null </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> TreeSet </td> 
 <td style="vertical-align:top;"> Set </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> CopyOnWriteArraySet </td> 
 <td style="vertical-align:top;"> Set </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> One null </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ConcurrentSkipListSet </td> 
 <td style="vertical-align:top;"> Set </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> HashMap </td> 
 <td style="vertical-align:top;"> Map </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 <td style="vertical-align:top;"> One null&nbsp;(key) </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> HashTable </td> 
 <td style="vertical-align:top;"> Map </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> LinkedHashMap </td> 
 <td style="vertical-align:top;"> Map </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 <td style="vertical-align:top;"> One null&nbsp;(key) </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> TreeMap </td> 
 <td style="vertical-align:top;"> Map </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ConcurrentHashMap </td> 
 <td style="vertical-align:top;"> Map </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ConcurrentSkipListMap </td> 
 <td style="vertical-align:top;"> Map </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N (key) </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ArrayDeque </td> 
 <td style="vertical-align:top;"> Deque </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> PriorityQueue </td> 
 <td style="vertical-align:top;"> Queue </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ConcurrentLinkedQueue </td> 
 <td style="vertical-align:top;"> Queue </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ConcurrentLinkedDeque </td> 
 <td style="vertical-align:top;"> Deque </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> ArrayBlockingQueue </td> 
 <td style="vertical-align:top;"> Queue </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> LinkedBlockingDeque </td> 
 <td style="vertical-align:top;"> Deque </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> PriorityBlockingQueue </td> 
 <td style="vertical-align:top;"> Queue </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 </tbody> 
</table>

**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 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**

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**

<table> 
 <tbody> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> 修饰符 </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Class </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Package </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Subclass </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> World </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> public </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> protected </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> no modifier </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> private </td> 
 <td style="vertical-align:top;"> Y </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 <td style="vertical-align:top;"> N </td> 
 </tr> 
 </tbody> 
</table>

**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**

<table> 
 <tbody> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.max(a,b) </td> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> a 和 b 的最大值 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.min(a,b) </td> 
 <td style="vertical-align:top;"> a 和 b 的最小值 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.abs(a) </td> 
 <td style="vertical-align:top;"> 绝对值 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.sqrt(a) </td> 
 <td style="vertical-align:top;"> a 的平方根 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.pow(a,b) </td> 
 <td style="vertical-align:top;"> b 的幂 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.round(a) </td> 
 <td style="vertical-align:top;"> 最接近的整数 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.sin(ang) </td> 
 <td style="vertical-align:top;"> 正弦 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.cos(ang) </td> 
 <td style="vertical-align:top;"> ang 的余弦 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.tan(ang) </td> 
 <td style="vertical-align:top;"> ang 的切线 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.asin(ang) </td> 
 <td style="vertical-align:top;"> ang 的反正弦 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.log(a) </td> 
 <td style="vertical-align:top;"> a 的自然对数 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.toDegrees(rad) </td> 
 <td style="vertical-align:top;"> 以度为单位的角度弧度 </td> 
 </tr> 
 <tr> 
 <td style="border-color:#e8e8e8;vertical-align:top;"> Math.toRadians(deg) </td> 
 <td style="vertical-align:top;"> 以弧度为单位的角度度 </td> 
 </tr> 
 </tbody> 
</table>

**Try/Catch/Finally**

try \{

// something

\} catch (Exception e) \{

e.printStackTrace();

\} finally \{

System.out.println("always printed");