From 75e091587bf3a81bf59d13d756ea7a0509ebcd85 Mon Sep 17 00:00:00 2001 From: jaywcjlove Date: Mon, 31 Oct 2022 16:21:31 +0000 Subject: [PATCH] feat: add `java.md` cheatsheet. (#12) a9896a265f3c1fc35d941627b54445adef247f7e --- docs/java.html | 1006 +++++++++++++++++++++++++++++++++++++++++++++++ index.html | 8 +- style/style.css | 11 + 3 files changed, 1023 insertions(+), 2 deletions(-) create mode 100644 docs/java.html diff --git a/docs/java.html b/docs/java.html new file mode 100644 index 00000000..4357dbf1 --- /dev/null +++ b/docs/java.html @@ -0,0 +1,1006 @@ + + + + +Java 备忘清单 + & java cheatsheet & Quick Reference + + + + + + +

+ + +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

+ +
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

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
数据类型大小默认范围
byte1 byte0-128 ^to^ 127
short2 byte0-2^15^ ^to^ 2^15^-1
int4 byte0-2^31^ ^to^ 2^31^-1
long8 byte0-2^63^ ^to^ 2^63^-1
float4 byte0.0fN/A
double8 byte0.0dN/A
char2 byte\u00000 ^to^ 65535
booleanN/Afalsetrue / false
+ +

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

+

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

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
搜集Interface有序已排序线程安全复制Nullable
ArrayListListYNNYY
VectorListYNYYY
LinkedListList, DequeYNNYY
CopyOnWriteArrayListListYNYYY
HashSetSetNNNNOne null
LinkedHashSetSetYNNNOne null
TreeSetSetYYNNN
CopyOnWriteArraySetSetYNYNOne null
ConcurrentSkipListSetSetYYYNN
HashMapMapNNNN (key)One null (key)
HashTableMapNNYN (key)N (key)
LinkedHashMapMapYNNN (key)One null (key)
TreeMapMapYYNN (key)N (key)
ConcurrentHashMapMapNNYN (key)N
ConcurrentSkipListMapMapYYYN (key)N
ArrayDequeDequeYNNYN
PriorityQueueQueueYNNYN
ConcurrentLinkedQueueQueueYNYYN
ConcurrentLinkedDequeDequeYNYYN
ArrayBlockingQueueQueueYNYYN
LinkedBlockingDequeDequeYNYYN
PriorityBlockingQueueQueueYNYYN
+ +

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

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
修饰符ClassPackageSubclassWorld
publicYYYY
protectedYYYN
no modifierYYNN
privateYNNN
+ +

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

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
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

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

另见

+ +
+ diff --git a/index.html b/index.html index 23b707b7..5462ee62 100644 --- a/index.html +++ b/index.html @@ -62,6 +62,10 @@ JSON + + + +Java Markdown @@ -161,11 +165,11 @@ Vue 2 - + -Vue 3 (欢迎参与建设)

+Vue 3

Nodejs

Express.js diff --git a/style/style.css b/style/style.css index 5ce73404..e857f686 100644 --- a/style/style.css +++ b/style/style.css @@ -365,6 +365,17 @@ body.home .max-container a.home-button:visited:hover { gap: 2rem; grid-template-columns: repeat(2, minmax(0, 1fr)); } +.home-card a.contributing { + position: relative; +} +.home-card a.contributing::after { + content: '待完善需要您的参与'; + display: block; + position: absolute; + font-size: 10px; + right: 5px; + bottom: -15px; +} [data-color-mode*='light'] body .home-card a { --text-opacity: 0.75;