diff --git a/README.md b/README.md index 7b34715..29e2681 100644 --- a/README.md +++ b/README.md @@ -23,6 +23,7 @@ Quick Reference [Golang](./docs/golang.md) [INI](./docs/ini.md) [JSON](./docs/json.md) +[Java](./docs/java.md) [Markdown](./docs/markdown.md) [MySQL](./docs/mysql.md) [Swift](./docs/swift.md) @@ -51,7 +52,7 @@ Quick Reference [RegEx](./docs/regex.md) [TypeScript](./docs/typescript.md) [Vue 2](./docs/vue2.md) -[Vue 3 (欢迎参与建设)](./docs/vue.md) +[Vue 3 ](./docs/vue.md) ## Nodejs diff --git a/docs/java.md b/docs/java.md new file mode 100644 index 0000000..5d488fb --- /dev/null +++ b/docs/java.md @@ -0,0 +1,774 @@ +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 + + +```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 + + +| 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 | + + +### 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 10) { + System.out.println("I don't"); +} else { + System.out.println("I also don't"); +} +``` + +查看: [Conditionals](#java-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 + + +```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 + + +- `+` _(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_ +- [?:](#三元运算符) _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 + +```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 + + +```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 + + +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_ + + +### ArrayList + +```java +List 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 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 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 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 | 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_ | + + +### 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 + + +- 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 + +```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)_ \ No newline at end of file diff --git a/scripts/assets/java.svg b/scripts/assets/java.svg new file mode 100644 index 0000000..6b59d3f --- /dev/null +++ b/scripts/assets/java.svg @@ -0,0 +1,3 @@ + + + diff --git a/scripts/style.css b/scripts/style.css index 5ce7340..e857f68 100644 --- a/scripts/style.css +++ b/scripts/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;