Java 备忘清单

public class Hello {

  // 主要方法

  public static void main(String[] args)

  {

    // 输出: Hello, world!

    System.out.println("Hello, world!");

  }

}

int num = 5;

float floatNum = 5.99f;

char letter = 'D';

boolean bool = true;

String site = "jaywcjlove.github.io";

String first = "John";

String last = "Doe";

String name = first + " " + last;

System.out.println(name);

String word = "QuickRef";

for (char c: word.toCharArray()) {

  System.out.print(c + "-");

}

// 输出: Q-u-i-c-k-R-e-f-

char[] chars = new char[10];

chars[0] = 'a'

chars[1] = 'b'

String[] letters = {"A", "B", "C"};

int[] mylist = {100, 200};

boolean[] answers = {true, false};

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

// 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

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");

}

Scanner in = new Scanner(System.in);

String str = in.nextLine();

System.out.println(str);

int num = in.nextInt();

System.out.println(num);

String str1 = "value"; 

String str2 = new String("value");

String str3 = String.valueOf(123);

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

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

String s1 = new String("QuickRef"); 

String s2 = new String("QuickRef"); 

s1 == s2          // false

s1.equals(s2)     // true

"AB".equalsIgnoreCase("ab")  // true

String str = "Abcd";

str.toUpperCase();     // ABCD

str.toLowerCase();     // abcd

str.concat("#");       // Abcd#

str.replace("b", "-"); // A-cd

"  abc ".trim();       // abc

"ab".toCharArray();    // {'a', 'b'}

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

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);

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;

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

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

String[] arr = {"a", "b", "c"};

for (int a: arr) {

    System.out.print(a + " ");

}

// 输出: a b c

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

char[] chars = {'b', 'a', 'c'};

Arrays.sort(chars);

// [a, b, c]

Arrays.toString(chars);

int k = 15;

if (k > 20) {

  System.out.println(1);

} else if (k > 10) {

  System.out.println(2);

} else {

  System.out.println(3);

}

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);

int a = 10;

int b = 20;

int max = (a > b) ? a : b;

// 输出: 20

System.out.println(max);

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

int[] numbers = {1,2,3,4,5};

for (int number: numbers) {

  System.out.print(number);

}

// 输出: 12345

int count = 0;

while (count < 5) {

  System.out.print(count);

  count++;

}

// 输出: 01234

int count = 0;

do {

  System.out.print(count);

  count++;

} while (count < 5);

// 输出: 01234

for (int i = 0; i < 5; i++) {

  if (i == 3) {

    continue;

  }

  System.out.print(i);

}

// 输出: 01245

for (int i = 0; i < 5; i++) {

  System.out.print(i);

  if (i == 3) {

    break;

  }

}

// 输出: 0123

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);

}

//lambda 打印元素

nums.forEach(e -> System.out.println(e.toString()));

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);

});

ConcurrentHashMap<Integer, String> m

            = new ConcurrentHashMap<>();

m.put(100, "Hello");

m.put(101, "Geeks");

m.put(102, "Geeks");

// 移除

m.remove(101, "Geeks");

// 如果不存在，就添加，存在就不变更

m.putIfAbsent(103, "Hello");

// 替换

m.replace(101, "Hello", "For");

System.out.println(m);

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);

}

set.forEach(e -> System.out.println(e.toString()));

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());

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("|"));

// 我是单行注释！

/*

而我是一个

多行注释！

*/

/**

  * 这个

  * 是

  * 文档

  * 注释

 */

try {

  // something

} catch (Exception e) {

  e.printStackTrace();

} finally {

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

}

/**

* 利用反射动态加载依赖库

* java9 及以上版本可用

* @param jar jar 文件

*/

Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);

method.setAccessible(true);

MethodHandle addURL = lookup.unreflect(method);

URL url = jar.toURI().toURL();

URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {url});

addURL.invoke(urlClassLoader, url);

//java8

Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);

method.setAccessible(true);

method.invoke(classLoader, url);

// 输入流转输出流

byte[] inputContent = "test".getBytes();

try (InputStream inputStream = new ByteArrayInputStream(inputContent);

     OutputStream outputStream = new ByteArrayOutputStream()) {

    byte[] buffer = new byte[1024];

    int len;

    while ((len = inputStream.read(buffer)) != -1) {

        outputStream.write(buffer, 0, len);

    }

} catch (IOException e) {

    throw new RuntimeException(e);

}

// 计算出现频率

List<Integer> list = new ArrayList<>();

list.add(1);

list.add(2);

list.add(2);

list.add(3);

list.add(3);

list.add(3);

int frequency = Collections.frequency(list, 2); // frequency = 2

// 统计词频

List<Integer> list = new ArrayList<>();

list.add(1);

list.add(1);

list.add(3);

list.add(2);

list.add(2);

list.add(2);

Map<Integer, Long> frequencyMap = list.stream().collect(Collectors.groupingBy(x -> x, Collectors.counting()));

// 1: 2

// 2: 3

// 3: 1