Java 知识点速查

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);

查看: Strings

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

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

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");
}

查看: [Conditionals](#条件语句 Conditionals)

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

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);

一旦创建就不能修改，任何修改都会创建一个新的String

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 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

用于循环数组或列表

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

// 实现Runnable接口
public class RunnableThread implements Runnable {
    @Override
    public void run() {
       // todo something
    }
}

// 实现Callable接口,T 替换成实际类型
public class CallableTask implements Callable<T> {
    @Override
    public T call() throws Exception {
        // todo something
        return null;
    }
}

// 继承Thrad类
public class ExtendsThread extends Thread {
    @Override
    public void run() {
       // todo something
    }
}

// 运行线程
public static void main(String[] args) throws ExecutionException, InterruptedException {
        new Thread(new RunnableThread()).start();
        new ExtendsThread2().start();
        FutureTask<Integer> integerFutureTask = new FutureTask<>(new CallableTask());
        integerFutureTask.run();
    }

/**
 *  corePoolSize: 核心线程数
 *  maximumPoolSize: 最大线程数
 *  keepAliveTime: 线程空闲时间
 *  timeUni: 线程空闲时间单位
 *  workQueue: 线程等待队列
 *  threadFactory: 线程创建工厂
 *  handler: 拒绝策略
 */
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                2, 5,
                5, TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(10),
                new DefaultThreadFactory("pollName"),
                new ThreadPoolExecutor.CallerRunsPolicy());

// 内置的线程池, 不推荐生产使用
Executors.newCachedThreadPool();
Executors.newFixedThreadPool(10);
Executors.newScheduledThreadPool(10);
Executors.newSingleThreadExecutor();

// 使用完之后一定要记得remove, 否则会内存泄露
ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
threadLocal.set(1);
threadLocal.get();
threadLocal.remove();

// 需要synchronized修饰的代码块才能使用
wait(); 
notify();
notifyAll();

// 使用lock的条件唤醒
ReentrantLock lock = new ReentrantLock();
Condition condition= lock.newCondition();
lock.lock();
try{
   // 当前线程唤醒或等待
    condition.await();
    condition.signal();
    condition.signalAll();
} finally {
  lock.unlock
}

// LockSupport,可以先unpark,后续park不会阻塞线程
LockSupport.park(obj);
LockSupport.unpark(thread);

// CountDownLatch
CountDownLatch countDownLatch = new CountDownLatch(2);
new Thread(() -> {
    try {
       ...
    }finally {
       countDownLatch.countDown();
    }
}).start();
countDownLatch.await();

// CompletableFuture
CompletableFuture<Void> task1 = CompletableFuture.runAsync(() -> {});
CompletableFuture<Void> task2 = CompletableFuture.runAsync(() -> {});
CompletableFuture<Void> task3 = CompletableFuture.runAsync(() -> {});
CompletableFuture.allOf(task1, task2, task3).get();

// Semaphore
Semaphore semaphore = new Semaphore(5);
try {
    semaphore.acquire();
} finally {
    semaphore.release(); 
}

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("|"));

查看: Regex in java

// 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);

• ArrayDeque 提供 resizable-array 并实现 Deque 接
• Arrays 包含一个静态工厂，允许将数组视为列表
• Collections 包含对集合进行操作或返回集合的静态方法
• Date 表示特定的时间瞬间，精度为毫秒
• Dictionary 是任何类的抽象父类，例如 Hashtable，它将键映射到值
• EnumMap 一个专门用于枚举键的 Map 实现
• EnumSet 一个专门用于枚举键的 Set 实现
• Formatter 提供对布局对齐和对齐、数字、字符串和日期/时间数据的常用格式以及特定于语言环境的输出的支持
• SecureRandom 实例用于生成安全的伪随机数流
• UUID 表示一个不可变的通用唯一标识符
• Vector 实现了一个可增长的对象数组
• LocalDate 表示没有时区的日期，只包含年月日，不可变并且线程安全的，java8 及以上版本可用
• LocalTime 表示没有时区的时间，只包含时分秒，不可变并且线程安全的，java8 及以上版本可用
• LocalDateTime 表示没有时区的日期时间，同时包含年月日时分秒，不可变并且线程安全的，java8 及以上版本可用

// 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