60个java常用的代码（能够帮助您更好地理解Java编程）

以下是60个常用的Java代码片段，涵盖了基础语法、常用类和一些实用功能：

基础语法

Hello World

java">public class HelloWorld {  
    public static void main(String[] args) {  
        System.out.println("Hello, World!");  
    }  
}

变量声明

java">int number = 10;  
String text = "Hello";

条件语句

java">if (number > 0) {  
    System.out.println("Positive");  
} else {  
    System.out.println("Negative or Zero");  
}

循环

java">for (int i = 0; i < 5; i++) {  
    System.out.println(i);  
}

数组
```
java">int[] numbers = {1, 2, 3, 4, 5};
```

方法与函数

方法定义

java">public int add(int a, int b) {  
    return a + b;  
}

方法重载

java">public int add(int a, int b) {  
    return a + b;  
}  
public double add(double a, double b) {  
    return a + b;  
}

递归

java">public int factorial(int n) {  
    if (n == 0) return 1;  
    return n * factorial(n - 1);  
}

面向对象编程

类与对象

java">class Dog {  
    String name;  
    void bark() {  
        System.out.println(name + " says woof!");  
    }  
}

构造函数

java">class Cat {  
    String name;  
    Cat(String name) {  
        this.name = name;  
    }  
}

继承

java">class Animal {}  
class Dog extends Animal {}

接口

java">interface Animal {  
    void sound();  
}  
class Cat implements Animal {  
    public void sound() {  
        System.out.println("Meow");  
    }  
}

集合框架

ArrayList

java">List<String> list = new ArrayList<>();  
list.add("Hello");

HashMap

java">Map<String, Integer> map = new HashMap<>();  
map.put("One", 1);  
map.put("Two", 2);

遍历集合

java">for (String item : list) {  
    System.out.println(item);  
}

异常处理

try-catch

java">try {  
    int result = 10 / 0;  
} catch (ArithmeticException e) {  
    System.out.println("Cannot divide by zero!");  
}

自定义异常

java">class MyException extends Exception {  
    public MyException(String message) {  
        super(message);  
    }  
}

输入输出

文件读取

java">try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {  
    String line;  
    while ((line = br.readLine()) != null) {  
        System.out.println(line);  
    }  
} catch (IOException e) {  
    e.printStackTrace();  
}

文件写入

java">try (BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"))) {  
    writer.write("Hello, World!");  
} catch (IOException e) {  
    e.printStackTrace();  
}

日期与时间

获取当前日期

java">LocalDate today = LocalDate.now();

日期格式化

java">DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");  
String formattedDate = today.format(formatter);

线程

创建线程

java">Thread thread = new Thread(() -> System.out.println("Thread running"));  
thread.start();

Runnable接口

java">class MyRunnable implements Runnable {  
    public void run() {  
        System.out.println("Runnable running");  
    }  
}  
new Thread(new MyRunnable()).start();

Lambda表达式

使用Lambda

java">List<String> names = Arrays.asList("Alice", "Bob", "Charlie");  
names.forEach(name -> System.out.println(name));

Stream API

使用Stream

java">List<String> filtered = names.stream()  
                              .filter(name -> name.startsWith("A"))  
                              .collect(Collectors.toList());

常用工具类

Math类
```
java">int max = Math.max(5, 10);
```

String manipulation

java">String uppercase = "hello".toUpperCase();

String split

java">String[] parts = "a,b,c".split(",");

UUID生成

java">String uuid = UUID.randomUUID().toString();

其他常用功能

JSON处理（使用Gson）

java">Gson gson = new Gson();  
String json = gson.toJson(yourObject);  
YourClass obj = gson.fromJson(json, YourClass.class);

正则表达式

java">boolean matches = "abc".matches("[a-z]+");

字符串连接

java">String result = String.join(", ", list);

数据库连接（JDBC）

连接数据库

java">Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "user", "password");

执行查询

java">Statement stmt = con.createStatement();  
ResultSet rs = stmt.executeQuery("SELECT * FROM users");

网络编程

连接到URL

java">URL url = new URL("http://example.com");  
HttpURLConnection conn = (HttpURLConnection) url.openConnection();

JavaFX (GUI)

创建窗口

java">Stage stage = new Stage();  
Scene scene = new Scene(new StackPane(), 300, 250);  
stage.setScene(scene);  
stage.show();

注解

自定义注解

java">@Retention(RetentionPolicy.RUNTIME)  
@Target(ElementType.METHOD)  
public @interface MyAnnotation {}

反射

获取类信息

java">Class<?> clazz = Class.forName("java.lang.String");

调用方法

java">Method method = clazz.getMethod("length");  
Object result = method.invoke("Hello");

Java 8 任意和方法引用

方法引用

java">names.forEach(System.out::println);

其他实用代码

版本判断

java">if (System.getProperty("java.version").startsWith("1.8")) {  
    System.out.println("Java 8");  
}

克隆对象
```
java">Object cloned = original.clone();
```

获取栈信息

java">new Exception().printStackTrace();

检查空值

java">if (obj == null) {  
    System.out.println("Object is null");  
}

避免重复元素（Set）

java">Set<String> set = new HashSet<>(list);

将集合转为数组

java">String[] array = list.toArray(new String[0]);

时间延迟
```
java">Thread.sleep(1000);
```

获取输入

java">Scanner scanner = new Scanner(System.in);  
String input = scanner.nextLine();

转换类型

java">int value = Integer.parseInt("123");

使用Optional防止NullPointerException

java">Optional<String> optional = Optional.ofNullable(name);  
optional.ifPresent(System.out::println);

使用StringBuilder

java">StringBuilder sb = new StringBuilder();  
sb.append("Hello").append(" World");  
String result = sb.toString();

多线程锁

java">synchronized (this) {  
    // synchronized block  
}

使用ExecutorService

java">ExecutorService executor = Executors.newFixedThreadPool(2);  
executor.submit(() -> System.out.println("Task"));

获取环境变量

java">String path = System.getenv("PATH");

打印当前线程名称

java">System.out.println(Thread.currentThread().getName());

基本数据类型转为字符串

java">String str = Integer.toString(100);

简化主函数

java">public static void main(String... args) {  
    // ...  
}

使用File类

java">File file = new File("file.txt");  
boolean exists = file.exists();

使用Property类

java">Properties properties = new Properties();  
properties.load(new FileReader("config.properties"));

使用String.format

java">String formatted = String.format("Hello, %s!", "World");

这些代码示例涵盖了Java的基本技能和一些高级特性，希望能够帮助您更好地理解Java编程。