1
// 错误写法：会抛 ConcurrentModificationException
2
Iterator<String> it = list.iterator();
3
while (it.hasNext()) {
4
    String s = it.next();
5
    if ("xxx".equals(s)) {
6
        list.remove(s);          // ❌
7
    }
8
}
9

10
// 正确写法：用迭代器自己的 remove()
11
Iterator<String> it = list.iterator();
12
while (it.hasNext()) {
13
    String s = it.next();
14
    if ("xxx".equals(s)) {
15
        it.remove();             // ✅
16
    }
17
}

1
// ❌ 错误
2
ArrayList<int> list = new ArrayList<>();
3

4
// ✅ 正确
5
ArrayList<Integer> list = new ArrayList<>();

1
public class ArrayList<E> {
2
    public boolean add(E e) { ... }
3
}

1
// 非静态：使用类上声明的 E
2
public class ArrayList<E> {
3
    public boolean add(E e) { ... }
4
}
5

6
// 静态：必须自己声明 <T>
7
public static <T> void printArray(T[] array) { ... }

1
public class Student implements Comparable<Student> {
2
    @Override
3
    public int compareTo(Student o) {
4
        return this.age - o.age;   // 按年龄升序
5
    }
6
}

1
@Override
2
public boolean equals(Object o) {
3
    if (this == o) return true;
4
    if (o == null || getClass() != o.getClass()) return false;
5
    Student student = (Student) o;
6
    return age == student.age && Objects.equals(name, student.name);
7
}
8

9
@Override
10
public int hashCode() {
11
    return Objects.hash(name, age);
12
}

1
public static int sum(int... nums) {     // 本质 int[] nums
2
    int total = 0;
3
    for (int n : nums) total += n;
4
    return total;
5
}
6

7
sum();              // 0
8
sum(1, 2, 3);       // 6
9
sum(new int[]{1,2});// 3

1
Set<String> keys = map.keySet();          // ① 获取所有键
2
for (String key : keys) {                 // ② 遍历 Set 集合
3
    String value = map.get(key);          // ③ 根据键找值
4
    System.out.println(key + "=" + value);
5
}

1
// Map.Entry 内部接口的方法
2
K getKey();      // 获取键
3
V getValue();    // 获取值
4

5
Set<Map.Entry<String, String>> entries = map.entrySet();
6
for (Map.Entry<String, String> entry : entries) {
7
    System.out.println(entry.getKey() + "=" + entry.getValue());
8
}

1
map.forEach((key, value) -> System.out.println(key + "=" + value));

1
ArrayList<String> list = new ArrayList<>();
2
Collections.addAll(list, "zhangsan,23", "lisi,24", "wangwu,25");
3

4
Map<String, Integer> map = list.stream()
5
        .collect(Collectors.toMap(
6
                s -> s.split(",")[0],   // 键：姓名
7
                s -> Integer.parseInt(s.split(",")[1])  // 值：年龄
8
        ));
9
// {zhangsan=23, lisi=24, wangwu=25}

1
// 演员类
2
public class Actor {
3
    private String name;
4
    public Actor(String name) { this.name = name; }
5
    public String getName() { return name; }
6
    public void setName(String name) { this.name = name; }
7
}
8

9
// 处理
10
ArrayList<String> males = new ArrayList<>(Arrays.asList(
11
        "蔡徐坤", "李易峰", "王俊凯", "易烊千玺", "陈伟霆", "邓伦"));
12
ArrayList<String> females = new ArrayList<>(Arrays.asList(
13
        "林允儿", "林志玲", "杨幂", "赵丽颖", "刘亦菲", "林心如"));
14

15
Stream<String> m = males.stream().filter(s -> s.length() == 3).limit(2);
16
Stream<String> f = females.stream().filter(s -> s.startsWith("林")).skip(1);
17
Stream.concat(m, f).map(Actor::new).forEach(a -> System.out.println(a.getName()));

1
try {
2
    // 可能会出现异常的代码
3
} catch (异常类型1 变量) {
4
    // 处理异常的方案
5
} catch (异常类型2 变量) {
6
    // 处理异常的方案
7
}

1
public void method() throws 异常1, 异常2, 异常3... { }

1
// 1. 自定义运行时异常
2
public class AgeOutOfRangeException extends RuntimeException {
3
    public AgeOutOfRangeException() { super(); }
4
    public AgeOutOfRangeException(String message) { super(message); }
5
}
6

7
// 2. 业务方法中抛出
8
public void setAge(int age) {
9
    if (age < 0 || age > 150) {
10
        throw new AgeOutOfRangeException("年龄超出范围: " + age);
11
    }
12
    this.age = age;
13
}
14

15
// 3. 调用方捕获
16
try {
17
    setAge(200);
18
} catch (AgeOutOfRangeException e) {
19
    System.out.println(e.getMessage());   // 年龄超出范围: 200
20
}

1
public class RecursionDemo1 {
2
    public static void main(String[] args) {
3
        int result = jc(5);
4
        System.out.println(result);   // 120
5
    }
6

7
    public static int jc(int num) {
8
        if (num == 1) {
9
            return 1;                    // 终止条件
10
        }
11
        return num * jc(num - 1);        // 自己调自己，参数规模更小
12
    }
13
}

1
public static int rabbit(int month) {
2
    if (month == 1 || month == 2) return 1;
3
    return rabbit(month - 1) + rabbit(month - 2);
4
}

1
public static void findJava(File dir, ArrayList<String> list) {
2
    File[] files = dir.listFiles();
3
    if (files == null) return;
4

5
    for (File f : files) {
6
        if (f.isFile() && f.getName().endsWith(".java")) {
7
            list.add(f.getAbsolutePath());
8
        } else if (f.isDirectory()) {
9
            findJava(f, list);                 // 递归
10
        }
11
    }
12
}

1
public static void deleteDir(File dir) {
2
    File[] files = dir.listFiles();
3
    if (files == null) return;
4

5
    for (File f : files) {
6
        if (f.isFile()) {
7
            f.delete();                       // 文件直接删
8
        } else if (f.isDirectory()) {
9
            deleteDir(f);                     // 递归
10
        }
11
    }
12
    dir.delete();                             // 最后删自己
13
}

1
public static long sizeOf(File dir) {
2
    long total = 0;
3
    File[] files = dir.listFiles();
4
    if (files == null) return 0;
5

6
    for (File f : files) {
7
        if (f.isFile()) {
8
            total += f.length();
9
        } else if (f.isDirectory()) {
10
            total += sizeOf(f);
11
        }
12
    }
13
    return total;
14
}

1
Integer i = 10;        // 自动装箱
2
int n = i;             // 自动拆箱
3
int result = i + 100;  // 拆箱后参与运算

1
String s = "10,50,30,20,40";
2
String[] arr = s.split(",");
3
int[] nums = new int[arr.length];
4
int max = nums[0];
5
for (int i = 0; i < arr.length; i++) {
6
    nums[i] = Integer.parseInt(arr[i]);   // 字符串 → int
7
    if (nums[i] > max) {
8
        max = nums[i];
9
    }
10
}
11
System.out.println("最大值: " + max);

1
BigDecimal divide = bd1.divide(参与运算的对象, 小数点后精确到多少位, 舍入模式);

1
BigDecimal a = new BigDecimal("10");
2
BigDecimal b = new BigDecimal("3");
3
BigDecimal r = a.divide(b, 2, RoundingMode.HALF_UP);  // 3.33

1
int[] arr = {11, 22, 33, 44, 55, 66, 77, 88, 99, 100};
2
int idx = Arrays.binarySearch(arr, 88);   // 7

1
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
2
Date d = new Date();
3
String s = sdf.format(d);                  // Date → 字符串
4
Date d2 = sdf.parse("2025-01-20 15:30:00"); // 字符串 → Date

1
LocalDateTime ldt = LocalDateTime.now();
2
LocalDateTime newLdt = ldt
3
        .withHour(10)               // 修改小时
4
        .plusDays(7)                // 加 7 天
5
        .minusHours(2);             // 减 2 小时

1
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
2
LocalDateTime ldt = LocalDateTime.now();
3
String s = dtf.format(ldt);                     // 格式化
4
LocalDateTime ldt2 = LocalDateTime.parse(s, dtf); // 解析

1
// 示例：写出数据
2
FileOutputStream fos = new FileOutputStream("D:\\A.txt", true);  // 追加写入
3
fos.write(97);                      // 写出单个字节 'a'
4
fos.write("Hello".getBytes());      // 写出整个字符串
5
fos.close();                        // 关闭流，释放资源

1
try (FileOutputStream fos = new FileOutputStream("D:\\A.txt")) {
2
    fos.write("Hello".getBytes());
3
} catch (IOException e) {
4
    e.printStackTrace();
5
}

1
// 方式一：单字节读取（效率低）
2
try (FileInputStream fis = new FileInputStream("D:\\A.txt")) {
3
    int b;
4
    while ((b = fis.read()) != -1) {
5
        System.out.print((char) b);
6
    }
7
} catch (IOException e) {
8
    e.printStackTrace();
9
}
10

11
// 方式二：字节数组读取（推荐，效率高）
12
try (FileInputStream fis = new FileInputStream("D:\\A.txt")) {
13
    byte[] bys = new byte[2];
14
    int len;
15
    while ((len = fis.read(bys)) != -1) {
16
        System.out.print(new String(bys, 0, len));
17
    }
18
} catch (IOException e) {
19
    e.printStackTrace();
20
}

1
Properties prop = new Properties();
2
prop.setProperty("username", "admin");
3
prop.setProperty("password", "123456");
4
System.out.println(prop.getProperty("username"));  // admin

1
// 把集合中的键值对写到配置文件
2
Properties prop = new Properties();
3
prop.setProperty("username", "admin");
4
prop.setProperty("password", "123456");
5
try (FileWriter fw = new FileWriter("config.properties")) {
6
    prop.store(fw, "user config");
7
}
8

9
// 从配置文件中读取键值对到集合
10
Properties prop2 = new Properties();
11
try (FileReader fr = new FileReader("config.properties")) {
12
    prop2.load(fr);
13
    System.out.println(prop2.getProperty("username"));
14
}

1
// 示例：获取并设置线程名
2
MyThread t1 = new MyThread("线程A");
3
MyThread t2 = new MyThread("线程B");
4

5
t1.setName("高铁");
6
t2.setName("飞机");
7

8
System.out.println(t1.getName());  // 高铁
9
System.out.println(t2.getName());  // 飞机
10

11
// Thread.currentThread()：获取当前线程对象
12
System.out.println(Thread.currentThread().getName());  // main

1
MyThread t1 = new MyThread("高铁");
2
MyThread t2 = new MyThread("飞机");
3

4
t1.setPriority(Thread.MIN_PRIORITY);  // 1
5
t2.setPriority(Thread.MAX_PRIORITY);  // 10
6

7
System.out.println(t1.getPriority());  // 1
8
System.out.println(t2.getPriority());  // 10

1
class Tickets implements Runnable {
2
    int tickets = 1;
3

4
    @Override
5
    public void run() {
6
        while (true) {
7
            if (tickets <= 0) {
8
                break;
9
            }
10
            System.out.println(Thread.currentThread().getName() + "卖出了第" + tickets + "号票");
11
            tickets--;
12
        }
13
    }
14
}