Java中的switch-case语句的语法、特征详解

switch (expression) {
    case value1:
        // 代码块1
        break;
    case value2:
        // 代码块2
        break;
    // 可以有任意数量的case语句
    default:
        // 默认代码块
}

int day = 3;
String dayName;
switch (day) {
    case 1:
        dayName = "星期一";
        break;
    case 2:
        dayName = "星期二";
        break;
    case 3:
        dayName = "星期三";
        break;
    case 4:
        dayName = "星期四";
        break;
    case 5:
        dayName = "星期五";
        break;
    default:
        dayName = "周末";
}
System.out.println(dayName); // 输出：星期三

int num = 2;
switch (num) {
    case 1:
        System.out.println("一");
    case 2:
        System.out.println("二"); // 会执行
    case 3:
        System.out.println("三"); // 也会执行
    default:
        System.out.println("其他"); // 也会执行
}
// 输出：
// 二
// 三
// 其他

String fruit = "apple";
switch (fruit) {
    case "banana":
        System.out.println("香蕉");
        break;
    case "apple":
        System.out.println("苹果");
        break;
    case "orange":
        System.out.println("橙子");
        break;
    default:
        System.out.println("未知水果");
}
// 输出：苹果

int day = 3;
String dayName = switch (day) {
    case 1 -> "星期一";
    case 2 -> "星期二";
    case 3 -> "星期三";
    case 4 -> "星期四";
    case 5 -> "星期五";
    default -> "周末";
};
System.out.println(dayName); // 输出：星期三

String fruit = "apple";
String description = switch (fruit) {
    case "banana" -> "黄色的弯曲水果";
    case "apple" -> {
        String color = "红色";
        yield color + "的圆形水果";
    }
    case "orange" -> "橙色的柑橘类水果";
    default -> "未知水果";
};
System.out.println(description); // 输出：红色的圆形水果

int month = 2;
int days = switch (month) {
    case 1, 3, 5, 7, 8, 10, 12 -> 31;
    case 4, 6, 9, 11 -> 30;
    case 2 -> {
        // 这里可以添加更复杂的逻辑
        yield 28; // 简化的闰年判断
    }
    default -> throw new IllegalArgumentException("无效月份: " + month);
};
System.out.println(days); // 输出：28

result = switch (expression) {
    case value1 -> expression1;
    case value2 -> {
        // 代码块
        yield result;
    }
    case value3, value4 -> expression3; // 多case
    default -> defaultExpression;
};

String input = null;
if (input == null) {
    System.out.println("输入不能为null");
    return;
}
// 后续switch处理

enum Color { RED, GREEN, BLUE }
Color color = Color.RED;
switch (color) {
    case RED -> System.out.println("红色");
    case GREEN -> System.out.println("绿色");
    case BLUE -> System.out.println("蓝色");
}

// 错误示例
double value = 1.0;
switch (value) { // 编译错误
    // ...
}

enum State { START, PROCESSING, FINISHED }
State current = State.START;
switch (current) {
    case START -> {
        System.out.println("开始处理");
        current = State.PROCESSING;
    }
    case PROCESSING -> {
        System.out.println("处理中...");
        current = State.FINISHED;
    }
    case FINISHED -> System.out.println("已完成");
}

String command = "help";
switch (command.toLowerCase()) {
    case "start" -> startService();
    case "stop" -> stopService();
    case "restart" -> restartService();
    case "help", "?" -> showHelp();
    default -> System.out.println("未知命令: " + command);
}

char input = 'w';
switch (input) {
    case 'w', 'W' -> movePlayer(0, 1);  // 上
    case 'a', 'A' -> movePlayer(-1, 0); // 左
    case 's', 'S' -> movePlayer(0, -1); // 下
    case 'd', 'D' -> movePlayer(1, 0);  // 右
    default -> System.out.println("无效输入");
}