控制流语句

提示

• Control Flow Statements
• Branching with Switch Statements
• Branching with Switch Expressions

if

public class IfStatement {
    public static void main(String[] args) {
        int key = 1;
        if (key == 1) {
            System.out.println("Hello World! 1");
        } else if (key == 2) {
            System.out.println("Hello World! 2");
        } else {
            System.out.println("Hello World! 3");
        }
    }
}

switch

语句

switch 语句接收一个 selector 变量作为参数，这个 selector 变量的类型必须是以下类型：

• byte, short, char, int primitive data types
• Character , Byte, Short, Integer wrapper types
• enumerated types
• String

注意不能用于 selector 变量的类型：boolean, long, float, double

int quarter = ...; // any value

String quarterLabel = null;
switch (quarter) {
    case 0: quarterLabel = "Q1 - Winter";
            break;
    case 1: quarterLabel = "Q2 - Spring";
            break;
    case 2: quarterLabel = "Q3 - Summer";
            break;
    case 3: quarterLabel = "Q3 - Summer";
            break;
    default: quarterLabel = "Unknown quarter";
};

switch 语句同样遵循遇到 break 语句时才会终止。

表达式

在 Java SE 14 ，支持 switch 表达式。

Day day = ...; // any day
int len =
    switch (day) {
        case MONDAY, FRIDAY, SUNDAY -> 6;
        case TUESDAY                -> 7;
        case THURSDAY, SATURDAY     -> 8;
        case WEDNESDAY              -> 9;
    };
System.out.println("len = " + len);

相比于传统的 switch 语句，switch 表达式需要穷尽枚举类型的所有可能值，否则会抛出 IncompatibleClassChangeError。

直接阅读：Branching with Switch Expressions

while

public class WhileStatement {
    public static void main(String[] args) {
        int i = 0;
        while (i < 3) {
            System.out.println("Hello World! " + i);
            i++;
        }
    }
}

do-while

public class DoWhileStatement {
    public static void main(String[] args) {
        int i = 0;
        do {
            System.out.println("Hello World! " + i);
            i++;
        } while (i < 3);
    }
}

for

public class ForStatement {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};

        // 第一种
        for (int i = 0; i < 3; i++) {
            System.out.println("Hello World! " + arr[i]);
        }

        // 第二种
        for (int i : arr) {
            System.out.println("Hello World! " + i);
        }
    }
}

其他

break

无标签

public class BreakStatement {
    public static void main(String[] args) {
        int i = 0;
        while (i < 3) {
            System.out.println("Hello World! " + i);
            if (i == 1) {
                break;
            }
            i++;
        }
    }
}

有标签

public class BreakStatement {
    public static void main(String[] args) {
        int i = 0;

        search:
            while (i < 3) {
                System.out.println("Hello World! " + i);
                if (i == 1) {
                    break search;
                }
                i++;
            }

            System.out.println("done " + i);
    }
}

// 结果：
// Hello World! 0
// Hello World! 1
// done 1

continue

无标签

public class ContinueStatement {
    public static void main(String[] args) {
        int i = 0;
        while (i < 3) {
            i++;
            if (i == 1) {
                continue;
            }
            System.out.println("Hello World! " + i);
        }
    }
}

有标签

public class ContinueStatement {
    public static void main(String[] args) {
        int i = 0;

        search:

            while (i < 3) {
                i++;
                if (i == 1) {
                    continue search;
                }
                System.out.println("Hello World! " + i);
            }

            System.out.println("done " + i);
    }
}

// 结果：
// Hello World! 2
// Hello World! 3
// done 3

---

return

从当前方法退出，控制流返回到调用方法的地方。

public class ReturnStatement {
    public static void main(String[] args) {
        int i = test(0);
    }

    public int test(int n) {
        if (n == 0) {
            return 0;
        } else {
            return n;
        }
    }
}

yield

• 从当前的 switch 语句中退出
• yield 后紧跟返回值的表达式，表达式不能为 void

public class YieldStatement {
    enum Day {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }

    public int calculate(Day day) {
        return switch (day) {
            case SATURDAY, SUNDAY -> 0;
            default -> {
                int remainingDays = 5 - day.ordinal();
                yield remainingDays;
            }
        }
    }
}

在可读性上，如果 yield 换成 return 可能是困惑的（不知道是退出方法还是该语句），所以有了 yield 语句。