【java17】java17新特性之switch表达式

Java17版本中switch表达式将允许switch有返回值，并且可以直接作为结果赋值给一个变量，等等一系列的变化。

下面有一个switch例子，依赖于给定的枚举值，执行case操作，故意省略break。

package com.morris.java17;

/**
 * switch省略break
 */
public class SwitchNotBreakDemo {
    public static void main(String[] args) {
        Month month = Month.APRIL;
        String season;
        switch (month) {
            case DECEMBER:
            case JANUARY:
            case FEBRUARY:
                season = "winter";
            case MARCH:
            case APRIL:
            case MAY:
                season = "spring";
            case JUNE:
            case JULY:
            case AUGUST:
                season = "summer";
            case SEPTEMBER:
            case OCTOBER:
            case NOVEMBER:
                season = "autumn";
            default:
                season = "unknown";
        }
        System.out.println(season); // unknown
    }
}
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运行结果如下：

unknown
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显然这不是期望的结果，因为我们需要在每个case里添加break防止所有的case都被执行。

package com.morris.java17;

/**
 * switch增加break
 */
public class SwitchBreakDemo {
    public static void main(String[] args) {
        Month month = Month.APRIL;
        String season;
        switch (month) {
            case DECEMBER:
            case JANUARY:
            case FEBRUARY:
                season = "winter";
                break;
            case MARCH:
            case APRIL:
            case MAY:
                season = "spring";
                break;
            case JUNE:
            case JULY:
            case AUGUST:
                season = "summer";
                break;
            case SEPTEMBER:
            case OCTOBER:
            case NOVEMBER:
                season = "autumn";
                break;
            default:
                season = "unknown";
        }
        System.out.println(season); // unknown
    }
}
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运行结果如下：

spring
1

可以通过switch表达式来进行简化。将冒号:替换为箭头->，并且switch表达式不需要break。

package com.morris.java17;

/**
 * switch表达式
 */
public class SwitchExpressDemo {
    public static void main(String[] args) {
        Month month = Month.APRIL;
        String season;
        switch (month) {
            case DECEMBER, JANUARY, FEBRUARY -> season = "winter";
            case MARCH, APRIL, MAY -> season = "spring";
            case JUNE, JULY, AUGUST -> season = "summer";
            case SEPTEMBER, OCTOBER, NOVEMBER -> season = "autumn";
            default -> season = "unknown";
        }
        System.out.println(season); // spring
    }
}
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switch表达式也可以返回一个值，比如上面的例子我们可以让switch返回一个字符串来表示我们要打印的文本。需要注意在switch语句的最后要加一个分号。

package com.morris.java17;

/**
 * switch表达式带返回值的使用
 */
public class SwitchReturnDemo {
    public static void main(String[] args) {
        Month month = Month.APRIL;
        String season = switch (month) {
            case DECEMBER, JANUARY, FEBRUARY -> "winter";
            case MARCH, APRIL, MAY -> "spring";
            case JUNE, JULY, AUGUST -> "summer";
            case SEPTEMBER, OCTOBER, NOVEMBER -> "autumn";
            default -> "unknown";
        };
        System.out.println(season); // spring
    }
}
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如果你想在case里想做不止一件事，比如在返回之前先进行一些计算或者打印操作，可以通过大括号来作为case块，最后的返回值使用关键字yield进行返回。

package com.morris.java17;

/**
 * switch表达式yield的使用
 */
public class SwitchYieldDemo {
    public static void main(String[] args) {
        Month month = Month.APRIL;
        String season = switch (month) {
            case DECEMBER, JANUARY, FEBRUARY -> "winter";
            case MARCH, APRIL, MAY -> {
                System.out.println("current season is spring");
                yield "spring";}
            case JUNE, JULY, AUGUST -> "summer";
            case SEPTEMBER, OCTOBER, NOVEMBER -> "autumn";
            default -> "unknown";
        };
        System.out.println(season); // spring
    }
}
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