使用“when”而不是“if else”是否会带来性能成本?

tl;医生:不,你不应该期望 when 放慢速度。

编译器重用if/else、switch和三元运算符构造,以表示 什么时候 声明。举个例子:

fun whenApplication(someObject: String?) = when (someObject) {
    null -> 0
    else -> 2
}

其字节码显示为Java代码:

public static final int whenApplication(@Nullable String someObject) {
   return someObject == null ? 0 : 2;
}

此处显示的测试稍微复杂一些:

fun whenApplication(someObject: Any?) = when (someObject) {
    is Int -> 2
    in 0..2 -> 4
    else -> 5
}

对应的字节码为Java:

public static final int whenApplication(@Nullable Object someObject) {
    int var10000;
    if (someObject instanceof Integer) {
       var10000 = 2;
    } else {
       byte var2 = 0;
       var10000 = CollectionsKt.contains((Iterable)(new IntRange(var2, 2)), someObject) ? 4 : 5;
    }

    return var10000;
}