“缺少返回声明”；切换后（枚举）-为什么？ [英] "Missing return statement" after switch(enum) - Why?

问题描述

编译器声称 MyClass.parse（）的结尾缺少返回语句。这是代码：

The compiler claims that a return statement is missing at the end of MyClass.parse(). Here is the code:

package de.hs_rm.safelyovertaken.ble;

import android.support.annotation.NonNull;

import java.util.Arrays;

class MyClass {

    @NonNull
    static MyClass parse(byte[] encoded) throws MyParseException {

        MyEnum myEnum = MyEnum.parse(Arrays.copyOfRange(encoded, 0, 2));

        switch (myEnum) {
            case A:
                return new MyClassA();

            case B:
                return new MyClassB();

            case C:
                return new MyClassC();
        }

        // compile error: "Missing return statement"

//        return null; // should never be reached
//        throw new AssertionError("Should never be reached");
    }
}

enum MyEnum {
    A, B, C;

    @NonNull
    static MyEnum parse(byte[] encoded) throws MyParseException {

        MyEnum result = null;

        // parse logic here

        if (result == null) {
            throw new MyParseException();
        }

        return result;
    }
}

class MyParseException extends Exception {
}

class MyClassA extends MyClass {
}

class MyClassB extends MyClass {
}

class MyClassC extends MyClass {
}

编译器正确吗？ （Android Studio）

Is the compiler right? (Android Studio)

如果是这样，在什么情况下可以达到该方法的目的？我认为 myEnum 不能为 null 并且所有枚举都包含在switch语句中，无论如何，返回语句都会离开方法。 myEnum 不能为 null ，因为 @NonNull 方法<$如果结果为 null ，则c $ c> MyEnum.parse（）会引发异常。

If so, under what circumstances could the end of the method be reached? I think myEnum cannot be null and all enum are covered in the switch statement where in any case a return statement will leave the method. myEnum cannot be null because the @NonNull method MyEnum.parse() throws an exception if the result is null.

如果没有，您是否会用 return null //标记方法的（希望）不可达结尾//永远不会到达或抛出 AssertionError ？

If not, would you mark the (hopefully) unreachable end of the method with return null // should never be reached or throw an AssertionError?

推荐答案

编译器正确吗？

Is the compiler right?

是的，因为它在编译时不验证枚举的覆盖范围。假设枚举位于另一个二进制文件中，并使用新的常量对其进行了更新。该方法将返回什么？

Yes, because it doesn't verify enum coverage at compile time. Say the enum lived in another binary and it was updated with a new constant. What would the method return?

myEnum 不能为 null ，因为 @NonNull 方法 MyEnum.parse（）如果结果返回异常是 null 。

myEnum cannot be null because the @NonNull method MyEnum.parse() throws an exception if the result is null.

编译器不够聪明，无法弄清楚（尽管您的IDE可能是）。但这是有争议的，因为打开 null 会导致NPE。

The compiler's not smart enough to figure that out (though your IDE might be). But it's a moot point because switching on null would result in a NPE.

如果没有，您是否将方法的（希望）结尾标记为 return null //永远不要到达或抛出 AssertionError ？

抛出 AssertionError 很常规。或者，考虑将在枚举常量中嵌入条件逻辑，而不要使用开关。

Throwing an AssertionError is pretty conventional. Alternatively, consider embedding conditional logic in the enum constants instead of using a switch.

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