返回null作为三元运算符允许的int值，但if语句则不允许 [英] Returning null as an int permitted with ternary operator but not if statement

问题描述

下面的代码段让我们看一下简单的Java代码:

Let's look at the simple Java code in the following snippet:

public class Main {

    private int temp() {
        return true ? null : 0;
        // No compiler error - the compiler allows a return value of null
        // in a method signature that returns an int.
    }

    private int same() {
        if (true) {
            return null;
            // The same is not possible with if,
            // and causes a compile-time error - incompatible types.
        } else {
            return 0;
        }
    }

    public static void main(String[] args) {
        Main m = new Main();
        System.out.println(m.temp());
        System.out.println(m.same());
    }
}

在这种最简单的Java代码中，即使函数的返回类型为int，temp()方法也不会发出编译器错误，并且我们正在尝试返回值null(通过语句return true ? null : 0; ).编译后，这显然会导致运行时异常NullPointerException.

In this simplest of Java code, the temp() method issues no compiler error even though the return type of the function is int, and we are trying to return the value null (through the statement return true ? null : 0;). When compiled, this obviously causes the run time exception NullPointerException.

但是，如果我们用if语句(如same()方法中)表示三元运算符，这似乎是错误的，而确实会发出编译时错误！为什么?

However, it appears that the same thing is wrong if we represent the ternary operator with an if statement (as in the same() method), which does issue a compile-time error! Why?

推荐答案

编译器将null解释为对Integer的空引用，将自动装箱/拆箱规则应用于条件运算符(如 Java语言规范，15.25 )，并快乐地前进.这将在运行时生成NullPointerException，您可以通过尝试进行确认.

The compiler interprets null as a null reference to an Integer, applies the autoboxing/unboxing rules for the conditional operator (as described in the Java Language Specification, 15.25), and moves happily on. This will generate a NullPointerException at run time, which you can confirm by trying it.

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