当返回的对象之一为空时，Java If 语句的简短形式返回 NullPointerException [英] Short form for Java If statement returns NullPointerException when one of the returned objects is null

问题描述

为什么这段代码返回错误:java.lang.NullPointerException

Why this code returns the error: java.lang.NullPointerException

Object obj = null;
Long lNull = null;
Long res = obj == null ? lNull : 10L;

但是下面的方法没有任何错误:

But the following way works without any errors:

Object obj = null;
Long res = obj == null ? null : 10L;

推荐答案

在第二种情况下，编译器可以推断 null 必须是 Long 类型.在第一种情况下，它没有 - 并假设表达式返回 long.您可以看到情况就是这样(即修复它)，例如

In the second case, the compiler can infer that the null must be a Long type. In the first case, it doesn't - and assumes that the expression returns a long. You can see that this is the case (i.e. fix it) like,

Long res = obj == null ? lNull : (Long) 10L;

不会产生 NullPointerException.

which does not yield a NullPointerException.

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