使用 Java 三元运算符时的奇怪行为 [英] Weird behaviour when using Java ternary operator

问题描述

当我像这样编写 Java 代码时:

When I write my java code like this:

Map<String, Long> map = new HashMap<>()
Long number =null;
if(map == null)
    number = (long) 0;
else
    number = map.get("non-existent key");

应用程序按预期运行，但当我这样做时:

the app runs as expected but when I do this:

Map<String, Long> map = new HashMap<>();
Long number= (map == null) ? (long)0 : map.get("non-existent key");

我在第二行得到一个 NullPointerException.调试指针从第二行跳转到java.lang.Thread类中的这个方法:

I get a NullPointerException on the second line. The debug pointer jumps from the second line to this method in the java.lang.Thread class:

 /**
     * Dispatch an uncaught exception to the handler. This method is
     * intended to be called only by the JVM.
     */
     private void dispatchUncaughtException(Throwable e) {
         getUncaughtExceptionHandler().uncaughtException(this, e);
     }

这里发生了什么?这两个代码路径完全相同，不是吗?

What is happening here? Both these code paths are exactly equivalent isn't it?

编辑

我使用的是 Java 1.7 U25

I am using Java 1.7 U25

推荐答案

它们并不等效.

这个表达式的类型

(map == null) ? (long)0 : map.get("non-existent key");

是 long 因为真正的结果是 long 类型.

is long because the true result has type long.

这个表达式属于 long 类型的原因来自 JLS 第 15.25 节:

The reason this expression is of type long is from section §15.25 of the JLS:

如果第二个和第三个操作数之一是原始类型T，而另一个的类型是对T应用装箱转换(第5.1.7节)的结果code>，则条件表达式的类型为T.

If one of the second and third operands is of primitive type T, and the type of the other is the result of applying boxing conversion (§5.1.7) to T, then the type of the conditional expression is T.

当您查找不存在的键时，map 返回 null.因此，Java 试图将其拆箱为 long.但它是null.所以它不能，你会得到一个 NullPointerException.您可以通过以下方式解决此问题:

When you lookup a non-existant key the map returns null. So, Java is attempting to unbox it to a long. But it's null. So it can't and you get a NullPointerException. You can fix this by saying:

Long number = (map == null) ? (Long)0L : map.get("non-existent key");

然后你会没事的.

然而，在这里，

if(map == null)
    number = (long) 0;
else
    number = map.get("non-existent key");

因为 number 被声明为 Long，所以永远不会发生对 long 的拆箱.

since number is declared as Long, that unboxing to a long never occurs.

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