分配给原始int的Null Integer对象抛出NullPointerException [英] Null Integer object assigned to primitive int throws NullPointerException

问题描述

Integer i = null;
int j = i;
System.out.println(j);

为什么抛出 NullPointerException 并且没有' t打印 j 的值为 0 ？

Why does it throw NullPointerException and doesn't print the value of j as 0?

推荐答案

整数是一个对象。因此可以为空。

Integer is an object. Therefore it is nullable.

Integer i = null;

是正确的。

<$ c另一方面，$ c> int 是原始值，因此不可为空。

int, on the other hand, is a primitive value, therefore not nullable.

int j = i;

相当于

int j = null;

这是不正确的，并抛出 NullPointerException 。

which is incorrect, and throws a NullPointerException.

感谢JNYRanger：

Expanding thanks to JNYRanger:

从原始值对象包装器到其原始等价物的隐式转换是称为拆箱并在对象成立后立即运行非空值。

This implicit conversion from a primitive value object wrapper to its primitive equivalent is called "unboxing" and works as soon as the object holds a not null value.

Integer i = 12;
int j = i;
System.out.println(j);

按预期输出12。

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