Android Studio:"xxx"的拆箱可能会产生"java.lang.NullPointerException" [英] Android Studio: Unboxing of 'xxx' may produce 'java.lang.NullPointerException'

问题描述

我正在关注Android图书示例:

I'm following the Android book example:

//Get the drink from the intent
int drinkIdd = (Integer)getIntent().getExtras().get(EXTRA_DRINKID);
Drink drink = Drink.drinks[drinkIdd];

该项目可以在Android Studio中运行，但在线上带有黄色警告:

And this project could be ran in Android Studio but with a yellow warning on line:

int drinkIdd = (Integer)getIntent().getExtras().get(EXTRA_DRINKID);

具有:

info: Unboxing of '(Integer)getIntent().getExtras().get(EXTRA_DRINKID)' may produce 'java.lang.NullPointerException'

据我了解，get(EXTRA_DRINKID)返回一个对象，然后(整数)将其转换为int类型以符合int drinkIdd.

From my understanding, get(EXTRA_DRINKID) return an Object, and (Integer) converts it to int type to meet with int drinkIdd.

• 您能否告诉我此信息的确切含义，初学者会感谢您的回答.

• 我可以这样写上一行吗?使用()将getIntent().getExtras().get()打包为一个整体，因为它最终返回了一个对象，然后将其转换为int.

• Could you tell me what this info means exactly, your answer will be appreciated for a beginner.

• And could I write upper line like this? Using ( ) to wrap getIntent().getExtras().get() as a whole one since it finally return an object, and then convert it to int.

int drinkIdd = (Integer)(getIntent().getExtras().get(EXTRA_DRINKID));

推荐答案

这是因为在调用以下代码时:

This is because when calling the following code:

getIntent().getExtras().get(EXTRA_DRINKID);

返回的对象可以为null.

The returning object can be null.

使用Integer转换值时，它不会抱怨，因为您可以将null转换为Integer.

When you casting the value with Integer, it will not complaining because you can cast null to Integer.

但是当您拨打以下电话时:

But when you call the following:

int drinkIdd = (Integer)getIntent().getExtras().get(EXTRA_DRINKID);

因为取消装箱，它会抱怨，因为您无法取消装箱 null 值.

because when you unboxing, it will complaining because you can't unboxing a null value.

您最好这样使用getInt()而不是get():

You better using getInt() instead of get() like this:

int drinkIdd = getIntent().getExtras().getInt(EXTRA_DRINKID);

因此您不会再收到警告，并使您的代码更健壮.

So you don't get the warning anymore and make your code more robust.

我不知道您正在读什么书，但我认为您需要更改您的书参考.看来这本书的作者还没有掌握Java和Android API的基本概念.因此，在作者完成工作之前，您需要使用另一本Android书籍进行学习；)

I don't know what book you're reading but I think you need to change your book reference. It seems the book Author haven't grasp the basic concept of Java and Android API. So, you need to use another Android book for your learning process until the Author has finished his/her job ;)

注意:
将包装器类型(Integer)的对象转换为其对应的原始(int)值称为拆箱.在自动装箱和取消装箱中了解更多信息.

Note:
Converting an object of a wrapper type (Integer) to its corresponding primitive (int) value is called unboxing. Read more at Autoboxing and Unboxing.

这篇关于Android Studio:"xxx"的拆箱可能会产生"java.lang.NullPointerException"的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！