What does java.lang.NullPointerException mean?
"java.lang.NullPointerException" mainly occurs at runtime. This exception belongs to a subclass of RuntimeException, so it is an exempt exception. When the application tries to use null where an object is required, This exception will be thrown.
Several reasons for "java.lang.NullPointerException"
1. The string variable is not initialized
2. Interface type objects are not initialized with a specific class, such as:
Map map // 会报错 Map map = new Map(); //则不会报错了
3. When the value of an object is empty, you do not judge it to be empty.
4. Comparison between strings and text. Text can be a string or an element of Enum. An exception will occur as follows
String str = null; if(str.equals(“Test”)){ //这里的代码将不会被触发,因为会抛出java.lang.NullPointerException异常。 }
5. Prioritize using the String.valueOf() method instead of toString( )
When program code requires a string representation of an object, avoid using the object's toString method. If the reference of your object is equal to null, NullPointerException will be thrown. Use the static String.valueOf method. This method will not throw any exception and print "null"
6. The class is declared type, default class = null; In this way, the system can only give you a null pointer exception when calling the method in the class. Just instantiate it: class = new Class();
7. Return null, the method Do not define the return value as a general type, but use an array. In this way, if you want to return null, you can avoid many unnecessary NullPointerException
Recommended tutorial: "Java Tutorial"
The above is the detailed content of What does java.lang.NullPointerException mean?. For more information, please follow other related articles on the PHP Chinese website!

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于结构化数据处理开源库SPL的相关问题,下面就一起来看一下java下理想的结构化数据处理类库,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于PriorityQueue优先级队列的相关知识,Java集合框架中提供了PriorityQueue和PriorityBlockingQueue两种类型的优先级队列,PriorityQueue是线程不安全的,PriorityBlockingQueue是线程安全的,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于java锁的相关问题,包括了独占锁、悲观锁、乐观锁、共享锁等等内容,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于多线程的相关问题,包括了线程安装、线程加锁与线程不安全的原因、线程安全的标准类等等内容,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于枚举的相关问题,包括了枚举的基本操作、集合类对枚举的支持等等内容,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于Java的相关知识,其中主要介绍了关于关键字中this和super的相关问题,以及他们的一些区别,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于java的相关知识,其中主要介绍了关于平衡二叉树(AVL树)的相关知识,AVL树本质上是带了平衡功能的二叉查找树,下面一起来看一下,希望对大家有帮助。

封装是一种信息隐藏技术,是指一种将抽象性函式接口的实现细节部分包装、隐藏起来的方法;封装可以被认为是一个保护屏障,防止指定类的代码和数据被外部类定义的代码随机访问。封装可以通过关键字private,protected和public实现。


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

Dreamweaver CS6
Visual web development tools

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

mPDF
mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),

PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool

Dreamweaver Mac version
Visual web development tools