空指针异常理解

只要明白了，什么是空指针异常，就知道怎么解决了？

在java中，更确切的说，应该是空引用异常，也就是一个对象，你没有给他分配内存，就开始使用这个对象，比如调用这个对象的方法，这时候，就会报空指针异常了。

怎么解决呢？很简单，不是没有分配内存吗，给他分配内存就行了。
怎么分配内存呢？也简单，初始化这个对象，这个对象分配了内存了。
如何初始化？你懂的...

知其所以然：NullPointerException 官方说明文档

/**

• Thrown when an application attempts to use null in a
• case where an object is required. These include:
• Calling the instance method of a null object.
• Accessing or modifying the field of a null object.
• Taking the length of null as if it were an array.
• Accessing or modifying the slots of null as if it
• were an array.
• Throwing null as if it were a Throwable
• value.

Applications should throw instances of this class to indicate other illegal uses of thenull object. * @author unascribed @version 1.20, 11/17/05 @since JDK1.0 */