java空指针异常：问题的根源和解决思路
@Java空指针异常 Exception in thread “main” java.lang.NullPointerException 前一段时间写实验，遇到了一个java空指针异常的问题，经过多方查询资料，中越解决了异常，因此决定记录一下本次异常的解决。
遇到的问题
程序的基本思想是读取文件，逐个将单词分开，将单词封装至Word类，然后判断单词的类型，将类型和单词本身写入输出文件中。但是这一句
if (words[i].word !=null) {
出現了空指针异常，我多次调试，总是找不到问题的根源在哪里。
解决的方法
搜索问题的时候，忽然看到了这篇文章，给我了启发 https://www.sohu.com/a/208264500_611601 文章中 这一句让我忽然意识到，是不是因为我的words[i]是一个null对象，于是我将代码修改成
if (words[i]!=null&&words[i].word !=null) {
也就是先判断words[i]这个对象是否为空，在进行words[i]的属性的判断，运行之后果然成功解决了。
查看源码
本着追本溯源的想法，想翻看对应的源码，看看还有什么原因可能会造成空指针。NullPointerException的源码如下：
/*
* Copyright (c) 1994, 2011, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
package java.lang;
/**
* Thrown when an application attempts to use {@code null} in a
* case where an object is required. These include:
*
- Calling the instance method of a {@code null} object.
*
- Accessing or modifying the field of a {@code null} object.
*
- Taking the length of {@code null} as if it were an array.
*
- Accessing or modifying the slots of {@code null} as if it
* were an array.
*
- Throwing {@code null} as if it were a {@code Throwable}
* value.
*
*
*
* Applications should throw instances of this class to indicate
* other illegal uses of the {@code null} object.
*
* {@code NullPointerException} objects may be constructed by the
* virtual machine as if {@linkplain Throwable#Throwable(String,
* Throwable, boolean, boolean) suppression were disabled and/or the
* stack trace was not writable}.
*
* @author unascribed
* @since JDK1.0
*/
public
class NullPointerException extends RuntimeException {
private static final long serialVersionUID = 5162710183389028792L;
/**
* Constructs a {@code NullPointerException} with no detail message.
*/
public NullPointerException() {
super();
}
/**
* Constructs a {@code NullPointerException} with the specified
* detail message.
*
* @param s the detail message.
*/
public NullPointerException(String s) {
super(s);
}
}
其中
1.
- Calling the instance method of a {@code null} object.
3.
- Accessing or modifying the field of a {@code null} object.
4.
- Taking the length of {@code null} as if it were an array.
5.
- Accessing or modifying the slots of {@code null} as if it
6. were an array.
7.
- Throwing {@code null} as if it were a {@code Throwable}
8. value.
9.
2.
不难看出，当以上五种情况的时候，会报空指针异常。 大致可以翻译为：
1.调用空对象的实例 2.访问或者修改空对象 3.测试一个空对象数组的长度 4.访问或者修改一个空数组的插槽（？水平有限，不太懂slots） 5.把一个空值当做抛出的参数
也就是说，避免出现空指针异常，要避免出现上述的五种情况才行。在很多规范编码的时候可以避免空指针异常。例如alibaba的编码规范中，一般在调用string1.equals(string2)的时候，如果string2是一个具体的字符串，而string1是一个对象的属性，此时应该把euqal反转，写成string2.euqals(string1). 例如，当你在idea中安装了编码规范，而写成了可能导致空指针的形式。idea就会提醒你 此时，应该把改成
"true".equals(resultItems.get("type")
这样就会避免可能会导致空指针的问题。
总结
遇到问题的时候，不要慌，多查查，多思考。大多数能够遇到的问题，应该都有前辈已经遇到过了，所以要多查，发现错误，吸取经验。
多看看源码，很多异常源码里面都有说明。
java源码链接：源码