@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:
* <ul>
* <li>Calling the instance method of a {@code null} object.
* <li>Accessing or modifying the field of a {@code null} object.
* <li>Taking the length of {@code null} as if it were an array.
* <li>Accessing or modifying the slots of {@code null} as if it
* were an array.
* <li>Throwing {@code null} as if it were a {@code Throwable}
* value.
* </ul>
* <p>
* 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. <ul>
2. <li>Calling the instance method of a {@code null} object.
3. <li>Accessing or modifying the field of a {@code null} object.
4. <li>Taking the length of {@code null} as if it were an array.
5. <li>Accessing or modifying the slots of {@code null} as if it
6. were an array.
7. <li>Throwing {@code null} as if it were a {@code Throwable}
8. value.
9. </ul>
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