为什么我的代码会生成NullPointerException？ [英] Why my code generate NullPointerException?

问题描述

我是java编程的初学者，所以我不讨论空指针异常请任何正文帮助我如何解决这类错误

-i有两个名为Student.java的第一类和另一个用SrDemo.java

-i在Student类中写gatter satter并使用这个是SrDemo类但是当我使用数组调用方法时它生成空指针异常我怎么能解决这个问题

，我用我的评论文本解决了这个问题，但是我希望这个解决方案使用数组，所以如果有人知道请提前告诉我谢谢..

这下面是我的两个班级的代码

i am beginner in java programing so i dont khow about the null pointer exception please any body help me how can i solve this type of error
-i have two class one with name Student.java and another with SrDemo.java
-i write gatter satter in Student class and use this is SrDemo class But when i call the method using array its generate null pointer exception how can i solve this
and i have solution this with my commented text but i want this solution with using array so please if anybody know tell me thanks in advance..
this below is my my code for both classes

SrDemo.java 
...........
public class SrDemo {
    public static void main(String args[]){
        Student sd[]=new Student[2];
        sd[0].setName("name");
        System.out.println(sd[0].getName());

         //this code solve the problem but i need solution with array how can i do that
        /*Student sd;
        for(int i=0;i<5;i++) {
            sd = new Student();
            sd.setName("jagdish " + i);
            sd.setCity("Baroda " + i);
            System.out.println(sd.getName());
            System.out.println(sd.getCity());
        }*/
    }
}

这是我的学生班，有这个gatter setter方法

and this is my Student class which have this gatter setter method

Student.java
...........

public class Student {
    private String name;
    private String city;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getCity() {
        return city;
    }
    public void setCity(String city) {
        this.city = city;
    }
}

error
.......
Exception in thread "main" java.lang.NullPointerException
	at Serialize.SrDemo.main(SrDemo.java:8)

推荐答案

这是我们遇到的最常见问题之一，也是我们最不能回答的问题，但你最有能力回答自己。



让我解释一下错误的含义：您尝试使用变量，属性或方法返回值，但它包含null - 这意味着变量中没有类的实例。

它有点像口袋：你的衬衫里有一个口袋，用来握笔。如果你进入口袋并发现那里没有笔，你就不能在一张纸上签名 - 如果你尝试的话，你会得到非常有趣的外观！空口袋给你一个空值（这里没有笔！）所以你不能做任何你检索笔通常做的事情。它为什么空？这就是问题 - 可能是你今天早上离开家时忘了拿起你的笔，或者你昨晚把它拿到昨天的衬衫口袋里时可能会把笔留下来。



我们无法分辨，因为我们不在那里，更重要的是，我们甚至看不到你的衬衫，更不用说口袋里的东西了！



回到计算机，你做了同样的事情，不知何故 - 我们看不到你的代码，更不用说运行它了，找不到包含null的东西。

但是你可以 - 而Visual Studio将在这里帮助你。在调试器中运行您的程序，当它失败时，VS将向您显示它发现问题的行。然后，您可以开始查看它的各个部分，以查看哪个值为null，并开始回顾代码以找出原因。因此，在包含错误行的方法的开头放置一个断点，然后从头再次运行程序。这一次，VS会在错误发生前停止，让你通过查看代码来查看你的价值来检查发生了什么。



但我们做不到那 - 我们没有您的代码，如果我们拥有它，我们不知道如何使用它，我们没有您的数据。所以尝试一下 - 看看你能找到多少信息！

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

很可能因为你声明了一个数组但从未初始化它。

这里：

Most probably because you are declaring an array but never initialize it.
Here:

Student sd[]=new Student[2];
sd[0].setName("name");

应为：

should read:

Student sd[]=new Student[2];
sd[0] = new Student();
sd[0].setName("name");

调试绝对是很快发现这类问题的方法。

Debugging is definitely the way to spot this kind of problems very quickly.

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