Why my code generate NullPointerException?

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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

Java

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());
        }*/
    }
}

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

Java

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)

Posted 4-Dec-15 7:21am

jagdish vaghela

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Foothill 4-Dec-15 13:49pm

You initialized the array but you never created objects for it.

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OriginalGriff · Accepted Answer · 2015-12-04T07:59:00

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!

phil.o · Accepted Answer · 2015-12-04T08:02:00

Solution 2

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

Java

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

should read:

Java

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

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

Posted 4-Dec-15 8:02am

phil.o

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jagdish vaghela 4-Dec-15 14:43pm

thanks sir it's work

Why my code generate NullPointerException?

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Solution 2

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