why i have this Exception in java : Exception in thread "main" java.lang.NullPointerException

Question

This is my main codes:

Scanner input= new Scanner(System.in);

Student[] starray=new Student[5];

for (int i=0; i<3; i++)
{
    System.out.println("enter:");
    starray[i].name=input.next();   
    System.out.println("enter:");
    starray[i].family=input.next(); 
    System.out.println("enter:");
    starray[i].sid=input.nextInt();
}      
for(int i=0; i<3; i++)
        System.out.println(starray[i].name);

and i have one class :

String name,family;
Integer sid;

Student(){
       name="kh";
       family="kh";
       sid=0;}

when i run it have Exception below: Exception in thread "main" java.lang.NullPointerException at testcodes.TestCodes.main(TestCodes.java:19) Java Result: 1

You are not creating any students in the array.... All i see is you are making the array but not creating a new Student() @Chrismas007 I think his question is in the title.... — 3kings
– 3kings, Commented Dec 5, 2014 at 17:00

Bathsheba · Accepted Answer · 2014-12-05 17:01:54Z

5

Student[] starray = new Student[5]; creates just the container. Every element within that container will be null.

You need to create each one in turn. Within your loop, consider

starray[i] = new Student();

Better still, build a strongly typed constructor to a Student, taking the name etc. as parameters. This will help to increase program stability.

answered Dec 5, 2014 at 17:01

Bathsheba

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Comments

frogatto · Accepted Answer · 2014-12-05 17:26:17Z

Java in this case is very similar to C++. In C++ when you declare an array of objects, all them still have not been initialized (these is no real objects within it), in other words the array is only a place holder for objects.

So your statement

Student[] starray = new Student[5];

in the form of visually could be

starray --> +------+------+------+------+------+
            | null | null | null | null | null |
            +------+------+------+------+------+

And after this statement

starray[0] = new Student();

would be

starray --> +------+------+------+------+------+
            |      | null | null | null | null |
            +---|--+------+------+------+------+
                |
                v
            +------------------+
            | Student Instance |
            +------------------+

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