Null Pointer Exception at Array Class

Question

Every time when I make a class and other class that will be an array where i will store the first class objects and i call a method ,i receive a nullPointerEception..

ex:

public class person {

    private String name,int age;
    public Class()//constructor

    public void print() {

        System.out.println(name);
        System.out.println(name);

        //.........others methods
    }
}

public class personList {

    person[] pl = new person[10];
    int i=0;

    public void add(person p) {

        pl[i]=p;
        i++
    }

    public void print() {

        for(int j=0;j<pl.length;j++)
        {
            pl[j].print();
        }
    }
}

The error is here: pl[j].print();

The object p[j] isn't null ,because I initiate in Main file(p=new person("Maxim",17),pl.add(p)).

Even if I intializate from Main like this p[0]=.....,p[1]=... I receive the same error.

What have I done wrong?

Not sure why this is getting so many downvotes. This is the same type of question that many new Java programmers have. — mttdbrd
– mttdbrd, Commented Mar 13, 2014 at 14:36
@mttdbrd it's maybe the lack of effort that went into the question. — Thomas
– Thomas, Commented Mar 13, 2014 at 14:45
Perhaps it's better to tell that to the OP than just hit downvote? — Chris Ballard
– Chris Ballard, Commented Mar 13, 2014 at 14:46
@Thomas, I guess it could be that. But this is a helping website. Downvotes don't help. Reformatting, asking questions, giving answers: that helps new users. — mttdbrd
– mttdbrd, Commented Mar 13, 2014 at 14:47
@mttdbrd that's true, at least one should explain downvotes if it hasn't been stated already. That said I also can only guess the reason behind those downvotes. — Thomas
– Thomas, Commented Mar 13, 2014 at 15:11

mttdbrd · Accepted Answer · 2014-03-13 14:33:58Z

2

The error is in this line:

person[] pl = new person[10];

You need to initialize each Person object in the array.

for(int i = 0; i < pl.length; i++)
{
    pl[i] = new Person();

}

Your constructor for Person is also wrong. It should be

public Person(){...}

not

public Class(){...}

answered Mar 13, 2014 at 14:33

mttdbrd

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Thomas · Accepted Answer · 2014-03-13 14:40:49Z

2

person[] pl = new person[10]; will create an array of length 10 with all elements being null.

Thus, if you don't initialize all 10 elements your print() method will eventually throw a NullPointerException.

You seem to only call add(p) once, i.e. you only intialize element at index 0 to a non-null value and thus when j reaches value/index 1, person[j] will be null.

To fix this you should change your loop to for(int j=0;j<i;j++), assuming that print() is a method of class personList (your posts formatting makes it hard to determine).

A few side notes:

Please check the Java coding conventions and stick to them, since that will make it easier for others to read your code (and it will probably even help you). At least stick to the convention on class names which should be camel case and start with an upper case letter, e.g. PersonList not personList.
Instead of using an array and a counter, e.g. Person[] and i, it would be better to use a list or some other collection, e.g. List<Person>. That way the loop could become for( Person p : pl) { p.print() } and you'd also not be restricted to 10 elements.

edited Mar 13, 2014 at 14:40

answered Mar 13, 2014 at 14:33

Thomas

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2 Comments

user3401592 Over a year ago

How can I create array with unknown numer of elements ?In C++ is with dynamic allocation.In java?

Thomas Over a year ago

@user3401592 in Java you'd use a collection, e.g. a List with one implementation being ArrayList, if you don't know the number of elements when creating the collection. If you do, you can just pass that number to the array constructor, e.g. int numElements = 3; int[] a = new int[numElements];.

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