Question

i have a 2 data classes

first:

public class Question {

    public CharSequence question;
    public Answer answers[];

}

second:

public class Answer {

    public CharSequence text;
    public int number;
}

now i want to save an Answer to Question:

Question qstn.answers = new Answer[2];

[...]

But i got an NullPointerException. Whats wrong? Can’t I change the length of an Array in an other class?

Answer 1

Have you created a new instance of Question first e.g.

Question qstn = new Question();
qstn.answers = new Answer[2];

Also unless you’re optimising judiciously, then you’d probably want to give encapsulation a go and make the instance variables private. Access through a public API e.g. getters or/and setters.

Also you may want to instantiate answers when the Object is created e.g.

public class Question {

    public CharSequence question;
    public Answer answers[] = new Answer[2];

}

Or you could do this in the constructor of Question (you’d need to add a Constructor, as the compiler will be generating you a non-arg Constructor by default with the way things stand).