Question

I have with two fields: long nr and a vector of long like below:

public class Pair {

    public long nr;
    public Vector<Long> lines;

    public Pair(long ap, long line){
        this.nr=ap;
        if (line!=0) lines.add(linie);
         else lines=null; 
    }

    public void create (long line){
        nr++;
        lines.add(line);
    }
}

I want to have a function(create) so it modifies the fields of the class. In the main class I have

Pair per1=new Pair(0,0);
Pair per2=new Pair(0,0);

per1.create(3);
per2.create(4);

The constructor works fine, but create doesn’t. What is the explanation, and how should the function look like?Thanks a lot.

Answer 1

You never created instance of Vector by calling new Vector<Long>() in your code. Moreover you are setting your class variable lines to null if line == 0.

Your code should like this:

public class Pair {

public long nr;
public Vector<Long> lines = new Vector<long>();

public  Pair(long ap, long line){
    this.nr=ap;
    if (line!=0) lines.add(linie);
}

public void create (long line){
    nr++;
    if (line!=0) lines.add(linie);
}

}