Question

I want to know why this piece of code gets a compilation error. I’m new to JAVA. I wanted to make a Set of pairs as in C++
What is the meaning of pair in inner class code

import java.io.*;
import java.util.Set;

public class UVa {
    public static boolean flag = false;
    public static Integer p0, p1;
    public static Set<pair<Integer,Integer> > sorter;

    public static class pair<first,second>{
        public first First;
        public second Second;
        private pair(first First,second Second){
            this.First = First;
            this.Second = Second;
        }
    }

    public static void main(String[] args) {
        try {
            while (true) {
                p0 = System.in.read();
                p1 = System.in.read();
                sorter.add(pair<p0, p1>); //<<Syntax error on token ">", Expression expected after this token

            }
        } catch (IOException e) {
            System.out.println(e.toString());
        }
    }
}

Answer 1

At line no 8:

public static Set<pair<Integer,Integer> > sorter;

Set is not initialize yet that’s why is throwing java.lang.NullPointerException , So for that you have ho initialize it first like following.

public static Set<pair<Integer,Integer> > sorter = new HashSet<UVa.pair<Integer,Integer>>();

And at line no 25
You are try to add pair<p0, p1> in to set like you are doing

 sorter.add(pair<p0, p1>); 

But its not a correct syntax . The correct syntax is .

sorter.add(new pair<Integer, Integer>(p0, p1));

or

sorter.add(new pair(p0, p1))