CODEWITHSUNDEEP
java
RofaelEmil
RofaelEmil

Reputation: 85

Set of pairs, compilation error

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());
        }
    }
}

Upvotes: 1

Views: 5343

Answers (7)

Sumit Singh
Sumit Singh

Reputation: 15906

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))

Upvotes: 2

Peter Lawrey
Peter Lawrey

Reputation: 533910

Welcome to Java. While Java is C++, the programming style is a little different. Try the following. Getting pair to compile is just the first of your problems. ;)

public class UVa {
    public static void main(String... args) {
        Scanner scanner = new Scanner(System.in);

        List<Pair<Integer, Integer>> sorter = new ArrayList<>();
        while (scanner.hasNext()) {
            int p0 = scanner.nextInt();
            int p1 = scanner.nextInt();
            sorter.add(Pair.of(p0, p1));
            scanner.nextLine();
        }

        Collections.sort(sorter, new Comparator<Pair<Integer, Integer>>() {
            @Override
            public int compare(Pair<Integer, Integer> o1, Pair<Integer, Integer> o2) {
                int cmp = Integer.compare(o1.first, o2.first);
                if (cmp == 0)
                    cmp = Integer.compare(o1.second, o2.second);
                return cmp;
            }
        });
        for (Pair<Integer, Integer> pair : sorter) {
            System.out.println(pair);
        }
    }
}

class Pair<First, Second> {
    public final First first;
    public final Second second;

    public Pair(First first, Second second) {
        this.first = first;
        this.second = second;
    }

    public static Pair<Integer, Integer> of(int i, int j) {
        return new Pair<Integer, Integer>(i, j);
    }

    @Override
    public String toString() {
        return "Pair{" +
                "first=" + first +
                ", second=" + second +
                '}';
    }
}

Upvotes: 0

Edd
Edd

Reputation: 3822

What you want to add to your Set is an object of type pair<Integer, Integer>. To create one of these you need to use the new keyword, and call the constructor with the two Integer variables p0 and p1.

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

It's conventional to name classes with a leading capital. I've rewritten your pair class below with these conventions, to try and show how it uses Java Generics more clearly:

public static class Pair<T1,T2>{
    public T1 first;
    public T2 second;
    private pair(T1 first, T2 second){
        this.first = first;
        this.second = second;
    }
}

Upvotes: 3

Bart van Heukelom
Bart van Heukelom

Reputation: 44124

It appears you're trying to construct a new pair using 

pair<p0, p1>

However, the correct syntax is

new pair<Integer, Integer>(p0, p1)

Upvotes: 5

Jason
Jason

Reputation: 1241

Changed your code :

 pair p= new pair<Integer, Integer>(p0, p1);
            sorter.add(p); //<<Syntax error on token ">", Expression expected after this token

Upvotes: 1

mbatchkarov
mbatchkarov

Reputation: 16119

In your source, pair<p0, p1> in the statement sorter.add(pair<p0, p1>); refers to the type pair. You are not actually creating an object to be added to the set. You might as well have written sorter.add(int);

Upvotes: 0

MadProgrammer
MadProgrammer

Reputation: 347332

Shouldn't 

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

Read

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

Upvotes: 0

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