Cannot instantiate the type Pair although not abstract

Question

Trying to write an iterator that returns a Pair: Part of my PairIterator

public Pair next() {
            this.counter ++;
            Pair p = new Pair(this.l.get(counter - 1), this.l.get(counter)); 
            //error occurs here

        }

public class Pair<E> {
    private E e1;
    private E e2;

    public Pair(E e1, E e2) {
        this.e1 = e1;
        this.e2 = e2;
    }
    public E first() {
        return this.e1;
    }
    public E second() {
        return this.e2;
    }
}

getting

Cannot instantiate the type Pair

... although Pair is not an abstract class/interface. Why is this happening?

Answer 1

Make sure to import the correct class to initialize your pair object.

import org.apache.commons.math3.util.Pair;

Concrete Class -> Can directly create object.

This gives an error:

import org.apache.commons.lang3.tuple.Pair;

Its an abstract class, so you can't create the object directly.

Answer 2

Try instantiating org.apache.commons.lang3.tuple.ImmutablePair (or org.apache.commons.lang3.tuple.MutablePair ) instead; those classes are concrete so they can be instantiated.

Answer 3

很可能是因为您没有向 Pair 添加泛型类型参数。