Why I get HashMap cannot be resolved to a type error?

Question

Here is my Java code:

import java.util.HashMap;
import java.util.Map;


    public class Polynomial<K> {
        Map<Integer, Object> polynomial;

        public Polynomial(){
            polynomial = new HashMap<K, V>();
        }
        public  Polynomial(int numberOfMembers){
            polynomial = new HashMap<K, V>(numberOfMembers);
        }
        public void addElm(int power, int coefficient){
            if (power < 0) {
                power = Math.abs(power);
                throw new RuntimeException("ERROR: The power must be an absolute number, converting to absolute");
            }
            for (Map.Entry m : polynomial.entrySet()) {
                if ((Integer) m.getKey() == power){
                    polynomial.put(power,m.getValue());
                }
            }
        }

    }

On this two rows:

polynomial = new HashMap<K, V>();

and this:

polynomial = new HashMap<K, V>(numberOfMembers);

I get this error:

HashMap<K,V> cannot be resolved to a type

Any idea what cause to the error above and how to fix it?

power = Math.abs(power); is a no-op since that value is promptly ignored. Likely the runtime optimizer will remove the line altogether, heck, the compiler might even do that, so you might as well, too. There's no point in keeping code that does nothing. — Lew Bloch
– Lew Bloch, Commented Jul 1, 2017 at 16:31

kmecpp · Accepted Answer · 2017-07-27 13:59:14Z

I don't think you actually intend to use the generics you are have.

When you create a new HashMap<K,V>(), those <K,V> values are the actual class names for the types that the HashMap will hold. In your case, those types are Integer and Object so you should write new HashMap<Integer, Object>().

Even that is unnecessary however because Java can infer the type of the HashMap since you have already defined it once. It is best to just use new HashMap<>();

public class Polynomial {

    Map<Integer, Object> polynomial;

    public Polynomial() {
        polynomial = new HashMap<>();
    }

    public Polynomial(int numberOfMembers) {
        polynomial = new HashMap<>(numberOfMembers);
    }

    public void addElm(int power, int coefficient) {
        if (power < 0) {
            power = Math.abs(power);
            throw new RuntimeException("ERROR: The power must be an absolute number, converting to absolute");
        }
        for (Entry<Integer, Object> m : polynomial.entrySet()) {
            if ((Integer) m.getKey() == power) {
                polynomial.put(power, m.getValue());
            }
        }
    }

}

Zabuzard · Accepted Answer · 2017-07-01 15:18:16Z

2

You write Map<Integer, Object> polynomial;. This means polynomial is a Map that needs Integer and Object but then you assign polynomial = new HashMap<K, V>();. The class K and V are not Integer and Object thus the error.

You could write

polynomial = new HashMap<Integer, Object>();

then it would work. What are the types you actually want? Why did you even wrote K, V in the first place? Or else, why the Integer, Object, why not stick to K and V? And also, if you would like to hold V generic you need to add it to class Polynomial<K, V> too.

edited Jul 1, 2017 at 15:18

answered Jul 1, 2017 at 14:34

Zabuzard

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