CODEWITHSUNDEEP
javaclassgenericsconstructor
Hubi
Hubi

Reputation: 560

How to build a class and constructor generic to use generic methods?

I have problems understanding how to build a class and a constructor generic. I'm pretty new to generics, but now I have a basic understanding.

The following method works:

@Override
public <T> List<T> groupProceeds(Collection<? extends T> e, Function<? super T, Integer> p) {
    int from = 10001;
    int to = 10070;
    int sum = 1010000;

    return buildGroup(e, p, from, to, sum);
}

How is it possible to use this method in a class? The class should have a constructor and generic properties.

This is one of my approaches, this doesn't work. But I also don't know how to solve it. 

public class StandardStructure<T, P> {

    private T entities;
    private P property;


    public StandardStructure (Collection<? extends T> entities, Function<? super T, Integer> property) {
        this.entities = entities;
        this.property = property;

    }

    @Override
    public <T> List<T> groupProceeds(Collection<? extends T> e, Function<? super T, Integer> p) {
        int from = 10001;
        int to = 10070;
        int sum = 1010000;

        return buildGroup(e, p, from, to, sum);
    }
}

When constructing the class, Collection entities and Function property should be passed to the method.

I need help setting up the class and the constructor generic.

Upvotes: 2

Views: 65

Answers (1)

Bentaye
Bentaye

Reputation: 9756

A few things I would think of:

  • You don't need the P type in your generics because the type of your property attribute is expressed in terms of T

  • If you have your entities and property as class variables, you don't need to pass them to your groupProceeds method

  • You can't use @Override here because you are not extending any class or implementing any interface

  • You should not make the method generic as it would hide the class generic. You want the same T as the class T.

Try it this way:

public class StandardStructure<T> {

    private Collection<? extends T> entities;
    private Function<? super T, Integer> property;

    public StandardStructure (Collection<? extends T> entities, Function<? super T, Integer> property) {
        this.entities = entities;
        this.property = property;
   }

    public List<T> groupProceeds() {
        int from = 10001;
        int to = 10070;
        int sum = 1010000;

        return buildGroup(entities, property, from, to, sum);
    }

    private List<T> buildGroup(Collection<? extends T> e,  Function<? super T, Integer> p, int from, int to, int sum) {
        /* Whatever that does */
    }
}

Or if you want to pass the parameters to the method, you don't need them as attributes on the class and don't even need the constructor anymore: 

public class StandardStructure<T> {

    public List<T> groupProceeds(Collection<? extends T> entities, Function<? super T, Integer> property) {
        int from = 10001;
        int to = 10070;
        int sum = 1010000;

        return buildGroup(entities, property, from, to, sum);
    }

    private List<T> buildGroup(Collection<? extends T> e,  Function<? super T, Integer> p, int from, int to, int sum) {
         /* Whatever that does */
    }
}

Upvotes: 2

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