如何创建具有两个泛型类型的类的实例？

Question

I'm trying to create an instance of a class ZipIterator which takes two arbitrary arrays as arguments and sets them equal to two private fields K[] and V[] via the constructor. In my Testing class' main method, I'm writing

import java.util.Iterator;

public class ZipIterator<K,V>
{
    private K[] arr1;
    private V[] arr2;
    private int pos = 0;

    public ZipIterator(K[] arr1, V[] arr2) {
        this.arr1 = arr1;
        this.arr2 = arr2;
    }

}

In my Testing class' main method, I'm trying to create a ZipIterator object like this

int[] arr1 = {1,5,3,1,6};
double[] arr2 = {2.3,42.1,1.6,6.43};
ZipIterator<int[],double[]> zip =  new ZipIterator<int[],double[]>(arr1,arr2);

but I keep getting the error:

error: incompatible types: int[] cannot be converted to int[][]

I'm not exactly sure what I am doing wrong. If anybody can help, it would be greatly appreciated!

Answer 1

Think about the signature of ZipIterator<K[], V[]> when K is int[] and V is double[] .

What you need to instantiate is ZipIterator<Integer, Double> ; you cannot parametrize a class with a primitive type.

Yes, it's inefficient because of boxing/unboxing. If you want higher performance, make wrapper classes that keep an array of a primitive type as an instance variable. Will be tricky to make it nicely parametric, though.

Answer 2

Look closely what happens:

You are using int[] as generic type K :

ZipIterator<int[], double[]> zip = new ZipIterator<int[],double[]>(arr1, arr2);

Your constructor, however, is accepting an array of K , so an array of int[] would require int[][] :

public ZipIterator(K[] arr1, V[] arr2) { ... }

---

Notes:

• You shouldn't use arrays with generics, use the List interface instead:

   public ZipIterator(List<K> arr1, List<V> arr2) { ... } 

  Or, like 4castle mentioned , you can even use the Iterable interface.

• Unlike what others said, an array is always an object, even if it is an array of a primitive type. So using int[] for K is perfectly valid, whilst int is not.

Answer 3

应该是ZipIterator<int,double> zip = new ZipIterator<int,double>(arr1,arr2);

Answer 4

Use the wrapper classes

Integer[] arr1 = {1,5,3,1,6};
Double[] arr2 = {2.3,42.1,1.6,6.43};
ZipIterator<Integer,Double> zip =  new ZipIterator<>(arr1,arr2);

Edited my answer based on the comment by @4castle