HashTable generic array creation error

Question

This is a grade 12 java HashTable assignment.

So my teacher gave me the template for doing this assignment, but it does not work :(. And he expects us to get the template to work and then do the assignment.

Here's what he gave us:

class MyHashTable<T>{
    private T[] vals;
    private int load;
    public MyHashTable(){
        load = 0;
        vals = new T[10];
    }
    public MyHashTable(int size){
        load = 0;
        vals = new T[size];
    }
    public void add(T obj){//don't care about negatives
        int size = vals.length;
        if((load+1.0)/size>0.6){
            size*=10;
            T[] tmp = new T[size];
            for(int i=0;i<size/10;i++){
                if(vals[i]!=null){
                    add(vals[i], tmp);
                }
            }
            vals = tmp;
        }
        add(obj, vals);
    }
    public void add(T obj, T[]vals){
        int loc = Math.abs(obj.hashCode())%vals.length;
        while(vals[loc]!=null){
            loc = (loc+1)%vals.length;
        }
        vals[loc] = obj;
    }
    /*public boolean contains(T obj){

    } */
}

it gives an error: error: generic array creation

Can anyone tell me what does that mean? With examples hopefully.

Due to type erasure, you cannot create instances of type parameters. — Reimeus, Apr 02 '13 at 14:41

score 2 · Answer 1 · answered Apr 02 '13 at 14:41

2

Due to the way that generics are implemented in Java, it is not possible to use generics such that type information is needed at runtime. See this.

answered Apr 02 '13 at 14:41

gparyani

1,958
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score 2 · Answer 2 · edited May 23 '17 at 10:32

2

error: generic array creation

You can not create arrays from generic types.

See also What's the reason I can't create generic array types in Java?

edited May 23 '17 at 10:32

Community

1
1

answered Apr 02 '13 at 14:43

Andreas Fester

36,091
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Pshemo · Accepted Answer · 2013-04-02T17:29:18.363

As everybody said generic type is erased at runtime:

T becomes Object,
T extends SomeClass becomes SomeClass.

So you have at least two options

You can use same pattern that was used in ArrayList<T> and store items in Object[] array rather then T[] array
```
vals = (T[]) new Object[size];
```
If you want to create array of real T type you would have to make user to pass instance of Class<T> object that will correspond T and use it like
```
vals = (T[]) java.lang.reflect.Array.newInstance(clazzT, size); 
```

where clazzT is Class<T> instance.

HashTable generic array creation error

3 Answers3