Creating an array of ArrayList which store object of Generic type

Question

I have to create an array of ArrayList which is storing an object of the generic type.

ArrayList<Entry>[] bucket;

when I initialize this to

bucket=(ArrayList<Entry>[])new Object[m];

I get java.lang.ClassCastException at runtime.

Entry class uses generics. Below is the code:

class Entry<K,V>{
    K key;
    V value;

    public Entry(K key,V value){
        this.key=key;
        this.value=value;
    }

After going through several posts on why Array of Objects cannot be cast to an ArrayList of a generic type, I have understood my problem. But I am not able to get my head around this to solve my particular case.

Some of the solutions involved:

Changing it FROM "an array of ArrayList" TO "an ArrayList of ArrayList",
but I can not do that in my case.

Full code:

import java.util.ArrayList;
class MyHashMap<K,V>  {

    int m;
    int loadFactor;
    int n;

    ArrayList<Entry>[] bucket;

    public MyHashMap(){
        this(10,2);
    }
    public MyHashMap(int m){
        this(m,2);
    }
    public MyHashMap(int m,int loadFactor){
        this.m=m;
        this.loadFactor=loadFactor;
        n=0;

        bucket=(ArrayList<Entry>[])new Object[m];

    }

    class Entry{
        K key;
        V value;

        public Entry(K key,V value){
            this.key=key;
            this.value=value;
        }

        public int hashCode(){
            return 0;
        }
    }


    public void put(K key, V value){    
        Entry entry=new Entry(key,value);
        int hash=entry.hashCode();
        int index=hash%m;
        bucket[index].add(entry);
    }

    public static void main(String[] args) {
        MyHashMap hashMap=new MyHashMap();

        hashMap.put("faisal",2);
    }
}

Why did you create a class called Entry which looks awfully similar to Java's Map.Entry? — Tim Biegeleisen
– Tim Biegeleisen, Commented Dec 26, 2016 at 7:26
why do you think you can cast a Object array into an array of ArrayList<Entry> ? — Scary Wombat
– Scary Wombat, Commented Dec 26, 2016 at 7:27
Please show the code in context where you plan to use this ArrayList. — Tim Biegeleisen
– Tim Biegeleisen, Commented Dec 26, 2016 at 7:27
Polymorphic type declaration doesn't work with collections.The type should be exact type.it cant be parent or subtype.but this is diff from with arrays.Entry[] bucket=new Object[m]; — gihan-maduranga
– gihan-maduranga, Commented Dec 26, 2016 at 7:28
@Tim I am actually implementing HashMap here,and I am using chaining. — Md Faisal
– Md Faisal, Commented Dec 26, 2016 at 7:28

Gurwinder Singh · Accepted Answer · 2016-12-26 07:33:45Z

2

You can't create arrays of generic type. Use this instead:

ArrayList<Entry>[] bucket = new ArrayList[m];

It shows unchecked warning though, which you can suppress using @SuppressWarnings("unchecked") like so:

@SuppressWarnings("unchecked")
ArrayList<Entry>[] bucket = new ArrayList[m];

answered Dec 26, 2016 at 7:33

Gurwinder Singh

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2 Comments

Md Faisal Over a year ago

The compiler does not complain nor there is any problem at runtime. I am now wondering if it is okay to do that.! I mean is this the only option left.

Gurwinder Singh Over a year ago

It's fine to do it if you know you are going to store only the ArrayList<Entry> objects in it. Better not to expose it outside your API class.

Kiran Kumar · Accepted Answer · 2016-12-26 07:29:19Z

0

Object can be cast to ArrayList only if the actual object it holds is an arrayList. i.e let's look at the below code,

Object obj = new ArrayList<String>();   // object variable holding array list type.
ArrayList<String> arr = (ArrayList<String>) obj;

answered Dec 26, 2016 at 7:29

Kiran Kumar

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1 Comment

Gurwinder Singh Over a year ago

Question is about array of array list

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