Creating generic arrays in Java

Question

public K[] toArray()
{
    K[] result = (K[])new Object[this.size()];
    int index  = 0;
    for(K k : this)
        result[index++] = k;
    return result;
}

This code does not seem to work, it will throw out an exception:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to ...

Could someone tell me how I can create an array with a generic type? Thanks.

Possible duplicate of http://stackoverflow.com/questions/395030/quick-java-question-casting-an-array-of-objects-into-an-array-of-my-intended-cla — Gaurav Saxena, Oct 25 '10 at 10:33
there are at least three duplicates of this. Search before posting. — Bozho, Oct 25 '10 at 10:43
For future reference: "throw an exception", not "through". As in the keyword "throw". — DJClayworth, Oct 25 '10 at 13:54
possible duplicate of [How to: generic array creation](http://stackoverflow.com/questions/529085/how-to-generic-array-creation) — Joeri Hendrickx, Jun 24 '14 at 07:10

score 10 · Accepted Answer · answered Oct 25 '10 at 10:45

10

You can't: you must pass the class as an argument:

public <K> K[] toArray(Class<K> clazz)
{
    K[] result = (K[])Array.newInstance(clazz,this.size());
    int index  = 0;
    for(K k : this)
        result[index++] = k;
    return result;
}

answered Oct 25 '10 at 10:45

Maurice Perry

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Gadolin · Answer 2 · 2010-10-25T10:43:00.330

1

Ok, this not works K[] result = new K[this.size()];

If you could hold class. Then:

  Class claz;
  Test(Class m) {
     claz = m;
  }

  <K>  K[] toArray() { 
K[] array=(K[])Array.newInstance(claz,this.size());
return array;
}

edited Oct 25 '10 at 10:43

answered Oct 25 '10 at 10:34

Gadolin

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score 1 · Answer 3 · edited May 23 '17 at 11:47

Your code throws that exception because it's actually giving you an array of type Object. Maurice Perry's code works, but the cast to K[ ] will result in a warning, as the compiler can't guarantee type safety in that case due to type erasure. You can, however, do the following.

import java.util.ArrayList;  
import java.lang.reflect.Array;  

public class ExtremeCoder<K> extends ArrayList<K>  
{  
   public K[ ] toArray(Class<K[ ]> clazz)  
   {  
      K[ ] result = clazz.cast(Array.newInstance(clazz.getComponentType( ), this.size( )));  
      int index = 0;  
      for(K k : this)  
         result[index++] = k;  
      return result;  
   }  
}

This will give you an array of the type you want with guaranteed type safety. How this works is explained in depth in my answer to a similar question from a while back.

Creating generic arrays in Java

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