How to convert arraylist to an array?

Question

I have this ArrayList in Java -

List<Map<String, Object>> list = new ArrayList<Map<String, Object>>();

To convert it to an array I invoke list.toArray() method, but it returns Object[]. This is not what I want. I want Map<String, Object>[].

I know about List.toArray(T[] a); It doesn't work with parameterized types.

The method signature of batchUpdate method in Spring framework is this -

int[] org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate.batchUpdate(String sql, Map<String, ?>[] batchValues)

If it is not possible to create array of Map objects why is Spring using it? And how are we supposed to use this method then?

@JigarJoshi That is my own question that was closed before I could get an answer. — Kshitiz Sharma
– Kshitiz Sharma, Commented Jun 7, 2012 at 4:51
There are correct answer in the comment in your previous question: No array with parameterized type — Adrian Shum
– Adrian Shum, Commented Jun 7, 2012 at 4:55
@AdrianShum Oh really??? I wonder what Spring developers were thinking.. — Kshitiz Sharma
– Kshitiz Sharma, Commented Jun 7, 2012 at 4:56

Gene · Accepted Answer · 2012-06-07 05:10:23Z

3

In a nutshell, you can't make arrays of concrete parameterized types. This is a pretty good explanation of what's going on. The Spring type is essentially the same as saying Map batchValues. The parameter types are for documentation only. This gaping hole in the Java type system is a tradeoff for performance.

answered Jun 7, 2012 at 5:10

Gene

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KV Prajapati · Accepted Answer · 2012-06-07 04:56:46Z

2

Try this,

 Map<String,Object>[] ar=list.toArray(new HashMap[list.size()]);

answered Jun 7, 2012 at 4:56

KV Prajapati

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1 Comment

Alex Calugarescu Over a year ago

This is the way to go... But you still lose type-safety by doing this... I think it would be more appropriate to use new Map[list.size()], this way any Map added to the list will go just fine.

Lai Xin Chu · Accepted Answer · 2012-06-07 05:04:30Z

1

Do it this way:

Map<String, Object> [] mp = new HashMap[list.size()]; 
list.toArray(mp);

This answer works. I tested it.

My full test code is as follows:

import java.util.*;
public class Test {

    public static void main (String [] args) {
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("Hello", "World");
        ArrayList<Map<String, Object>> list = new ArrayList<Map <String, Object>>();
        list.add(map);

        Map<String, Object> [] mp = new HashMap[list.size()]; 
        list.toArray(mp);   
        System.out.println(mp[0]);    // prints out {Hello=World}
    }
}

edited Jun 7, 2012 at 5:04

answered Jun 7, 2012 at 4:58

Lai Xin Chu

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UVM · Accepted Answer · 2012-06-07 05:08:09Z

1

You can try like this:

Map<String, Object> [] mp = list.toArray(new HashMap[list.size()]);

edited Jun 7, 2012 at 5:08

answered Jun 7, 2012 at 4:54

UVM

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liuzhijun · Accepted Answer · 2012-06-07 04:59:12Z

0

HashMap[] map = list.toArray(new HashMap[0]);

answered Jun 7, 2012 at 4:59

liuzhijun

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MTilsted · Accepted Answer · 2012-06-07 04:55:28Z

-2

You can't do it.

List.toArray(T[] a) is your only option, but it does not work with parameterized types because they are not preserved at runtime.

answered Jun 7, 2012 at 4:55

MTilsted

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3 Comments

MTilsted Over a year ago

What about it? Do you have an example where Spring does this?

Kshitiz Sharma Over a year ago

int[] org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate.batchUpdate(String sql, Map<String, ?>[] batchValues)

MTilsted Over a year ago

Hu? That spring function does something completely different. The problem which prevent the original author from doing what he want, is is that the generic part of the type don't exists at runtime. So you can't create a new object of the same type.

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