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?

Answer 1

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.

Answer 2

试试这个，

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

Answer 3

你可以尝试这样：

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

Answer 4

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}
    }
}

Answer 5

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

Answer 6

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.