Conversion of different Collection data types : Collections Framework « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. Collections Framework

Implementations of the Collection interface normally have a constructor that accepts a Collection object. This enables you to convert a Collection to a different type of Collection, such as a Queue to a List, or a List to a Set, etc. Here are the constructors of some implementations:

public ArrayList (Collection c)
     public HashSet (Collection c)
     public LinkedList (Collection c)

As an example, the following code converts a Queue to a List.

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

public class MainClass {

  public static void main(String[] args) {
    Queue queue = new LinkedList();
    queue.add("Hello");
    queue.add("World");
    List list = new ArrayList(queue);

    System.out.println(list);
  }

}

[Hello, World]

And this converts a List to a Set.

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class MainClass {

  public static void main(String[] args) {
    List myList = new ArrayList();
    myList.add("Hello");
    myList.add("World");
    myList.add("World");
    Set set = new HashSet(myList);

    System.out.println(set);
  }

}

[World, Hello]