Singleton Collection Classes

Singletons are useful to provide a unique source of data or functionality to other Java Objects. Collection framework provides singleton classes for each collection class like SingletonList and SingletonMap.  

In this article we will discuss about SingletonList which is equally applicable to other singleton collection classes.

Collections Class provide method to convert normal list into singletonList like below syntax,

List<String> mySingletonList = Collections.singletonList("AAA");

Java documentation says "Returns an immutable list containing only the specified object. The returned list is serializable."

Why we need this type single object Immutable list?

Below is the code to remove all null values from list - in conventional style:

package singleton;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class SingletonListTest {

	public static void main(String[] args) {
		List <String> myList = new ArrayList<String>(){
		{add("a"); add(null); add("c"); add(null);}};

		System.out.println("Original List" + myList);
		//Creating nullObject List to provide to removeAll method
                List<String> nullList = new ArrayList<String>();
		nullList.add(null);

		System.out.println(myList.removeAll(nullList));
		System.out.println("Modified List" + myList);
	}
}

Output:

Original List[a, null, c, null]
true
Modified List[a, c]

In above code

• we created an empty list object


• add an null element to it, and


• passed it to removeAll method argument.

We can solve the problem with below one line change..

package singleton;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class SingletonListTest {

	public static void main(String[] args) {
		List <String> myList = new ArrayList<String>(){
		{add("a"); add(null); add("c"); add(null);}};

		System.out.println("Original List" + myList);
		System.out.println(myList.removeAll(Collections.singleton(null)));
		System.out.println("Modified List" + myList);	
	}
}

Output: Same as above.

Hope this is helpful article. Happy coding.