How to create and initialize List or ArrayList in one line in Java? Example

creating and initializing List at the same time

Sometimes we want to create and initialize a List like ArrayList or LinkedList in one line much like creating an array and initializing it on the same line. If you look at The array on Java programming language you can create and initialize both primitive and object arrays e.g. String array very easily in just one line but in order to create a List equivalent of that array, you need to type a lot of code. This is also one of the tricky Java question sometimes appears in Interview as Write Java code to create and initialize ArrayList in the same line.

In this Java tips we will see this trick which allow you to create and initialize List much like an Array. This tip can also save a lot of time while creating a test program or quickly trying some stuff.

Java Tip to create and initialize List in one line

In our post 3 ways to convert Array to ArrayList in Java, we have discussed Arrays.asList() method. You can use Arrays.asList() method to create and initialize List at the same line. java.util.Arrays class act as a bridge between Array and List in Java and by using this method you can quickly create a List from Array, which looks like creating and initializing List in one line, as shown in the below program.

package test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 *
 * Java program to show
 * How to create and initialize a List in one line in Java
 * Arrays.asList() our go-to method to initialize List
 * in the same line.
 *
 * @author http://java67.blogspot.com
 */
public class ArrayListDemo{

    public static void main(String args[]) {
     
        //You can create and initialize Array in just one line in Java
        String[] coolStringArray
                = new String[]{"Java" , "Scala" , "Groovy"};
        System.out.println(" Array : "
                             + Arrays.toString(coolStringArray));
   
        //Now If you want to create
        // an ArrayList with three elements
        List<String> notSoCoolStringList = new ArrayList<String>();
        notSoCoolStringList.add("Java");
        notSoCoolStringList.add("Scala");
        notSoCoolStringList.add("Groovy");
     
        //It took four-line to create and initialize List
        System.err.println(" List : " + notSoCoolStringList);
     
        //Now here is simple Java tips to
       // create and initialize List in one line
        List<String> coolStringList = Arrays.asList("Java",
                          "Scala", "Groovy");
        System.err.println(" List created and initialized
                    at same line : " + coolStringList);
    }
}

Output:
 Array : [Java, Scala, Groovy]
 List : [Java, Scala, Groovy]
 List created and initialized at same line : [Java, Scala, Groovy]

So this was our Java tip to create and initialize a List in the same line. Just remember that Arrays.asList() return java.util.List and not ArrayList or LinkedList. 

Another worth noting point is that List returned by Arrays.asList() is a fixed-length list that doesn't allow you to add or remove elements from it. add() and remove() method will throw UnSupportedOperationException if you try to add or remove elements from the List.  

Some programs mistook this List as a read-only List, which is not because it allows set() operation which can be used to change elements in the List. Only legitimate way to create  read-only Collection is Collections.unmodifiableXXX() method.

You can also see these Java Collections and Stream Courses to learn more about ArrayList and other collection classes in Java. 

That's all on this Java tip about how to create and initialize a List in the same line in Java. This is a great tip that can save a lot of coding time if you frequently create and initialize a List with test data. You can use this trick to quickly create ArrayList with test data for your unit testing and demos. It really saves a lot of keystrokes in long run. 

 Other Java Collection tutorials from java67 blog

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 Difference between TreeSet and TreeMap in Java

 How to sort ArrayList in Java