Java Program to Concatenate Two List

Concatenating two lists means merging two lists into a single list.

Consider the given lists:

LIST 1

LIST 2

LIST AFTER CONCATENATION

There are several methods to perform concatenation operation:

1. Using addAll() method
2. Using stream
3. Using union()

Method 1: Using addAll() method

Syntax:

addAll ( list name ) 

This method takes name of list as argument and add all the elements of the specified list in the same order as the original list. 

• Create a new empty list ( concatenated_list)
• Use addAll () method to concatenate the given list1 and list2 into the newly created list.

concatenated_list.addAll (list1) // concatenates first list 

concatenated_list.addAll (list2) // concatenates second list 

After performing the above steps our empty list now contains both the list.

list1: [1, 2, 3, 4]
list2: [5, 6, 7, 8]
Concatenated list: [1, 2, 3, 4, 5, 6, 7, 8]

Method 2: Using streams

Stream.concat(list1.stream(),list2.stream()).collect(Collectors.toList())

It takes two streams as argument and creates a concatenated stream out of them.Second list is appended to the first list. 

list1: [1, 2, 3, 4]
list2: [5, 6, 7, 8]
Concatenated list: [1, 2, 3, 4, 5, 6, 7, 8]

Method 3: Using union() 

ListUtils.union( list1, list2) 

 It takes two list as argument and return new concatenated list. Second list is appended to the first list. 

Output:

Prerequisites for running the above code:

• Download and Install the following Apache Commons Collections library http://commons.apache.org/proper/commons-collections/download_collections.cgi
• Now go to the properties of your project.
• Select Java Build Path.
• Under Java Build Path go to libraries.
• Under libraries select Classpath.
• Now go to add external JARs and add the JAR executable file, downloaded from the given link.