Split a List into Two Halves in Java
Last Updated :
10 Aug, 2022
Here we are given a list and the task is to split it into two news lists as one can better perceive from the below illustration as follows:
Illustration:
Input : list = {1, 2, 3, 4, 5, 6}
Output : first = {1, 2, 3}, second = {4, 5, 6}
Input : list = {1, 2, 3, 4, 5}
Output : first = {1, 2}, second = {3, 4, 5}
Methods:
- Using loops(Naive Approach)
- Using subList() method of List class
- Using partitioningBy() method of Collectors class
- Using Google guava Library
Let us do discuss the above-defined methods to details alongside implementing via clean java programs as follows:
Method 1: Using loops
Approach:
- Create two new empty lists and assign the first half element of the original list.
- Reset into the second empty list.
Example:
Java
import java.util.ArrayList;
import java.util.List;
public class GFG {
public static List[] split(List<String> list)
{
List<String> first = new ArrayList<String>();
List<String> second = new ArrayList<String>();
int size = list.size();
for ( int i = 0 ; i < size / 2 ; i++)
first.add(list.get(i));
for ( int i = size / 2 ; i < size; i++)
second.add(list.get(i));
return new List[] { first, second };
}
public static void main(String[] args)
{
List<String> list = new ArrayList<String>();
list.add( "Geeks" );
list.add( "Practice" );
list.add( "Contribute" );
list.add( "IDE" );
list.add( "Courses" );
List[] lists = split(list);
System.out.println(lists[ 0 ]);
System.out.println(lists[ 1 ]);
}
}
|
Output
[Geeks, Practice]
[Contribute, IDE, Courses]
Method 2: Using subList() method of List class
It returns a view of the portion of this list between the specified index(include) to another index (exclude). For example, let us take arbitrarily from 2 to 5, Here index 2 will include only. In case if both specified indexes are equal, the returned list is empty. The List.subList() returned a list, so non-structural changes in the returned list.
Example:
Java
import java.util.ArrayList;
import java.util.List;
public class GFG {
public static List[] split(List<String> list)
{
int size = list.size();
List<String> first
= new ArrayList<>(list.subList( 0 , (size) / 2 ));
List<String> second = new ArrayList<>(
list.subList((size) / 2 , size));
return new List[] { first, second };
}
public static void main(String[] args)
{
List<String> list = new ArrayList<String>();
list.add( "Geeks" );
list.add( "Practice" );
list.add( "Contribute" );
list.add( "IDE" );
list.add( "Courses" );
List[] lists = split(list);
System.out.println(lists[ 0 ]);
System.out.println(lists[ 1 ]);
}
}
|
Output
[Geeks, Practice]
[Contribute, IDE, Courses]
Method 3: Using partitioningBy() method of Collectors class
Java 8 Collectors.partitioningBy() is a method that partitions the elements of stream always in two-part, unlike Naive and List.subList(). It returns a Collector that stores the values in a Map. The key of the map can be only in Boolean.
Syntax: partitioningBy() method
public static Collector<T, ?, Map<Boolean, List>>
partitioningBy(Predicate predicate)
Example:
Java
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
public class GFG {
public static List[] split(List<String> list)
{
int midIndex
= ((list.size() / 2 )
- (((list.size() % 2 ) > 0 ) ? 0 : 1 ));
List<List<String> > lists = new ArrayList<>(
list.stream()
.collect(Collectors.partitioningBy(
s -> list.indexOf(s) > midIndex))
.values());
return new List[] { lists.get( 0 ), lists.get( 1 ) };
}
public static void main(String[] args)
{
List<String> list = new ArrayList<String>();
list.add( "Geeks" );
list.add( "Practice" );
list.add( "Contribute" );
list.add( "IDE" );
list.add( "Courses" );
List[] lists = split(list);
System.out.println(lists[ 0 ]);
System.out.println(lists[ 1 ]);
}
}
|
Output
[Geeks, Practice, Contribute]
[IDE, Courses]
Method 4: Using Google guava Library
Guava is an open-source Java-based library which is developed by Google Inc. In the Guava library, we can use Lists.partition() method that splits the list into consecutive sublists, every list specified size. In order to split the list into two sublists, In our case, we can pass the size that is equal to half the size of our list.
Example
Java
import com.google.common.collect.Iterables;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class GFG {
public static List[] split(List<String> list)
{
Iterator<List<String> > itr
= Iterables.partition(list, (list.size()) / 2 )
.iterator();
return new List[] { new ArrayList<>(itr.next()),
new ArrayList<>(itr.next()) };
}
public static void main(String[] args)
{
List<String> list = new ArrayList<String>();
list.add( "Geeks" );
list.add( "Practice" );
list.add( "Contribute" );
list.add( "IDE" );
list.add( "Courses" );
List[] lists = split(list);
System.out.println(lists[ 0 ]);
System.out.println(lists[ 1 ]);
}
}
|
Output
[Geeks, Practice]
[Contribute, IDE]
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