In Java we have different ways to reverse arraylist . In earlier post learn arraylist in java that it is resistible array backed list and maintain insertion order. We can reverse an arraylist by using following ways
- Using Collections.reverse() method.
- Using Recursion
- Using add() + get() method – extra space require
- Using add() + remove() method – No extra space require
1. Using Collections.reverse() method –
Collection.reverse() method can use for reverse a list in java .
Sysntax –
public static void reverse(List<?> list) {
}This method reverses the order of the elements in the specified list.This method runs in linear time.
It takes list as parameter , whose elements are to be reversed.It throws UnsupportedOperationException – if the given list or its list-iterator does not support the set operation.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
public class ArrayListReverseByCollections {
public static void main(String[] args) {
List<String> languages = new ArrayList<>(
Arrays.asList("PHP", "JAVA", "Python"));
//print list
System.out.println("element of list ="+ languages);
/*
Reverses the order of the elements in the specified list.
*/
Collections.reverse(languages);
//print reverse list
System.out.println("element of reverse list ="+ languages);
}
}Output:
element of list =[PHP, JAVA, Python]
element of reverse list =[Python, JAVA, PHP]2. Reverse Arraylist Using Recursion
Now we will reverse arraylist using recursion
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class ReverseListByRecursion {
public static<String> void reverseList(List<String> list)
{
// Base Case: list is empty or only one element is left
if (list == null || list.size() <= 1)
return;
// remove first element from list
String value = list.remove(0);
// recur for remaining items of list
reverseList(list);
// insert the top element back to list after recurse for remaining items
list.add(value);
}
public static void main(String[] args) {
List<String> languages = new ArrayList<>(
Arrays.asList("PHP", "JAVA", "Python"));
reverseList(languages);
System.out.println(languages);
}
}Output :
[Python, JAVA, PHP]3. Using add() + get() method – extra space require
We will write our own method , in this we reverse arraylist in java using for loop , we read list element from last to first one by one and set to new arraylist.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class ReverseListByMethod {
public static void main(String[] args) {
List<String> languages = new ArrayList<>(
Arrays.asList("PHP", "JAVA", "Python"));
ArrayList<String> reverseList = new ArrayList<String>();
for (int i = languages.size() -1 ; i > -1 ; i--) {
reverseList.add( languages.get(i));
}
//print reverse list
System.out.println("reverse list is ="+ reverseList);
}
}reverse list is =[Python, JAVA, PHP]4. Using add() + remove() method – No extra space require
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class ReverseList {
public static void main(String[] args)
{
List<String> languages = new ArrayList<>(
Arrays.asList("PHP", "JAVA", "Python"));
for (int i = 0, j = languages.size() - 1; i < j; i++) {
languages.add(i, languages.remove(j));
}
System.out.println(languages);
}
}[Python, JAVA, PHP]