How in Java rotate List elements or move an element to specified position? Java Collections Framework has nice solutions for this problem.

In the following example we use Collections.rotate(List), from java.util package, to rotate List elements by specified number of positions:

package com.farenda.java;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsRotate {

    public static void main(String[] args) {
        List<String> items = Arrays.asList("a", "b", "c", "d", "e");
        System.out.println("Items: " + items);

        forwardBy(items, 1);

        forwardBy(items, 1);

        backwardBy(items, 2);

        moveXbyY(items, 1, 3);
    }

    private static void moveXbyY(List<String> items, int x, int y) {
        String e = items.get(x);
        // +1 because index 'to' is exclusive
        Collections.rotate(items.subList(x, x+y+1), -1);
        System.out.printf("Move '%s' by %d: %s%n", e, y, items);
    }

    private static void backwardBy(List<String> items, int distance) {
        Collections.rotate(items, -distance);
        System.out.printf("Backward %d: %s%n", distance, items);
    }

    private static void forwardBy(List<String> items, int distance) {
        Collections.rotate(items, distance);
        System.out.printf("Forward %d: %s%n", distance, items);
    }
}

As you can see, to move backward, all you have to do is to provide negative number of steps.

Collections.rotate(List) in conjunction with List.subList(from, to) creates a nice idiom to move an element to specified position. It works fast, because subList creates only a view backed by List, so changes in the view are reflected in the original List.

Here’s the result of running the Java code:

Items: [a, b, c, d, e]
Forward 1: [e, a, b, c, d]
Forward 1: [d, e, a, b, c]
Backward 2: [a, b, c, d, e]
Move 'b' by 3: [a, c, d, e, b]

The Java idiom rotate + subView is worth remembering.