The Collections class allows for you to move objects around in the list using various methods (ls is the List):

Reversing a list:

Collections.reverse(ls);

Rotating positions of elements in a list

The rotate method requires an integer argument. This is how many spots to move it along the line by. An example of this is below:

List<String> ls = new ArrayList<String>();
ls.add(" how");
ls.add(" are");
ls.add(" you?");
ls.add("hello,");
Collections.rotate(ls, 1);

for(String line : ls) System.out.print(line);
System.out.println();

This will print "hello, how are you?"

Shuffling elements around in a list

Using the same list above, we can shuffle the elements in a list:

Collections.shuffle(ls);

We can also give it a java.util.Random object that it uses to randomly place objects in spots:

Random random = new Random(12); 
Collections.shuffle(ls, random);

Syntax

• ls.add(E element); //Adds an element
• ls.remove(E element); //Removes an element
• for(E element : ls){} //Iterates over each element
• ls.toArray(new String[ls.length]); //Converts a List of Strings to an array of Strings
• ls.get(int index); //Returns the element at the specified index.
• ls.set(int index, E element); //Replaces the element at a specified position .
• ls.isEmpty(); //Returns true if the array contains no elements, otherwise it returns false.
• ls.indexOf(Object o); //Returns the index of the first location of the specified element o, or, if it is not present, returns -1.
• ls.lastIndexOf(Object o); //Returns the index of the last location of the specified element o, or, if it is not present, returns -1.
• ls.size(); //Returns the number of elements in the List.

Remarks

A list (opens new window) is an object which stores a an ordered collection of values. "Ordered" means the values are stored in a particular order--one item comes first, one comes second, and so on. The individual values are commonly called "elements". Java lists typically provide these features:

• Lists may contain zero or more elements.
• Lists may contain duplicate values. In other words, an element can be inserted into a list more than once.
• Lists store their elements in a particular order, meaning one element comes first, one comes next, and so on.
• Each element has an index indicating its position within the list. The first element has index 0, the next has index 1, and so on.
• Lists permit inserting elements at the beginning, at the end, or at any index within the list.
• Testing whether a list contains a particular value generally means examining each element in the list. This means that the time to perform this check is O(n) (opens new window), proportional to the size of the list.

Adding a value to a list at some point other than the end will move all of the following elements "down" or "to the right". In other words, adding an element at index n moves the element which used to be at index n to index n+1, and so on. For example:

List<String> list = new ArrayList<>();
list.add("world");
System.out.println(list.indexOf("world"));      // Prints "0"
// Inserting a new value at index 0 moves "world" to index 1
list.add(0, "Hello");
System.out.println(list.indexOf("world"));      // Prints "1"
System.out.println(list.indexOf("Hello"));      // Prints "0"