Java’s Collections class is packed with powerful utility methods that simplify working with data structures, but many developers only scratch the surface of what it can do. 

Did you know there are several lesser-known methods in the Collections class that can save you time, improve your code efficiency, and solve tricky problems effortlessly?

In this article, we’re uncovering some of the hidden methods of the Collections class .

You’ll learn how these methods work, when to use them, and how they can level up your Java development skills.

Whether you’re an aspiring developer or an experienced professional, these methods will give you a new appreciation for the hidden power of the Collections class. 

Let’s jump in and explore!

1. Creating Empty List

Suppose you want to create an empty list. This is what you will probably do. 

List<Integer> numbers = new ArrayList<>();

Instead there is a method emptyList() in collections class that will create an empty list for you.

List<Integer> numbers = Collections.emptyList();

Remember that this list is an empty list and you also cant add an element to it. 

Adding an element will give you UnsupportedOperationException.
So, the question arises what is the use of this list and method.

This method is extremely useful when you have to return a list from a method but the data size is 0. For example, some rows fetched from database

public List<String> names() {
  if(isEmpty()) {
    return new ArrayList<>();
  }
}

Instead of returning null, return an empty list to prevent NullPointerException.
This is also memory efficient as it will return same object every time but if you return a new ArrayList this way, it will create a new object every time.

2. Counting Element Occurrence

This is a list of some elements. 

List<Integer> numbers = Arrays.asList(1,3,1,2,1);

Suppose I ask you to tell me the frequency or number of times a particular element occurs, lets say 1, in the list.

You would loop over the list and check if the element matches with the one we want to count and increment a counter like this.

int count = 0;
for(Integer num: numbers) {
  if(num == 1) {
    count++;
  }
}


Instead of all this, use frequency() method in collections class and pass it the list and the element whose frequency we want to know as below.

int count = Collections.frequency(numbers, 1); // will be 2

3. Reversing List

You can easily reverse a list elements with Collections.reverse() method passing it the list as argument. 

List<String> list = Arrays.asList("a", "x", "b", "y");
Collections.reverse(list); 
// list is now "y", "b", "x", "a"

Reversing means changing the order of list elements such that the first element becomes the last element, second element becomes the second last element and so on. 

Remember that reverse() method modifies the original list and this is the reason that the return type of this method is void. 
print the list. 

4. Shuffling Elements

If you want to change the order of list elements in random order, then use shuffle() method of collections passing it the list as argument.

shuffle() also modifies the original list. Every time you call shuffle(), the order of list elements will be random.

List<String> list = Arrays.asList("a", "x", "b", "y");
Collections.shuffle(list); // "b", "a", "y", "x"
Collections.shuffle(list); // "y", "a", "b", "x"

5. Interchanging Elements

Lets say you want to interchange the position of two list elements without modifying the other list elements.
Collections class has a swap() method which takes 3 arguments, the list and the indexes of the elements to be swapped.

List<String> list = Arrays.asList("a", "x", "b", "y");

So, if we want to swap a with y in this case, the first index will be 0 and the second one will be 4, the index of y since the indexes begin at 0.

Collections.swap(list, 0, 4);

6. Min and Max

Finding minimum and maximum element in a list is a commonly required task. 

Generally, we write our own logic to do this and that involves iterating over the list, comparing all the elements with each other.

There is a shorter way using Collections class.

Collections class has min() and max() methods which accept a collection argument and return the minimum and maximum element according to the natural ordering.

Natural ordering means that if the collection is of numbers, then the min and max elements will be determined by comparing numbers.

And if the collection is of string elements, then min and max elements will be determined based on alphabetic comparison. Example,

List<String> list = Arrays.asList("a", "x", "b", "y");
String min = Collections.min(list); // a
String max = Collections.max(list); // y

List<Integer> numbers = Arrays.asList(1,3,1,2,1);
Integer minNum = Collections.min(numbers); // 1
Integer maxNum = Collections.min(numbers); // 3

7. Moving Elements

Lets say you want to move list elements by some places. 

Use rotate() method with the number of places the elements need to be moved.

So, if I write Collections.rotate(), supply it the list and 2, then it will move all the elements two places forward. 

The elements at the end will be rotated back from the start.

rotate() will modify the list. 

List<Integer> numbers = Arrays.asList(1,2,3,4,5);
Collections.rotate(numbers, 2); // 4, 5, 1, 2, 3

8. Immutable List

When you want an immutable list that can not be modified and has a single element, then instead of creating a new ArrayList and adding element, simply use singletonList() method, passing it one element.

List<Integer> numbers = Collections.singletonList(1);

The singletonList() is more memory-efficient than creating an ArrayList for a single element, as it uses a minimal internal structure to store the value.
Similarly, there are singletonSet() and singletonMap() methods to create an immutable set and map.

9. Resetting List

Have you ever needed to quickly reset or initialize all elements of a list to a specific value without writing a loop? 

That’s where the Collections.fill() method comes in! 

It’s a simple yet powerful utility in Java’s Collections class that allows you to replace all the elements in a list with a single specified value.
fill() takes 2 arguments: the list and the element with which you want to fill.

List<Integer> numbers = Arrays.asList(1,2,3,4,5);
Collections.fill(numbers, 1); // 1,1,1,1,1

While fill() might seem basic at first glance, it makes all the elements in the list the same — it can save you a lot of time in scenarios like resetting a list, initializing placeholder values, or creating mock data for testing.

10. Duplicate Element List

Suppose you have to quickly create a list with a same element, then use nCopies() method.

nCopies() accepts two arguments: the element itself and the number of times it needs to be added to the list. Example,

List<Integer> ones= Collections.nCopies(1, 5); // 1,1,1,1,1

The returned list is immutable but is serializable and can be used to add to another list with addAll().

Wrapping Up

The methods that we learnt might seem simple at first glance, but when used effectively, they can make your code cleaner, more efficient, and easier to maintain.