Java Collections Methods
In Java, the Collections class provides useful static methods to operate on collection objects such as ArrayList, LinkedList, HashSet, etc.
The Collections class belongs to the java.util package. It contains methods for sorting, searching, reversing, shuffling, and more.
1. Importing Collections
import java.util.Collections;
import java.util.ArrayList;
2. sort()
The sort() method sorts a collection in ascending order.
ArrayList numbers = new ArrayList();
numbers.add(5);
numbers.add(1);
numbers.add(8);
Collections.sort(numbers);
System.out.println(numbers);
3. reverse()
The reverse() method reverses the order of elements.
Collections.reverse(numbers);
System.out.println(numbers);
4. shuffle()
The shuffle() method randomly changes the order of elements.
Collections.shuffle(numbers);
System.out.println(numbers);
5. binarySearch()
The binarySearch() method searches for an element in a sorted list.
Collections.sort(numbers);
int index = Collections.binarySearch(numbers, 5);
System.out.println(index);
Important: The list must be sorted before using binarySearch().
6. max() and min()
The max() method returns the maximum element.
The min() method returns the minimum element.
System.out.println(Collections.max(numbers));
System.out.println(Collections.min(numbers));
7. frequency()
The frequency() method counts how many times an element appears.
numbers.add(5);
System.out.println(Collections.frequency(numbers, 5));
8. replaceAll()
The replaceAll() method replaces all occurrences of a value with another value.
Collections.replaceAll(numbers, 5, 10);
System.out.println(numbers);
9. copy()
The copy() method copies elements from one list to another.
ArrayList newList = new ArrayList();
newList.add(0);
newList.add(0);
newList.add(0);
Collections.copy(newList, numbers);
System.out.println(newList);
10. fill()
The fill() method replaces all elements with a specified value.
Collections.fill(numbers, 100);
System.out.println(numbers);
Why Collections Class is Important?
- Provides ready-made utility methods.
- Reduces manual coding.
- Improves performance and readability.
- Works with all List implementations.
- Commonly used in real-world applications.
Difference Between Collection and Collections
- Collection is an interface.
- Collections is a utility class.
- Collection stores objects.
- Collections provides methods to operate on collections.
Important Points
- All methods in Collections class are static.
- No need to create an object of Collections.
- Some methods require sorted data.
- Works mainly with List interface.
Conclusion
The Collections class provides powerful methods like sort(), reverse(), shuffle(), max(), min(), and more.
It makes working with collections easier and more efficient.
Tip: Always use Collections utility methods instead of writing long manual logic.