Comparator Methods and Examples

The Comparator interface in Java is used to define custom sorting logic for objects. It allows sorting collections based on different attributes without modifying the original class. It is used:

• When a class needs to be sorted in multiple ways
• When sorting, logic should be external to the class
• When the class does not implement Comparable

[1, 2, 5, 8]

Explanation:

• A Comparator is implemented using a lambda expression to define custom sorting logic for integers.
• Collections.sort() applies this comparator to sort the list in ascending order.

Syntax

public interface Comparator<T> {
int compare(T o1, T o2);
}

Commonly used Comparator Methods

1. compare(T o1, T o2)

Compares two objects to determine their sorting order. It returns a negative, zero, or positive value based on the comparison result.

5

Syntax:

int compare(T o1, T o2)

Description:

• Returns a negative value if o1 < o2
• Returns zero if o1 == o2
• Returns a positive value if o1 > o2

2. comparing()

Creates a comparator using a key extractor function, allowing objects to be sorted based on a specific field.

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21
23

Syntax:

static <T, U extends Comparable<? super U>>
Comparator<T> comparing(Function<? super T, ? extends U> keyExtractor)

Description:

• Creates a comparator using a key extractor function
• Sorts objects based on a specific field
• Commonly used with method references or lambdas

3. comparingInt(), comparingLong(), comparingDouble()

Primitive-specific comparator methods that improve performance by avoiding unnecessary boxing.

1 100 4.9
2 200 4.5

Syntax:

static <T> Comparator<T> comparingInt(
ToIntFunction<? super T> keyExtractor)

static <T> Comparator<T> comparingLong(
ToLongFunction<? super T> keyExtractor)

static <T> Comparator<T> comparingDouble(
ToDoubleFunction<? super T> keyExtractor)

Description:

• Used for sorting based on primitive fields
• Avoids auto-boxing overhead
• Improves performance compared to comparing()

4. reversed()

Returns a comparator that sorts elements in the reverse order of the original comparator.

[20, 10, 5]

Syntax:

default Comparator<T> reversed()

Description:

• Reverses the sorting order of an existing comparator
• Useful for descending order sorting
• Does not modify the original comparator

5. thenComparing()

Used for multi-level sorting by applying a secondary comparison when the primary comparison results in equality.

25 Adam
25 Bob
30 Carl

Syntax:

default Comparator<T> thenComparing(Comparator<? super T> other)

Description:

• Applies a secondary comparison when values are equal
• Enables multi-level sorting
• Used after a primary comparator

Other Comparator Methods