TakeWhile
The ‘takeWhile’ method on Java Stream API is used for extracting elements from a stream based on specified conditions. The ‘takeWhile‘ is particularly useful when we want to limit the elements taken from the beginning of a stream until a certain condition becomes false.
Signature: Stream<T> takeWhile(Predicate<? super T> predicate)
Example:
List<Integer> numbers = List.of(1,2,3,4,5,6,7,8);
numbers.stream()
.takeWhile(x -> x < 5)
.forEach(System.out :: println); # 1234
DropWhile
The ‘dropWhile’ is just the opposite of the ‘takeWhile’ method, the ‘dropWhile’ method returns a stream consisting of the remaining elements of the stream after dropping the longest prefix of elements that satisfy the given predicate.
Signature: Stream<T> dropWhile(Predicate<? super T> predicate)
Example:
private static void dropWhileDemo(){
List<Integer> numbers = List.of(1,2,3,4,5,6,7,8);
numbers.stream()
.dropWhile(x -> x < 5)
.forEach(System.out :: print);
}
#5678
When To Use Them:
TakeWhile:
Use ‘takeWhile‘ when you want to extract elements from the beginning of a stream as long as a certain condition holds. It’s useful for creating a subset of elements that satisfy specific criteria.
DropWhile:
Use ‘dropWhile‘ when you want to skip elements from the beginning of a stream until a certain condition becomes false. It’s handy when you want to exclude a prefix of elements based on a condition.
Traditional For Loop and ‘takeWhile’ Example
Say we have a list of integers, and you want to find and print all elements until a certain condition is met using a traditional for loop.
List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8);
int condition = 5;
for (int number : numbers) {
if (number < condition) {
System.out.print(number);
} else {
break;
}
}
Let’s achieve the same result using the ‘takeWhile’ method.
List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8);
int condition = 5;
numbers.stream()
.takeWhile(num -> num < condition)
.forEach(System.out :: print);
Traditional For Loop and ‘dropWhile’ Example
List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8);
int condition = 5;
for (int number : numbers) {
if (number > condition) {
System.out.print(number);
}
continue;
}
let’s achieve the same result using the ‘dropWhile’ method.
numbers.stream()
.dropWhile(x -> x <= condition)
.forEach(System.out :: print);
Key Points
- Both methods stop processing as soon as the predicate condition is no longer met. This can lead to efficiency gains, especially for large collections or infinite streams.
- If the first element already fails the predicate, takeWhile returns an empty collection. dropWhile returns an empty collection if all elements satisfy the predicate.
- The order of elements in the resulting collection is preserved from the original collection.
- We can combine the ‘takeWhile’ and ‘dropWhile’ functions with the ‘limit’ function to further control the number of elements processed. takeWhile and limit: Say we have a list of numbers, extract until a certain condition is met, and limit the result to a specific count. dropWhile and limit: Skip numbers until a certain condition is met and limit the result to a specific count.
List<Integer> numbers = List.of(1,2,3,4,5,6,7,8,9);
List<Integer> takeWhileAndLimit = numbers.stream()
.takeWhile(x -> x < 6)
.limit(3)
.collect(Collectors.toList());
System.out.println("TakeWhile and Limit: " + takeWhileAndLimit);
// Using dropWhile and limit
List<Integer> dropWhileAndLimit = numbers.stream()
.dropWhile(x -> x < 6)
.limit(3)
.collect(Collectors.toList());
System.out.println("DropWhile and Limit: " + dropWhileAndLimit);
# TakeWhile and Limit: [1, 2, 3]
# DropWhile and Limit: [6, 7, 8]
Common Use Cases for Both ‘takeWhile’ and ‘dropWhile’
– Filtering and Transforming Data
– Processing large or potentially infinite streams efficiently
– Parsing and Validation
– Domain Specific Use-Cases, such as fetching database records until a condition is met.
Key Advantages of ‘takeWhile’ and ‘dropWhile’
- Conciseness and Readability: Replacing traditional loop with takeWhile and dropWhile simplifies code and improves readability. Focuses on the intent of filtering rather than loop mechanisms.
- Functional Programming style: Aligns with functional programming principles by emphasizing immutable collections and pure functions, it also promotes composability and reusability of code when working with collections.
- Efficiency: Both methods stop processing as soon as the predicate condition is no longer met