Absolutely! Here’s a crash course on Java Streams — a powerful feature introduced in Java 8 that allows for functional-style operations on collections of data.
🔹 What is a Java Stream?
A Stream is not a data structure, but rather a sequence of elements from a source (like a collection) that supports aggregate operations.
Think of it like a pipeline for transforming data.
🔹 Stream Basics
To use streams, you usually follow 3 steps:
- Create a Stream
- Apply Intermediate Operations (like
filter,map, etc.) - Apply a Terminal Operation (like
collect,forEach, etc.)
🔸 Example: Filtering and printing names
List<String> names = List.of("John", "Jane", "Jack", "Tom");
names.stream()
.filter(name -> name.startsWith("J"))
.forEach(System.out::println);
Output:
John
Jane
Jack
🔹 Stream Types
- Sequential Stream: processes elements one by one
- Parallel Stream: processes elements in parallel (multi-threaded) → use with caution
list.parallelStream()
🔹 Common Stream Operations
🔸 filter()
Filters elements based on a condition
stream.filter(x -> x > 10)
🔸 map()
Transforms each element
stream.map(String::toUpperCase)
🔸 sorted()
Sorts the elements
stream.sorted()
🔸 distinct()
Removes duplicates
stream.distinct()
🔸 limit() / skip()
Limit or skip elements
stream.limit(3)
🔹 Terminal Operations
🔸 collect()
Collects results into a collection
List<String> result = stream.collect(Collectors.toList());
🔸 forEach()
Performs an action for each element
stream.forEach(System.out::println);
🔸 count()
Counts elements
long count = stream.count();
🔸 reduce()
Reduces elements to a single value
int sum = stream.reduce(0, Integer::sum);
🔹 Example: Convert List of names to uppercase and collect them
List<string> names = List.of("john", "jane", "jack");<br><br>List<string> upperNames = names.stream()<br> .map(String::toUpperCase)<br> .collect(Collectors.toList());<br></string></string>🔹 Advanced: Grouping and Partitioning
Map<Boolean, List<Integer>> partitioned = numbers.stream()
.collect(Collectors.partitioningBy(n -> n % 2 == 0));
🔹 Stream vs Traditional Loop
Loop:
List<String> result = new ArrayList<>();
for (String name : names) {
if (name.length() > 3) {
result.add(name.toUpperCase());
}
}
Stream:
List<String> result = names.stream()
.filter(name -> name.length() > 3)
.map(String::toUpperCase)
.collect(Collectors.toList());
🔹 Notes
- Streams are lazy: intermediate operations don’t execute until a terminal operation is called.
- Streams cannot be reused: once consumed, they are gone.
✅ Summary
| Operation Type | Examples |
|---|---|
| Intermediate | filter(), map(), sorted() |
| Terminal | collect(), forEach(), reduce() |
| Collector Helpers | toList(), joining(), groupingBy() |
If you want, I can turn this into a cheat sheet PDF or a practice task sheet. Would you like that?
