Java Tutorial
Java Streams
Streams let you filter, map, and reduce collections as a pipeline. Nothing runs until a terminal operation like collect or forEach.
filter and map
Call .stream() on a collection, then chain intermediate ops. filter keeps some elements; map transforms each one. The stream is not a new list until you collect or forEach.
Example
import java.util.List;
public class Main {
public static void main(String[] args) {
List.of(1, 2, 3, 4).stream()
.filter(n -> n % 2 == 0)
.map(n -> n * n)
.forEach(n -> System.out.print(n + " "));
System.out.println(); // 4 16
}
}collect into a List
Collectors.toList() (or .toList() on newer JDKs) materializes the pipeline into a real list you can keep.
Example
import java.util.List;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
List names = List.of("Ada", "Lin", "Al");
List longOnes = names.stream()
.filter(s -> s.length() > 2)
.map(String::toUpperCase)
.collect(Collectors.toList());
System.out.println(longOnes); // [ADA, LIN]
}
} sum and match
mapToInt plus sum totals numbers. anyMatch /allMatch answer yes/no questions about the stream.
Example
import java.util.List;
public class Main {
public static void main(String[] args) {
List nums = List.of(8, 11, 5);
int sum = nums.stream().mapToInt(n -> n).sum();
boolean anyBig = nums.stream().anyMatch(n -> n > 10);
System.out.println(sum); // 24
System.out.println(anyBig); // true
}
} Intermediate ops are lazy. If you never call a terminal op, nothing in the pipeline runs — handy when you build a stream and pass it along.
sorted and distinct
sorted orders elements; distinct drops duplicates according toequals. Chain them when cleaning input.
Example
import java.util.List;
public class Main {
public static void main(String[] args) {
List.of(3, 1, 2, 1, 3).stream()
.distinct()
.sorted()
.forEach(n -> System.out.print(n + " "));
System.out.println(); // 1 2 3
}
}A stream can be consumed only once. After forEach or collect, create a new stream from the source if you need another pass.
Try It Yourself
Exercise: From List.of(2, 5, 8, 11), keep numbers greater than 5, double them, and print each with forEach.
Show solution
import java.util.List;
List.of(2, 5, 8, 11).stream()
.filter(n -> n > 5)
.map(n -> n * 2)
.forEach(n -> System.out.print(n + " "));
System.out.println();filter keeps 8 and 11; map turns them into 16 and 22.
Key Takeaways
- Streams are pipelines: intermediate ops transform, terminal ops trigger execution.
filter,map,sorted,distinctare common steps.collect,forEach,sum, and match methods finish the pipeline.- A stream is single-use — stream again from the collection for another pass.
Worked examples
The short listings above are there so you can see the grammar. The programs here use the same statements on quantities that already have units: a speed, a pH, a count of bases. They are classroom numbers. Air resistance is ignored. g is 9.81 m/s² unless a line says otherwise.
Open them in the Java editor at /java/try. Change one measurement and check whether the result still has the right unit.
Statistics
Mean via stream
A stream maps values to a double average without an explicit loop. mapToInt then average returns an OptionalDouble.
x̄ = (Σxᵢ) / n
Example
import java.util.List;
public class Main {
public static void main(String[] args) {
double mean = List.of(2, 4, 6, 8).stream()
.mapToInt(n -> n)
.average()
.orElse(0);
System.out.println(mean);
}
}Maths
Product with reduce
reduce folds a list into one value. Starting from 1, multiply each term to get a product.
Π xᵢ
Example
import java.util.List;
public class Main {
public static void main(String[] args) {
int product = List.of(2, 3, 4).stream().reduce(1, (a, b) -> a * b);
System.out.println(product);
}
}