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Java Stream API

Theory

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Java Stream API

Stream API is modern Java (5/8 toolkit). Lambdas are short functions: (n) -> print. They implement a functional interface (one abstract method). Streams let you filter/map/collect a list without a manual loop.

For Stream API: generics keep types safe: List<String>. Autoboxing turns int into Integer when needed. varargs is String... args. Optional avoids some null checks — don’t overuse it.

Keep Stream API small. Don’t dump business logic inside forEach. Know one tiny example you can write from memory.

Let's take this on the board with one tiny Main class — no extra files. Java Stream API — output: [4, 16]. 1..5 → keep evens 2,4 → square → 4 and 16. Start from main, go line by line, and stop at each print. That output is the proof for Stream API.

Exam tip

Explain map/filter/collect as a 3-step stream story.

Example

import java.util.*;
import java.util.stream.*;
public class Main {
  public static void main(String[] args) {
    List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);
    List<Integer> evenSq = nums.stream()
      .filter(n -> n % 2 == 0)
      .map(n -> n * n)
      .collect(Collectors.toList());
    System.out.println(evenSq); // [4, 16]
  }
}

Java Stream API — output: [4, 16]. 1..5 → keep evens 2,4 → square → 4 and 16.

Short notes

DefStream API is part of modern Java (generics / Java 8 style).
RuleStream API — lambda = (args) -> body on a functional interface.
UseStream API — streams filter/map/collect. Generics = List<String>.
TrapStream API — huge logic inside lambdas, or forgetting generics.

import java.util.*; import java.util.stream.*; public class Main { public static void main(String[] args) { List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5); List<Integer> evenSq = nums.stream() .filter(n -> n % 2 == 0) .map(n -> n * n) .collect(Collectors.toList()); System.out.println(evenSq); // [4, 16] } }

Java Stream API

Example

Short notes

Questions

Java Stream API

Example

Short notes

Questions