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Java Stream Filter

Theory

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Java Stream Filter

Stream Filter 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 Filter: 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 Filter 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 Filter — 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 Filter.

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

Short notes

DefStream Filter is part of modern Java (generics / Java 8 style).
RuleStream Filter — lambda = (args) -> body on a functional interface.
UseStream Filter — streams filter/map/collect. Generics = List<String>.
TrapStream Filter — 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 Filter

Example

Short notes

Questions

Java Stream Filter

Example

Short notes

Questions