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Collectors class in Java

Theory

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Collectors class in Java

Collectors class 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 Collectors class: 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 Collectors class 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. Collectors class in Java — 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 Collectors class.

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]
  }
}

Collectors class in Java — output: [4, 16]. 1..5 → keep evens 2,4 → square → 4 and 16.

Short notes

DefCollectors class is part of modern Java (generics / Java 8 style).
RuleCollectors class — lambda = (args) -> body on a functional interface.
UseCollectors class — streams filter/map/collect. Generics = List<String>.
TrapCollectors class — 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] } }

Collectors class in Java

Example

Short notes

Questions

Collectors class in Java

Example

Short notes

Questions