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Functional Interfaces in Java

Theory

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Functional Interfaces in Java

Functional Interfaces 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 Functional Interfaces: 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 Functional Interfaces 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. Functional Interfaces 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 Functional Interfaces.

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]
  }
}

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

Short notes

DefFunctional Interfaces is part of modern Java (generics / Java 8 style).
RuleFunctional Interfaces — lambda = (args) -> body on a functional interface.
UseFunctional Interfaces — streams filter/map/collect. Generics = List<String>.
TrapFunctional Interfaces — 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] } }

Functional Interfaces in Java

Example

Short notes

Questions

Functional Interfaces in Java

Example

Short notes

Questions