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Java Default Methods

Theory

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Java Default Methods

Default Methods 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 Default Methods: 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 Default Methods 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 Default Methods — 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 Default Methods.

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

Short notes

DefDefault Methods is part of modern Java (generics / Java 8 style).
RuleDefault Methods — lambda = (args) -> body on a functional interface.
UseDefault Methods — streams filter/map/collect. Generics = List<String>.
TrapDefault Methods — 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 Default Methods

Example

Short notes

Questions

Java Default Methods

Example

Short notes

Questions