5. Java Generics, Object Methods, Lambdas & Streams
Chapters 17-20: Generic Classes/Wildcards, equals & hashCode contract, Lambdas, Functional Interfaces & Stream API
Module 1: Generics, Wildcards & equals/hashCode Contract
Achieving compile-time type safety and maintaining HashMap equality contracts.
🇬🇧 English Definition
Generics provide compile-time type safety. If two objects are equal according to equals(), they MUST return the same hashCode() to ensure correct HashSet/HashMap behavior.
🇮🇳 Hinglish Definition
Generics compile-time par type safety dete hain. Map/Set me correct working ke liye equals() aur hashCode() dono consistent hone chahiye.
Core Concepts & Rules
- Generic Class: class Box
{ private T value; } - hashCode Contract: Equal objects must have identical hashCodes.
🔍 Line-by-Line Code Breakdown
- Line 1-5: Generic Box
class storing value of parameterized type T. - Line 8: Instantiating Box
with type safety.
🎯 Interview Focus & Questions
- Q: What is the equals and hashCode contract?
A: If two objects are equal via equals(), their hashCode() values MUST be equal.
class Box<T> {
private T value;
public void set(T value) { this.value = value; }
public T get() { return value; }
}
public class GenericsDemo {
public static void main(String[] args) {
Box<String> b = new Box<>();
b.set("Java Generics");
System.out.println(b.get());
}
}Module 2: Lambda Expressions & Stream API Processing
Functional programming pipelines using filter, map, collect, reduce, and distinct operations.
🇬🇧 English Definition
A Lambda expression is a concise implementation of a Functional Interface (@FunctionalInterface). Streams provide a declarative pipeline to filter, transform, collect, and reduce sequences of data.
🇮🇳 Hinglish Definition
Lambda expression anonymous behavior ko short form me likhti hai. Stream API collection data ko functional pipeline through filter, map aur collect karti hai.
Core Concepts & Rules
- filter: Keeps elements matching predicate (n -> n % 2 == 0).
- map: Transforms each element into another value (String::toUpperCase).
- collect: Accumulates results into a List or Set (Collectors.toList()).
🔍 Line-by-Line Code Breakdown
- Line 2: List.of(1, 2, 3, 4) creates immutable list.
- Line 3-5: Stream pipeline filters even numbers, squares them, and prints results.
🎯 Interview Focus & Questions
- Q: Intermediate vs Terminal Stream operations?
A: Intermediate operations (filter, map) are lazy and return Streams; Terminal operations (collect, forEach) trigger execution.
import java.util.List;
public class StreamDemo {
public static void main(String[] args) {
List<Integer> nums = List.of(1, 2, 3, 4, 5);
nums.stream()
.filter(n -> n % 2 == 0)
.map(n -> n * n)
.forEach(System.out::println);
}
}