Java Generics

Java Generics

1. What are generics in Java and why were they introduced?

Generics allow us to write classes, interfaces, and methods that operate on specific data types while still being reusable. They were introduced to provide type safety and avoid runtime ClassCastException.

List<String> names = new ArrayList<>();

Now only String values can be added.

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2. How do generics provide type safety?

Generics ensure the type of data is checked at compile time. This prevents adding the wrong data type into a collection.

List<Integer> list = new ArrayList<>();
list.add("Hello"); // Compile-time error → safe

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3. What is the difference between generics in compile-time vs run-time?

• Compile-time: Generics enforce type checking.
• Run-time: Generic type information is removed (due to type erasure).

List<Integer> a = new ArrayList<>();
List<String> b = new ArrayList<>();
System.out.println(a.getClass() == b.getClass()); // true

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4. Difference between List<Object> and List<?>

Feature	List<Object>	List<?>
Accepts	Any object	Any generic list
Add elements	Yes	No (except null)
Use case	Insert values	Read-only access

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5. What are bounded type parameters? Example using extends and super

Bounded type parameters restrict the type that can be used.

<T extends Number>  // Upper bound: T must be Number or its subclass
<? super Integer>   // Lower bound: Type must be Integer or its parent

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6. Meaning of <T extends Number>

T can only be:

• Number or its subclasses (Integer, Double, Float, etc.)

public class Test<T extends Number> { }

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7. When should you use ? extends and when ? super?

Use	Meaning	Purpose
? extends T	Any subclass of T	Read Only (Producer)
? super T	Any superclass of T	Write Allowed (Consumer)

Easy rule: PECS → Producer Extends, Consumer Super

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8. What is a generic method? Example

A method that defines its own type parameter.

public static <T> void print(T value) {
    System.out.println(value);
}

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9. Can we use primitive types with generics? Why not?

No, because generics work only with objects, and primitives are not objects. We use wrapper classes:

Primitive	Wrapper
int	Integer
double	Double

List<int>      // ❌
List<Integer>  // ✅

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10. Can a generic class have multiple type parameters? Example <K, V>

Yes. Commonly used in Map implementations.

class Pair<K, V> {
    K key;
    V value;
}

Pair<String, Integer> p = new Pair<>();

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