Generics

Analogy: label

Generics are like Labels on Boxes.

If you label a box "Books", you shouldn't be able to put a "Sandwich" in it.

Without Generics, every box is a "Mystery Box" (Object), and you might get a nasty surprise when you reach in!

The Magic Box

Define the type of the box (<T>) and see what fits.

Box<String>: Only accepts text.
Box<Integer>: Only accepts numbers.
Box<Fruit>: Only accepts fruit.

Magic Box (Generics)

1. Define Box Type <T>

2. Try to Insert Item

Box<String>

The Code

Use angle brackets <T> to define a generic type.

Java Example

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// A Generic Class
class Box<T> {
    private T content;
    public void set(T content) { this.content = content; }
    public T get() { return content; }
}

Box<String> stringBox = new Box<>();
stringBox.set("Hello"); // OK
// stringBox.set(123); // Compile Error!

Advanced Concepts

Bounded Types

Sometimes you want to restrict what types can be used.

Upper Bound: <T extends Number> (Must be Number or subclass).

Java Example

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// Only accepts Numbers (Integer, Double, etc.)
class Calculator<T extends Number> {
    // ...
}

Wildcards

When you don't know the exact type.

Unbounded: <?> (Any type).
Upper Bounded: <? extends Number> (Number or subclass).
Lower Bounded: <? super Integer> (Integer or superclass).

Java Example

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// Wildcard: ? means "Any Type"
void printBox(Box<?> box) {
    System.out.println(box.get());
}