java-generics

What this skill does

# Java Generics

Master Java's generics system for writing type-safe, reusable code with
compile-time type checking, generic classes, methods, wildcards, and
type bounds.

## Introduction to Generics

Generics enable types to be parameters when defining classes, interfaces,
and methods, providing compile-time type safety.

**Basic generic class:**

```java
public class Box<T> {
    private T content;

    public void set(T content) {
        this.content = content;
    }

    public T get() {
        return content;
    }

    public static void main(String[] args) {
        // Type-safe box for String
        Box<String> stringBox = new Box<>();
        stringBox.set("Hello");
        String value = stringBox.get(); // No casting needed

        // Type-safe box for Integer
        Box<Integer> intBox = new Box<>();
        intBox.set(42);
        Integer number = intBox.get();
    }
}
```

**Generic with multiple type parameters:**

```java
public class Pair<K, V> {
    private K key;
    private V value;

    public Pair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public K getKey() { return key; }
    public V getValue() { return value; }

    public static void main(String[] args) {
        Pair<String, Integer> pair = new Pair<>("age", 30);
        String key = pair.getKey();
        Integer value = pair.getValue();
    }
}
```

## Generic Methods

Generic methods can be defined independently of generic classes.

**Basic generic method:**

```java
public class GenericMethods {
    // Generic method
    public static <T> void printArray(T[] array) {
        for (T element : array) {
            System.out.println(element);
        }
    }

    // Generic method with return type
    public static <T> T getFirst(T[] array) {
        if (array.length > 0) {
            return array[0];
        }
        return null;
    }

    public static void main(String[] args) {
        String[] strings = {"a", "b", "c"};
        Integer[] numbers = {1, 2, 3};

        printArray(strings);  // T inferred as String
        printArray(numbers);  // T inferred as Integer

        String first = getFirst(strings);
        Integer firstNum = getFirst(numbers);
    }
}
```

**Generic method with multiple type parameters:**

```java
public class MultiplTypeParams {
    public static <K, V> Map<K, V> createMap(K key, V value) {
        Map<K, V> map = new HashMap<>();
        map.put(key, value);
        return map;
    }

    public static <T, R> R transform(T input, Function<T, R> transformer) {
        return transformer.apply(input);
    }

    public static void main(String[] args) {
        Map<String, Integer> map = createMap("count", 10);

        String result = transform(42, num -> "Number: " + num);
        // Result: "Number: 42"
    }
}
```

## Bounded Type Parameters

Type bounds restrict the types that can be used as type arguments.

**Upper bounded type parameters:**

```java
public class UpperBound {
    // T must be Number or subclass of Number
    public static <T extends Number> double sum(List<T> numbers) {
        double total = 0;
        for (T num : numbers) {
            total += num.doubleValue();
        }
        return total;
    }

    // Multiple bounds
    public static <T extends Comparable<T> & Serializable> T max(T a, T b) {
        return a.compareTo(b) > 0 ? a : b;
    }

    public static void main(String[] args) {
        List<Integer> integers = List.of(1, 2, 3, 4, 5);
        double sum = sum(integers); // 15.0

        List<Double> doubles = List.of(1.5, 2.5, 3.5);
        double doubleSum = sum(doubles); // 7.5

        String maxStr = max("apple", "banana"); // "banana"
    }
}
```

**Class with bounded type parameter:**

```java
public class NumberBox<T extends Number> {
    private T number;

    public NumberBox(T number) {
        this.number = number;
    }

    public double doubleValue() {
        return number.doubleValue();
    }

    public boolean isZero() {
        return number.doubleValue() == 0.0;
    }

    public static void main(String[] args) {
        NumberBox<Integer> intBox = new NumberBox<>(42);
        NumberBox<Double> doubleBox = new NumberBox<>(3.14);

        // Compile error: String is not a Number
        // NumberBox<String> stringBox = new NumberBox<>("fail");
    }
}
```

## Wildcards

Wildcards provide flexibility when working with generic types.

**Unbounded wildcard:**

```java
public class UnboundedWildcard {
    // Accept any List
    public static void printList(List<?> list) {
        for (Object elem : list) {
            System.out.println(elem);
        }
    }

    public static int size(List<?> list) {
        return list.size();
    }

    public static void main(String[] args) {
        List<String> strings = List.of("a", "b", "c");
        List<Integer> integers = List.of(1, 2, 3);

        printList(strings);
        printList(integers);

        System.out.println(size(strings));  // 3
        System.out.println(size(integers)); // 3
    }
}
```

**Upper bounded wildcard:**

```java
public class UpperBoundedWildcard {
    // Accept List of Number or any subclass
    public static double sum(List<? extends Number> numbers) {
        double total = 0;
        for (Number num : numbers) {
            total += num.doubleValue();
        }
        return total;
    }

    public static void main(String[] args) {
        List<Integer> integers = List.of(1, 2, 3);
        List<Double> doubles = List.of(1.5, 2.5);
        List<Number> numbers = List.of(1, 2.5, 3);

        System.out.println(sum(integers)); // 6.0
        System.out.println(sum(doubles));  // 4.0
        System.out.println(sum(numbers));  // 6.5
    }
}
```

**Lower bounded wildcard:**

```java
public class LowerBoundedWildcard {
    // Accept List of Integer or any superclass
    public static void addIntegers(List<? super Integer> list) {
        for (int i = 1; i <= 5; i++) {
            list.add(i);
        }
    }

    public static void main(String[] args) {
        List<Integer> integers = new ArrayList<>();
        addIntegers(integers);
        System.out.println(integers); // [1, 2, 3, 4, 5]

        List<Number> numbers = new ArrayList<>();
        addIntegers(numbers);
        System.out.println(numbers); // [1, 2, 3, 4, 5]

        List<Object> objects = new ArrayList<>();
        addIntegers(objects);
        System.out.println(objects); // [1, 2, 3, 4, 5]
    }
}
```

## PECS Principle

Producer Extends, Consumer Super - guideline for using wildcards.

**PECS in action:**

```java
public class PECSExample {
    // Producer - reading from source (extends)
    public static <T> void copy(
        List<? extends T> source,
        List<? super T> destination
    ) {
        for (T item : source) {
            destination.add(item);
        }
    }

    // Producer - extends for reading
    public static double sumNumbers(List<? extends Number> numbers) {
        double sum = 0;
        for (Number num : numbers) { // Reading (producing values)
            sum += num.doubleValue();
        }
        return sum;
    }

    // Consumer - super for writing
    public static void addNumbers(List<? super Integer> list) {
        for (int i = 1; i <= 3; i++) {
            list.add(i); // Writing (consuming values)
        }
    }

    public static void main(String[] args) {
        List<Integer> source = List.of(1, 2, 3);
        List<Number> destination = new ArrayList<>();

        copy(source, destination);
        System.out.println(destination); // [1, 2, 3]
    }
}
```

## Generic Interfaces

Interfaces can be generic, providing contracts for generic types.

**Generic interface:**

```java
public interface Repository<T, ID> {
    T findById(ID id);
    List<T> findAll();
    void save(T entity);
    void delete(ID id);
}

public class UserRepository implements Repository<User, Long> {
    private Map<Long, User> storage = new HashMap<>();

    @Override
    public