Introduction to Generics

Generics were introduced in Java 5 as a way to provide stronger type checking and eliminate the need for explicit type casting when using collections and other generic classes. Generics enable you to write more flexible, reusable, and type-safe code.

// Without generics
List names = new ArrayList();
names.add("Alice");
names.add("Bob");
String name = (String) names.get(0); // Explicit type casting required

// With generics
List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
String name = names.get(0); // No type casting required

Benefits of Using Generics

1. Type safety: Generics provide compile-time type checking, reducing the likelihood of runtime errors.

2. Code reusability: Generic classes and methods can be used with different data types, making them more reusable.

3. Elimination of type casts: Generics eliminate the need for explicit type casting when retrieving elements from a collection.

Generic Syntax and Type Parameters

In Java, angle brackets (<>) are used to define a generic type. The type parameter (typically T for "type") represents a placeholder for the actual type that will be used when creating an instance of the generic class.

public class Box<T> {
    private T item;

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

    public T get() {
        return item;
    }
}

Generic Classes and Interfaces

You can create generic classes and interfaces by specifying a type parameter in the class or interface definition. The type parameter can then be used as the type for fields, method arguments, and method return values.

public interface Stack<E> {
    void push(E item);
    E pop();
    E peek();
    boolean isEmpty();
}

Generic Methods

Generic methods can be defined within both generic and non-generic classes. To define a generic method, you need to declare a type parameter before the method's return type.

public class ArrayUtil {
    public static <T> void swap(T[] array, int i, int j) {
        T temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
}

Bounded Type Parameters

Bounded type parameters are used to restrict the types that can be used as arguments for a generic class or method. You can specify an upper bound using the extends keyword.

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

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

    public T get() {
        return number;
    }
}

Wildcards and Bounded Wildcards

Wildcards allow you to create more flexible methods that work with different generic types. The wildcard character ? represents an unknown type. You can use wildcards to specify upper and lower bounds using the extends and super keywords, respectively.

Upper-bounded Wildcard

Upper-bounded wildcards are used when you want to ensure that the type argument is a subtype of a specific class or interface. You can specify an upper bound using the extends keyword.

public static double sumOfList(List<? extends Number> numbers) {
    double sum = 0.0;
    for (Number number : numbers) {
        sum += number.doubleValue();
    }
    return sum;
}

Lower-bounded Wildcard

Lower-bounded wildcards are used when you want to ensure that the type argument is a supertype of a specific class or interface. You can specify a lower bound using the super keyword.

public static void addIntegersToList(List<? super Integer> numbers, int count) {
    for (int i = 0; i < count; i++) {
        numbers.add(i);
    }
}

Unbounded Wildcard

Unbounded wildcards are useful when you want to work with a collection of any type. They provide the most flexibility, as they do not restrict the type argument in any way.

public static void processElements(List<?> list) {
    for (Object item : list) {
        System.out.println(item);
    }
}

By using wildcards and bounded wildcards, you can create more flexible and versatile methods that work with a wide range of generic types, while still maintaining type safety.

Generic Inheritance and Subtyping

Generics in Java also have rules for inheritance and subtyping. A generic class or interface with a subtype parameter is not a subtype of the same class or interface with a different type parameter.

List<String> stringList = new ArrayList<>();
List<Object> objectList = stringList; // Compile-time error

Type Inference

Type inference is a feature in Java that allows the compiler to determine the appropriate generic type parameters based on the context. This feature simplifies the code by reducing the need for explicit type parameters when using generics.

Java SE 7 introduced the "diamond" operator (<>), which allows the Java compiler to infer the type arguments for a generic class instantiation based on the context.

// Before Java SE 7
Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();

// With Java SE 7 and later (diamond operator)
Map<String, List<Integer>> map = new HashMap<>();

Java SE 8 introduced improved type inference in lambda expressions and generic method invocations.

// Java SE 8: Type inference in lambda expressions
Comparator<String> comparator = (s1, s2) -> s1.length() - s2.length();

// Java SE 8: Type inference in generic method invocations
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
Collections.sort(names, Comparator.comparingInt(String::length));

Type Erasure

Type erasure is a Java compiler's process of removing generic type information during compilation, replacing type parameters with their bounds or Object if the type parameters are unbounded. Type erasure ensures that generic Java code remains compatible with older Java versions that do not support generics.

public class Box<T> {
    private T item;

    // After type erasure, it becomes:
    // public class Box {
    //     private Object item;
}

Restrictions and Limitations of Generics

1. Primitive types: Generic type parameters cannot be primitive types, such as int, double, or char. You need to use their corresponding wrapper classes instead, such as Integer, Double, or Character.

2. Static fields and methods: Type parameters cannot be used in static fields or methods within a generic class.

3. Overloading: You cannot overload methods that differ only by their generic type parameters.

4. Exceptions: Generic classes cannot extend Throwable or its subclasses, and type parameters cannot be used as exception types in a throws clause.

Practical Use Case: A Generic Data Structure

Here's an example of a generic data structure: a simple, type-safe stack implementation using generics.

public class GenericStack<E> {
    private LinkedList<E> list = new LinkedList<>();

    public void push(E item) {
        list.addFirst(item);
    }

    public E pop() {
        if (isEmpty()) {
            throw new NoSuchElementException("Stack is empty");
        }
        return list.removeFirst();
    }

    public E peek() {
        if (isEmpty()) {
            throw new NoSuchElementException("Stack is empty");
        }
        return list.getFirst();
    }

    public boolean isEmpty() {
        return list.isEmpty();
    }

    public int size() {
        return list.size();
    }
}

Advanced Topics: Type Tokens and Super Type Tokens

Type tokens and super type tokens are advanced techniques used to overcome type erasure limitations when working with generics. They involve creating instances of the java.lang.reflect.Type interface to represent generic types at runtime.

// Type token
Type type = new TypeToken<List<String>>(){}.getType();

// Super type token
Type type = new TypeToken<List<String>>(){}.getSupertype(ArrayList.class);

Sample Project: Building a Generic Repository

In this sample project, we will demonstrate the capabilities of generics by creating a simple, in-memory generic repository.

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

public class InMemoryRepository<T, ID> implements Repository<T, ID> {
    private Map<ID, T> storage = new HashMap<>();

    @Override
    public T save(T entity) {
        // You will need a way to generate or extract the ID from the entity
        // For simplicity, we assume the entity has a getId() method
        ID id = entity.getId();
        storage.put(id, entity);
        return entity;
    }

    @Override
    public T findById(ID id) {
        return storage.get(id);
    }

    @Override
    public List<T> findAll() {
        return new ArrayList<>(storage.values());
    }

    @Override
    public void deleteById(ID id) {
        storage.remove(id);
    }

    @Override
    public void deleteAll() {
        storage.clear();
    }
}

Conclusion

Generics in Java are an incredibly powerful feature that enables developers to write type-safe, reusable, and flexible code. This blog post has covered the fundamentals of generics, their syntax, benefits, and limitations. Through practical examples and a sample project, you've seen how generics can be used to create versatile data structures, methods, and interfaces. With this knowledge, you can now leverage the power of generics to write more efficient, maintainable, and robust Java applications.