Java Generics

Last updated on Jan 13, 2023

I. Introduction

A. Explanation of what generics in Java are

Generics in Java are a powerful feature that allows developers to create reusable classes and methods that can work with multiple types of data. They were first introduced in Java SE 5.0 and have since become an essential tool for writing efficient and maintainable code.

The main advantage of generics is that they allow developers to create classes and methods that can operate on any type of data, without having to write separate versions for each type. This makes code more flexible and easier to maintain. For example, a developer can create a single class that can be used to store a list of any type of data, such as integers, strings, or custom objects.

Generics are defined using angle brackets (< and >) and the type parameter, which is a placeholder for the actual type that will be used when the class or method is instantiated. For example, a generic class called "MyList" might be defined as:

class MyList<T> {
    private T[] data;
    // ...
}

Here, the type parameter T is used to define the type of data that the MyList class can store. When the class is instantiated, a specific type can be specified, such as:

MyList<String> myStringList = new MyList<String>();

This creates an instance of the MyList class that can only store strings.

Generics can also be used with methods. For example, a generic method called "swap" might be defined as:

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

This method can be used to swap any two elements in an array of any type. The type parameter T is used to specify the type of data in the array.

Generics in Java also include the concept of wildcards which allows the developer to specify a range of types that the class or method can work with. This is useful when working with a hierarchy of classes. For example, a method that takes a List<Number> could also take a List<Integer> because Integer is a subclass of Number.

B. Benefits of using generics

Type safety: One of the main benefits of using generics in Java is that they provide type safety. By specifying the type of data that a class or method can work with, the compiler can catch errors at compile-time, rather than at runtime. This makes code more robust and less prone to errors.
Improved code readability: Generics make the code more readable by making the intent of the code clear. The type parameter tells the developer the type of data that the class or method works with, which makes the code more self-documenting.
Reusability: Generics allow developers to create reusable classes and methods that can work with multiple types of data. This makes code more flexible and easier to maintain, since developers don't have to write separate versions for each type of data.
Performance: Using generics can also improve the performance of the code. By specifying the type of data that a class or method works with, the compiler can generate more efficient code, which can result in faster execution times.
Working with collections: One of the most common use cases for generics is working with collections. The Java Collections Framework provides a wide range of classes and interfaces that can be used to work with collections of data. By using generics, developers can create collections that can store any type of data, which makes the code more flexible and easier to maintain.
Wildcards: Generics in Java also include the concept of wildcards which allows the developer to specify a range of types that the class or method can work with. This is useful when working with a hierarchy of classes. For example, a method that takes a List<Number> could also take a List<Integer> because Integer is a subclass of Number.

II. Generic Classes

A. Syntax for declaring a generic class

Generics in Java allow developers to create reusable classes and methods that can work with multiple types of data. In this article, we will discuss the syntax for declaring a generic class in Java.

To declare a generic class in Java, you must use angle brackets (< and >) and a type parameter. The type parameter is a placeholder for the actual type that will be used when the class is instantiated. For example, a generic class called "MyGenericClass" might be defined as:

class MyGenericClass<T> {
    // class body
}

Here, the type parameter T is used to define the type of data that the MyGenericClass class can work with. When the class is instantiated, a specific type can be specified, such as:

MyGenericClass<Integer> myIntegerObject 
= new MyGenericClass<Integer>();

This creates an instance of the MyGenericClass class that can only work with integers.It's also possible to use multiple type parameters to define a generic class. For example:

It's also possible to specify a bound for the type parameter by using the "extends" keyword. For example:

class MyGenericClass {// class body}

This means that the type parameter T must be a subclass of Number.A generic class can also have generic methods, which can also be defined using type parameters. For example:

class MyGenericClass<T> {
    public T getValue() {
        // method body
    }
}

B. Example of a generic class

This class is a basic implementation of a dynamic array that can store any type of data. The type parameter T is used to define the type of data that the class can store. The class has methods to add an element, get an element at a specific index, and get the size of the list.

class MyGenericList<T> {
    private T[] data;
    private int size;

    public MyGenericList(int capacity) {
        data = (T[]) new Object[capacity];
        size = 0;
    }

    public void add(T element) {
        data[size++] = element;
    }

    public T get(int index) {
        return data[index];
    }

    public int size() {
        return size;
    }
}

To use this class, a specific type can be specified when the class is instantiated, such as

MyGenericList<Integer> myIntegerList = new MyGenericList<Integer>(10);
myIntegerList.add(5);
myIntegerList.add(7);
int element = myIntegerList.get(0); // element will be 5

MyGenericList<String> myStringList = new MyGenericList<String>(10);
myStringList.add("Hello");
myStringList.add("World");
String element = myStringList.get(1); // element will be "World"

In this example, the class MyGenericList is defined as a generic class that can store any type of data. The type parameter T is used to define the type of data that the class can store. It's also possible to use the same class for different types of data by instantiating it with different types.

C. Restrictions on types that can be used as type parameters

When declaring a generic class or method in Java, certain restrictions apply to the types that can be used as type parameters.

Primitive types: Primitive types, such as int, char, and boolean, cannot be used as type parameters. Instead, their corresponding wrapper classes, such as Integer, Character, and Boolean, must be used.
Arrays: Arrays cannot be used as type parameters because they are not objects and do not have a common superclass. Instead, the Java Collections Framework provides a wide range of classes and interfaces that can be used to work with collections of data.
Interfaces: An interface cannot be used as a type parameter unless it is bounded by a class or another interface. For example,

class MyGenericClass<T extends Comparable<T>>{ //class body}

Here T must implement the Comparable interface.

4. Generic type parameters: Generic type parameters cannot be used as type parameters for classes or methods. This is because the type parameter does not specify a concrete type, so it cannot be used to create an instance of a class or invoke a method.

Anonymous classes: Anonymous classes cannot be used as type parameters because they do not have a name that can be used to refer to them.
A final class can't be used as a type parameter, a final class is a class that cannot be sub classed and therefore cannot be used as a type parameter.

III. Generic Methods

A. Syntax for declaring a generic method

One of the key features of generics is the ability to declare generic methods. In this article, we will discuss the syntax for declaring a generic method in Java.

To declare a generic method in Java, you must use angle brackets (< and >) and a type parameter. The type parameter is a placeholder for the actual type that will be used when the method is invoked. For example, a generic method called "swap" might be defined as:

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

Here, the type parameter T is used to define the type of data that the swap method can work with. When the method is invoked, a specific type can be specified, such as:

Integer[] myIntegerArray = {1, 2, 3, 4}; MyClass.swap(myIntegerArray, 0, 1);

This invokes the swap method on an integer array, the type parameter T is inferred as Integer.

A generic method can also be declared within a generic class. In this case, the type parameter of the method can be inferred from the class definition. For example:

class MyGenericClass<T> {
    public T getValue() {
        // method body
    }
}

It's also possible to specify a bound for the type parameter by using the "extends" keyword. For example:

class MyClass {
    public static <T extends Number> T sum(T[] array) {
        T result = 0;
        for (T value : array) {
            result += value;
        }
        return result;
    }
}

This means that the type parameter T must be a subclass of Number.

B. Example of a generic method

Here is an example of a generic method called "printArray" that can print the elements of an array of any type:

class MyClass {
    public static <T> void printArray(T[] array) {
        for (T element : array) {
            System.out.print(element + " ");
        }
        System.out.println();
    }
}

This method takes an array of any type as its parameter and prints each element of the array. The type parameter T is used to define the type of data that the array can store. When the method is invoked, a specific type can be specified, such as:

Integer[] myIntegerArray = {1, 2, 3, 4};
MyClass.printArray(myIntegerArray); // prints "1 2 3 4"

In this example, the method printArray is defined as a generic method that can print the elements of an array of any type. The type parameter T is used to define the type of data that the array can store. It's possible to use the same method for different types of data by invoking it with different types of arrays.

C. Type inference in generic methods

Type inference in generic methods is a feature in Java that allows the compiler to automatically determine the type of a generic method's type parameter based on the method's arguments. This feature was introduced in Java SE 7, and it makes it possible to invoke a generic method without explicitly specifying the type parameter.

For example, consider the following generic method:

class MyClass {
    public static <T> T getMiddleElement(T[] array) {
        return array[array.length / 2];
    }
}

This method takes an array of any type as its parameter and returns the middle element of the array. To invoke this method with an integer array, you can do this:

Integer[] myIntegerArray = {1, 2, 3, 4};
Integer middle = MyClass.getMiddleElement(myIntegerArray);

With type inference, you can invoke the same method without specifying the type parameter:

Integer middle = MyClass.getMiddleElement(myIntegerArray);

In this example, the compiler can infer the type parameter T as Integer based on the type of the argument passed to the method.

It's also possible to use type inference with multiple type parameters, for example:

class MyClass {
    public static <T, S> void copy(T[] source, S[] destination) {
        for (int i = 0; i < source.length; i++) {
            destination[i] = (S) source[i];
        }
    }
}

This method takes two arrays as its parameters, one source array and one destination array. The type parameter T is used to define the type of data in the source array and the type parameter S is used to define the type of data in the destination array. The method copies the elements of the source array to the destination array.

It's also possible to use type inference with the wildcard character "?" which can be used to specify a range of types that the class or method can work with.

IV. Wildcard Types
A. Explanation of wildcard types

Wildcard types in Java are a feature of generics that allow developers to specify a range of types that a class or method can work with. They are represented by the "?" character and are used to indicate that a type parameter can be any type. Wildcard types are useful when working with a hierarchy of classes.

There are two types of wildcards:

Upper bound wildcards: represented by the "? extends" syntax. These are used to specify that a type parameter can be any type that is a subclass of a specific class or interface. For example, a method that takes a List<? extends Number> could also take a List<Integer> because Integer is a subclass of Number.

public static double sum(List<? extends Number> list) {
    double sum = 0;
    for (Number n : list) {
        sum += n.doubleValue();
    }
    return sum;
}

Lower bound wildcards: represented by the "? super" syntax. These are used to specify that a type parameter can be any type that is a superclass of a specific class or interface. For example, a method that takes a List<? super Integer> could also take a List<Number> because Integer is a subclass of Number.

public static void copy(List<? super Integer> dest, List<? extends Integer> src) {
    for (Integer i : src) {
        dest.add(i);
    }
}

It's also possible to use a wildcard with no bounds, this is known as an unbounded wildcard.

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

B. Upper bounded wildcards

Upper bounded wildcards in Java are a feature of generics that allow developers to specify that a type parameter can be any type that is a subclass of a specific class or interface. They are represented by the "? extends" syntax and are used to indicate that a type parameter can be any type that is a subclass of a specific class or interface.

For example, consider the following method:

public static double sum(List<? extends Number> list) {
    double sum = 0;
    for (Number n : list) {
        sum += n.doubleValue();
    }
    return sum;
}

This method takes a List of any type that is a subclass of Number as its parameter, and it sums all the elements of the list. You can call this method with a list of any type that extends Number. For example:

List<Integer> integers = Arrays.asList(1, 2, 3, 4);
List<Double> doubles = Arrays.asList(1.0, 2.0, 3.0, 4.0);
double intSum = sum(integers);
double doubleSum = sum(doubles);

The upper bound wildcard allows the developer to write a method that can work with a variety of types while still maintaining type safety.

It's also possible to use an upper bound wildcard with a class definition. For example, consider the following class:

class MyClass<T extends Number> {
    // class body
}

In this example, the class MyClass can work with any type that is a subclass of Number.

C. Lower bounded wildcards

Lower bounded wildcards in Java are a feature of generics that allow developers to specify that a type parameter can be any type that is a superclass of a specific class or interface. They are represented by the "? super" syntax and are used to indicate that a type parameter can be any type that is a superclass of a specific class or interface.

For example, consider the following method:

public static void copy(List<? super Integer> dest, List<? extends Integer> src) {
	for (Integer i : src){
    	dest.add(i);
    }
 }

This method takes two lists as its parameters: a destination list and a source list. The destination list can be of any type that is a superclass of Integer, while the source list can be of any type that is a subclass of Integer. It copies the elements of the source list to the destination list.

You can call this method with lists of different types, for example:

List integers = Arrays.asList(1, 2, 3, 4);
List numbers = new ArrayList<>();
copy(numbers, integers);

The lower bound wildcard allows the developer to write a method that can work with a variety of types while still maintaining type safety.

It's also possible to use a lower bound wildcard with a class definition. For example, consider the following class:

class MyClass<T super Number> {
    // class body
}

In this example, the class MyClass can work with any type that is a super class of Number.

D. Unbounded wildcards

Unbounded wildcards in Java are a feature of generics that allow developers to specify that a type parameter can be any type without specifying a specific class or interface. They are represented by the "?" character and are used to indicate that a type parameter can be any type.

For example, consider the following method:

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

This method takes a list of any type as its parameter and prints all the elements of the list. You can call this method with a list of any type:

List<Integer> integers = Arrays.asList(1, 2, 3, 4);
List<String> strings = Arrays.asList("Hello", "World");
print(integers);
print(strings);

The unbounded wildcard allows the developer to write a method that can work with a variety of types without specifying a specific class or interface.

It's also possible to use an unbounded wildcard with a class definition, for example:

class MyClass<T> {
    public void add(List<?> list) {
        // method body
    }
}

In this example, the method add can work with a list of any type.

However, it's important to note that, while unbounded wildcards make it easy to write code that can work with a variety of types, they also make it harder to reason about the types of data that are being passed to a method or class, and it can make the code less type-safe.

V. Type Erasure

A. Explanation of type erasure

Type erasure is a technique used by the Java compiler to implement generics. The Java Virtual Machine (JVM) does not have native support for generics, so the compiler replaces all generic type information with non-generic type information, such as the Object class, before the code is executed. This process is called type erasure.

For example, consider the following generic class:

class MyGenericClass<T> {
    private T value;
    public T getValue() {
        return value;
    }
    public void setValue(T value) {
        this.value = value;
    }
}

During compilation, the compiler replaces the type parameter T with the Object class. The resulting byte code looks like this:

class MyGenericClass {
    private Object value;
    public Object getValue() {
        return value;
    }
    public void setValue(Object value) {
        this.value = value;
    }
}

Type erasure also applies to generic methods. For example, consider the following generic method:

public static <T> void printArray(T[] array) {
    for (T element : array) {
        System.out.print(element + " ");
    }
    System.out.println();
}

During compilation, the compiler replaces the type parameter T with the Object class. The resulting byte code looks like this:

public static void printArray(Object[] array) {
    for (Object element : array) {
        System.out.print(element + " ");
    }
    System.out.println();
}

This process of type erasure has some implications for the use of generics in Java. For example, since the type parameter is replaced with the Object class, it's not possible to use primitive types as type parameters. Additionally, the type parameter information is not available at runtime, so it's not possible to use reflection to determine the type of a generic class or method.

B. How type erasure affects generic code

Type erasure affects generic code in a few ways:

Type Safety: Since the type parameter information is erased during compilation, the JVM cannot enforce type safety at runtime. This means that it's possible to pass an incompatible type to a generic class or method, and a ClassCastException will only be thrown at runtime.
Reflection: The type parameter information is not available at runtime, so it's not possible to use reflection to determine the type of a generic class or method. For example, the following code will not work:

MyGenericClass<Integer> myObject = new MyGenericClass<Integer>();
Class<?> clazz = myObject.getClass();

This is because myObject.getClass() will return the raw type MyGenericClass.class instead of MyGenericClass<Integer>.class

Primitive types: Since the type parameter is replaced with the Object class during type erasure, it's not possible to use primitive types as type parameters. For example, the following code will not work:

MyGenericClass<int> myObject = new MyGenericClass<int>();

Arrays: Generics and arrays do not mix well in Java. When creating a generic array, it's possible to create an array of a non-reifiable type, which can lead to a runtime exception.

List<Integer>[] array = new List<Integer>[10];
//compile-time error

Overriding and Overloading: Java does not distinguish between a method that is overridden and a method that is overloaded. This can lead to unexpected behaviour when using generics with method overriding and overloading.

C. How to handle type erasure in generic code

There are a few ways to handle type erasure in generic code:

Use bounds: By using bounds, you can specify the types that a class or method can work with. For example, you can use the "extends" keyword to specify that a type parameter must be a subclass of a specific class, or the "super" keyword to specify that a type parameter must be a superclass of a specific class.

class MyClass<T extends Number> {
    // class body
}

Use wrapper classes: Instead of using primitive types, you can use wrapper classes. For example, use Integer instead of int.
Use a type token: A type token is a way of encapsulating a type in an object, so that it can be passed around as a value. You can use a class like TypeReference from the library Gson or Class from java.lang.

class MyClass<T> {
    private final Type type;
    public MyClass() {
        this.type = new TypeReference<T>() {}.getType();
    }
}

3. Use the Class<T> parameter: You can pass an explicit Class<T> parameter to your class or method and use it to obtain the class object of the type parameter.

class MyClass {
	public MyClass(Class clazz) {
    // use clazz}
}

4. Use the @SuppressWarnings("unchecked") annotation: This annotation can be used to suppress the warning that the compiler generates when it encounters a type

VI. Conclusion

A. Summary of key points

Generics in Java is a feature that allows developers to write code that can work with a variety of types while still maintaining type safety.
The Java Virtual Machine (JVM) does not have native support for generics, so the compiler replaces all generic type information with non-generic type information, such as the Object class, before the code is executed. This process is called type erasure.
The type parameter information is not available at runtime, so it's not possible to use reflection to determine the type of a generic class or method.
Type erasure affects generic code by removing the type information during compilation, which makes it harder to enforce type safety, and makes it difficult to use reflection, primitive types and arrays with generics.
To handle type erasure in generic code, developers can use bounds, wrapper classes, type tokens, Class<T> parameter and @SuppressWarnings("unchecked") annotation.

B. Best practices for using generics in Java

Here are some best practices for using generics in Java:

Use bounds: Use the "extends" keyword to specify that a type parameter must be a subclass of a specific class, or the "super" keyword to specify that a type parameter must be a superclass of a specific class. This helps to ensure type safety and makes the code more readable.
Prefer interfaces over classes: When defining a generic type, use interfaces instead of classes. Interfaces provide a more flexible way to define the behavior of a class, and they can be implemented by multiple classes.
Avoid using raw types: Raw types are non-generic classes or interfaces that are used in place of a generic type. Avoid using raw types as they can lead to type safety issues and unexpected behavior.
Use the "? extends" and "? super" wildcards judiciously: While wildcards are useful for making code more flexible and maintainable, they can also make the code more difficult to understand. Use them only when necessary and make sure to document their use.
Be mindful of the limitations of type erasure: Keep in mind that the type parameter information is not available at runtime and that it's not possible to use reflection to determine the type of a generic class or method.
Avoid creating generic arrays: Generics and arrays do not mix well in Java, it's possible to create an array of a non-reifiable type, which can lead to a runtime exception.
Be careful when overriding and overloading methods: Java does not distinguish between a method that is overridden and a method that is overloaded. This can lead to unexpected behavior when using generics with method overriding and overloading.
Keep it simple: Avoid using complex generic constructs unless they are absolutely necessary. Simple, easy-to-understand code is often more maintainable and less prone to errors.

In conclusion, following best practices when using generics in Java can help to ensure that your code is type-safe, maintainable, and easy to understand. Use bounds, interfaces, avoid raw types, be mindful of type erasure, be careful when overriding and overloading methods and keep it simple.

C. Additional resources for learning more about generics in Java.

Here are some additional resources for learning more about generics in Java:

Oracle's official Java tutorial on generics: This tutorial provides a comprehensive introduction to generics in Java, including examples and explanations of key concepts such as type parameters, bounds, and wildcards. https://docs.oracle.com/en/java/javase/14/docs/tutorial/java/generics/index.html
Java Generics and Collections by Maurice Naftalin and Philip Wadler: This book provides a detailed examination of generics and collections in Java, including in-depth coverage of type erasure and best practices for using generics effectively.
Effective Java, Third Edition by Joshua Bloch: This book includes a section on generics that provides best practices for using generics in Java, including how to avoid common pitfalls and how to make the most of the features of generics.
Java Generics FAQs: This is a collection of frequently asked questions about generics in Java, along with answers and explanations. It's a great resource for understanding the intricacies of generics in Java. https://docs.oracle.com/en/java/javase/14/docs/technotes/guides/language/generics.html
Java Generics and Collections by Neal Gafter: This is a series of blog posts that cover a wide range of topics related to generics in Java, including advanced uses of wildcards and how to work with collections using generics. https://gafter.blog/2006/12/11/reified-generics-in-java-se-6/
Java Generics and Type Inference by Angelika Langer: This article provides a detailed explanation of the type inference feature in Java and how it is used with generics. https://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html

These resources will help you gain a deeper understanding of generics in Java, and will provide practical guidance on how to use them effectively in your own code.

I. Introduction

A. Explanation of what generics in Java are

B. Benefits of using generics

II. Generic Classes

A. Syntax for declaring a generic class

B. Example of a generic class

C. Restrictions on types that can be used as type parameters

III. Generic Methods

A. Syntax for declaring a generic method

B. Example of a generic method

C. Type inference in generic methods

IV. Wildcard TypesA. Explanation of wildcard types

B. Upper bounded wildcards

C. Lower bounded wildcards

D. Unbounded wildcards

V. Type Erasure

A. Explanation of type erasure

B. How type erasure affects generic code

C. How to handle type erasure in generic code

VI. Conclusion

A. Summary of key points

B. Best practices for using generics in Java

C. Additional resources for learning more about generics in Java.

IV. Wildcard Types
A. Explanation of wildcard types