How to use Generics in Java? [In-Depth Tutorial]


Written by - Bashir Alam
Reviewed by - Deepak Prasad

Generics is a feature of Java that allows you to write code that can work with any data type. It was introduced in Java 5 to provide type safety and eliminate the need for type casting. With Generics, you can write code that is more flexible, reusable, and easier to read and maintain.

The guide will begin by introducing the basics of generics, including the syntax and its advantages. Then, it will delve deeper into the various ways in which generics can be used, including defining generic classes, creating generic methods, and implementing interfaces using generics. It will also cover the use of wildcard types, bounded type parameters, and type erasure, among other topics. By the end of this guide, you should have a solid understanding of how to use generics in Java to write more flexible and efficient code.

 

What is Generics in Java?

Generics is a feature in Java that allows you to write code that can work with different types of objects while maintaining type safety. It was introduced in Java 5 to improve code reusability, readability, and reliability.

Before Generics, Java programmers used raw types, which were classes that could hold objects of any type. Raw types lacked type safety because they could store any type of object, which could lead to runtime errors if the wrong type of object was stored in the container. Generics provide a way to specify the type of objects that a class can hold or manipulate at compile time, which eliminates the need for runtime type casting.

The syntax of Generics in Java involves the use of type parameters. A type parameter is a placeholder for a type that is specified when an instance of a generic class or method is created. The syntax for defining a generic class is:

public class MyClass<T> {
  // Class implementation
}

In this example, the type parameter T can be replaced with any type when an instance of the class is created. For example:

MyClass<String> myStringClass = new MyClass<String>();

Here, the type parameter T is replaced with the type String.

Generics can also be used with methods. The syntax for defining a generic method is similar to that of a generic class, except that the type parameter is defined before the return type of the method:

public <T> T myGenericMethod(T parameter) {
  // Method implementation
}

In this example, the type parameter T is defined before the return type of the method. This allows the method to accept any type of object as a parameter and return an object of the same type.

 

Benefits of using Generics in Java

Generics is a powerful feature in Java that provides several benefits, including:

  1. Type safety: One of the main benefits of Generics is that it provides type safety. By using Generics, you can ensure that the correct type of object is stored in a container or passed as a parameter to a method. This helps to eliminate runtime errors that can occur when the wrong type of object is used.
  2. Code reusability: Generics allow you to write code that can work with different types of objects. This reduces the need for duplicate code and makes it easier to write and maintain software. With Generics, you can write a single piece of code that can work with multiple types of objects.
  3. Readability: Generics make code easier to read and understand. By providing a clear indication of what types of objects a class or method is designed to work with, Generics makes it easier for developers to understand how to use the code.
  4. Performance: Generics can improve performance by eliminating the need for typecasting at runtime. Without Generics, you would need to cast objects to their correct types, which can be slow and reduce the performance of your code. With Generics, the correct type is specified at compile time, eliminating the need for runtime type casting.
  5. Code maintainability: Generics make code more maintainable. By reducing the amount of duplicate code and making it easier to understand, Generics can make it easier to maintain software over time. This can lead to a reduction in the amount of time and effort required to maintain software and can improve the overall quality of the code.

 

Wildcards in Generics in Java

Wildcards in Generics is a feature in Java that provides a flexible way of defining generic types that can work with multiple types. Wildcards are represented by the question mark symbol "?", and they allow you to define generic types that can work with any type that meets certain criteria.

There are two types of wildcards in Java:

 

Upper-bounded wildcards

Upper-bounded wildcards allow you to define generic types that can work with any type that extends a particular class or implements a particular interface. This is useful when you want to define a generic type that can work with any type that is a subclass of a particular class or implements a particular interface.

Here's an example of an upper-bounded wildcard:

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

In this example, the upper bounded wildcard "? extends Number" allows you to define a generic type that can work with any type that extends the Number class, such as Integer, Double, or Float.

 

Lower bounded wildcards

Lower bounded wildcards allow you to define generic types that can work with any type that is a superclass of a particular class. This is useful when you want to define a generic type that can work with any type that is a superclass of a particular class.

Here's an example of a lower-bounded wildcard:

public static void addNumbers(List<? super Integer> list) {
    list.add(10);
    list.add(20);
    list.add(30);
}

In this example, the lower bounded wildcard "? super Integer" allows you to define a generic type that can work with any type that is a superclass of the Integer class, such as a Number or Object.

 

What is the difference between a wildcard and a type parameter in Java generics?

In Java Generics, both type parameters and wildcards are used to provide type safety and flexibility to classes and methods. However, there is a difference between the two.

A type parameter is a placeholder for a specific type. It is declared using the <T> syntax, where T can be replaced with any valid identifier. The type parameter can then be used in the class or method to represent a specific type. For example, the following class uses a type parameter to represent a list of elements of type T:

public class MyList<T> {
    private List<T> elements;
    //...
}

In the above example, T is a type parameter that represents the element type of the list. When an instance of MyList is created, the type argument for T is specified, such as MyList<Integer> list = new MyList<>().

A wildcard, on the other hand, is a placeholder for an unknown type or a set of types. It is declared using the ? syntax. There are two types of wildcards in Java: the unbounded wildcard and the bounded wildcard.

The unbounded wildcard represents an unknown type and is denoted by the ? symbol. It is used when the specific type is not important, but type safety is still desired. For example, the following method accepts a list of any type:

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

The above method accepts a list of any type using the unbounded wildcard. This allows it to be used with lists of any type, such as List<Integer>, List<String>, etc.

The bounded wildcard is used to restrict the type of a wildcard to a specific class or interface, denoted by <? extends Class> or <? super Class>. For example, the following method accepts a list of elements that extend Number:

public static void sumList(List<? extends Number> list) {
    double sum = 0;
    for (Number n : list) {
        sum += n.doubleValue();
    }
    System.out.println("Sum: " + sum);
}

In the above example, the bounded wildcard <? extends Number> restricts the list to contain only elements that extend Number, such as Integer, Double, etc.

 

What is type erasure in Java generics, and how does it work?

Type erasure is a process in which the generic type information is removed at runtime, making the code compatible with pre-existing Java code that does not support generics. This is done to maintain backward compatibility with older Java versions. The compiler replaces type parameters with their upper bounds, and replaces wildcard types with their upper or lower bounds as necessary.

For example, consider the following generic class:

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

When this class is compiled, the compiler replaces the type parameter T with its upper bound Object, resulting in the following code:

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

As you can see, the type parameter T has been replaced with Object. This is why, at runtime, the type information for T is not available. This can cause issues if you need to access the type information at runtime, such as when using reflection.

Type erasure also affects the use of generics with inheritance and polymorphism. For example, consider the following code:

List<Integer> myList = new ArrayList<>();
List rawList = myList;

In the above code, myList is a list of integers, but when it is assigned to the raw rawList variable, the type information is lost. This means that if you try to retrieve an element from rawList, you will get a raw Object instead of an Integer.

 

Restrictions and Limitations of Generics

While Generics provides several benefits, there are also some restrictions and limitations that developers need to be aware of when using Generics in Java. Here are some of the main restrictions and limitations:

  • Type erasure: One of the main limitations of Generics is that it uses type erasure. This means that the type information is removed at compile time and is not available at runtime. This can limit the ability to perform certain operations at runtime, such as determining the type of an object.
  • Cannot use primitive types: Generics cannot be used with primitive types such as int, float, or boolean. This is because Generics require objects, and primitive types are not objects in Java.
  • Cannot create instances of type parameters: It is not possible to create instances of type parameters. This means that you cannot create an array of a generic type or create a new instance of a generic class with a specific type parameter.
  • Cannot use static fields or methods with type parameters: Generics cannot be used with static fields or methods that use type parameters. This is because static fields and methods are associated with a class, and type parameters are associated with instances of a class.
  • Cannot overload methods based on type parameters: It is not possible to overload methods based on their type parameters. This is because type erasure removes the type information at compile time, so the methods would have the same signature at runtime.
  • Type parameter bounds: Type parameters can have bounds that restrict the types that can be used with the parameter. However, this can limit the flexibility of the code and make it more difficult to write generic code that works with a wide range of types.

 

Summary

Java Generics is a feature of the Java programming language that allows classes and methods to be parameterized with a type. It was introduced in Java 5 to provide stronger type safety and to make the code more reusable.

The main benefit of generics is that they allow the type of data to be specified at compile-time, rather than at runtime. This makes the code more type-safe and less prone to runtime errors. Generics also allow for more flexible and reusable code, since classes and methods can be parameterized with any type.

In summary, Java Generics is a powerful feature that provides stronger type safety and more flexibility in the Java programming language. Generics are implemented using type parameters and wildcard types, which allow classes and methods to be parameterized with any type. By using generics, developers can create more reusable and type-safe code that is less prone to runtime errors.

 

Further Readings

Generics in Java

 

Bashir Alam

He is a Computer Science graduate from the University of Central Asia, currently employed as a full-time Machine Learning Engineer at uExel. His expertise lies in OCR, text extraction, data preprocessing, and predictive models. You can reach out to him on his Linkedin or check his projects on GitHub page.

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