Generics in Java

This article describes Generics in Java

11/14/20234 min read

Introduction to Generics

Generics in Java is a powerful feature introduced in Java 5 to enhance the type safety and reusability of code. They allow developers to write classes, interfaces, and methods that operate on different data types while providing compile-time type checking. Generics enable the creation of classes and methods that are parameterized by one or more types, allowing for increased flexibility and code clarity.

Purpose of Generics

The primary goals of generics in Java include:

Type Safety: Generics ensure that the data types used in a class or method are known at compile time. This reduces the chances of runtime errors related to type mismatches.

Code Reusability: By using generics, you can write code that works with different data types without sacrificing type safety. This promotes the creation of more generic and reusable components.

Compile-Time Type Checking: Generics enable the compiler to perform type checking at compile time, providing early detection of potential type-related errors.

Syntax of Generics

Generic Classes

A generic class in Java is declared with one or more type parameters within angle brackets (<>). These type parameters represent the types that the class can work with. Here's a basic example of a generic class:

public class Box<T> {

private T value;

public void setValue(T value) {

this.value = value;


public T getValue() {

return value;



In this example, Box is a generic class with a type parameter T. This class can be instantiated with different data types, providing flexibility and type safety.

Generic Methods

Similar to generic classes, generic methods allow you to write methods that operate on different types. The type parameters are specified before the return type of the method. Here's an example:

public class Utils {

public <T> void printArray(T[] array) {

for (T item : array) {

System.out.print(item + " ");





In this example, the printArray method is a generic method that can print arrays of any type.

Bounded Type Parameters

Bounded type parameters restrict the types that can be used as arguments in a generic class or method. There are two types of bounded type parameters: upper-bounded and lower-bounded.

Upper-Bounded Wildcard (<? extends T>):

public class Box<T extends Number> {

// Class body


In this example, the type parameter T is bounded to be a subtype of Number or Number itself.

Lower-Bounded Wildcard (<? super T>):

public void processList(List<? super Integer> list) {

// Method body


In this example, the method accepts a list of elements of type Integer or any superclass of Integer.

Generics in Action

Let's explore some practical examples to illustrate the usage of generics in different scenarios.

Generic Classes

Consider a generic class representing a pair of elements:

public class Pair<T, U> {

private T first;

private U second;

public Pair(T first, U second) {

this.first = first;

this.second = second;


public T getFirst() {

return first;


public U getSecond() {

return second;



This Pair class can be used to create pairs of different data types:

Pair<String, Integer> person = new Pair<>("John Doe", 30);

Pair<Double, String> coordinates = new Pair<>(1345, "45.678");

Generic Methods

Consider a utility class with a generic method to find the maximum of two elements:

public class MathUtils {

public static <T extends Comparable<T>> T max(T a, T b) {

return a.compareTo(b) > 0 ? a : b;



This method can find the maximum of integers, doubles, or any other type that implements the Comparable interface:

int maxInt = MathUtils.max(5, 8);

double maxDouble = MathUtils.max(3.14, 71);

Bounded Type Parameters

Consider a method that computes the sum of a list of numbers with an upper-bounded wildcard:

public static double sumOfList(List<? extends Number> numbers) {

double sum = 0.0;

for (Number number : numbers) {

sum += number.doubleValue();


return sum;


This method can accept a list of integers, doubles, or any other type that extends Number:

List<Integer> integers = Arrays.asList(1, 2, 3);

List<Double> doubles = Arrays.asList(1, 2, 3.3);

double sumIntegers = sumOfList(integers);

double sumDoubles = sumOfList(doubles);

Type Erasure

Java uses type erasure to implement generics, which means that generic type information is erased during compilation. This is done to maintain backward compatibility with code written before the introduction of generics. While this allows for seamless integration with existing code, it also imposes some limitations.


Cannot Instantiate Generic Arrays:

// This results in a compilation error

List<String>[] arrayOfLists = new List<String>[10];

Due to type erasure, creating an array of a generic type is not allowed.

Cannot Use Primitive Types as Type Arguments:

// This results in a compilation error

Box<int> intBox = new Box<>();

Generics in Java cannot be instantiated with primitive types; instead, you should use their corresponding wrapper classes.

Type Erasure and Overloaded Methods:

public class Example {

public void process(List<String> strings) {

// Process strings


public void process(List<Integer> integers) {

// Process integers



Due to type erasure, these two methods would have the same erasure, leading to a compilation error. Overloaded methods with generic parameters can be confusing and should be avoided.

Best Practices and Considerations

Use Generics for Type Safety

Generics provide a way to make your code more type-safe by catching errors at compile time rather than runtime. Take advantage of this feature to create robust and reliable code.

Be Mindful of Type Erasure

While type erasure allows for backward compatibility, it also imposes limitations. Understanding these limitations and working within them is crucial for effective use of generics.

Consider Bounded Type Parameters

Bounded type parameters can enhance the flexibility of your generic classes and methods while maintaining type safety.

Use them when appropriate to provide more refined constraints on the types used.


Generics in Java play a crucial role in enhancing the type safety and flexibility of code. By allowing classes and methods to work with different data types while providing compile-time type checking, generics contribute to the creation of more reusable and error-resistant software. While type erasure introduces certain limitations, the benefits of generics far outweigh these constraints. As you continue your journey in Java development, a solid understanding of generics will empower you to write more robust and scalable code.

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