Generics in Java

Generics in Java are a powerful feature introduced in Java 5 that enables developers to write flexible, type-safe, robust, and reusable code. By allowing classes, interfaces, and methods to operate on typed parameters, the need for explicit type casting is eliminated by generics, and this reduces the chances of runtime errors such as ClassCastException.

This blog will cover topics like what generics are, why they are used, their features, advantages and limitations, best practices, and how they are compared with similar concepts like C++ templates.

Table of Contents:

• What is Generic in Java?
• Why Use Generics in Java?
• Features of Java Generics
• Types of Java Generics
• How Java Generics Work Internally
• Java Generics with Collections
• Differences Between Raw Types and Generic Types
• Java Generics and Type Safety
• Java Generics vs Templates in C++
• Types Parameter Naming Convention
• Advantages of Generics in Java
• Limitations of Generics in Java
• Common Mistakes with Java Generics
• Java Generics Best Practices
• Conclusion

What are Generics in Java?

In Java, the parameterized types that allow us to create classes, interfaces, and methods in which the type of data they are dealing with will be specified as a parameter are called Generics. Generics were added to provide compile-time type checking and remove the risk of ClassCastException, which was very common while working with collection classes. If we are using generics, then we don’t need to write multiple versions of the same code for different data types.  

Syntax of Generics in Java

The general syntax for defining generics in Java is:

class ClassName {
// T can be used as a type within this class
}

where,

• T is generally a type parameter (you can also use E, K, V, etc., based on the convention)
• You can define multiple type parameters separated by commas, like <T, U, V>.
• The actual type is specified when creating an object of the class.

Here is a Java program showing how Generics are used in Java:

Output:

Why Use Generics in Java?

• One of the main reasons to use generics is that they help catch errors too early and reduce the runtime exceptions that occur due to incorrect type casting.
• Java Generics also enhances code reusability by allowing developers to write methods, classes, or interfaces that work with any data type without rewriting the same logic multiple times for different types.
• It helps to eliminate the need for explicit type casting, which makes the code cleaner and less error-prone, and by removing the unchecked casts, the code becomes more readable and maintainable.
• These generics help to support the wildcards and bounds, which allow a certain kind of flexibility while still maintaining safety.
• They also help in better integration with functional programming, like it works seamlessly with type erasure, lambda expressions, and functional interfaces, enabling flexible APIs like map and filter.

Features of Java Generics

The key features of Java generics:

1. Type Safety: Generics enforce compile-time type checking, which helps to avoid ClassCastException at runtime.

2. Code Reusability: Generics enable us to write a single class or a method that can operate on different types, which eliminates the need for duplicating logic for each data type.

3. Elimination of Type Casting: There is no need for explicit casting when retrieving the objects from collections.

4. Generic Interfaces: Generics help to design flexible APIs and structures. Generics support the bounded types using extends (for upper bounds) or super (for lower bounds) so that the types are restricted from being used.

6. Wildcard Types: Wildcards provide greater flexibility when we are using generics with unknown or partially known types.

7. Compile-Time Checking and Better Error Messages: Generics give clearer and more helpful compiler errors, which makes debugging and development easier.

8. Integration with Collections Framework: The entire Collection framework (like list, set, and map) of Java is built using generics, which enables type-safe and flexible data structures.

9. No Overhead at Runtime (Type Erasure): Generics are implemented by Java via type erasure, which means generic type information is removed at runtime. This ensures backward compatibility with the older versions of Java but limits the features like the generic arrays and instanceof checks with generic types.

Types of Java Generics        

1. Generic Method in Java

A Generic Method in Java is a method that is defined with one or more type parameters, which allows it to operate on objects of various types while maintaining compile-time type safety. These type parameters are declared before the return type of the method and can be used as placeholders for actual data types. These can be used inside both generic and non-generic classes. Given below is the code for the same:

Output:

2. Generic Class in Java

A Generic Class is a class that defines one or more type parameters within the angle brackets as a part of its declaration. These type parameters are used to create class members (fields, methods, etc.) that can operate on objects of various types without sacrificing type safety. When an object of the class is created, the actual type is always specified, which makes the class flexible and reusable for different data types. Given below is the code for the same:

Output:

How Java Generics Work Internally

Generics in Java are a compile-time feature that was introduced to provide type safety and eliminate the need for type casting. However, Java implements generics using a technique known as Type Erasure, which means generic type information is removed at runtime. This is how it works:

1. Type Erasure in Java

Type Erasure is used by Java to implement generics, which means that the generic type parameters are removed during the time of compilation and replaced with their upper bound. Let’s take an example:

Due to type erasure, at compile time, Java replaces T with Object. Let’s see the code after type erasure

2. Type Casting Is Done Automatically

Java inserts type casts automatically to maintain type safety because the type information is erased. 

For example,

The generic T is erased and replaced with the object after compilation. Hence, the code becomes 

Java automatically inserts type casting to maintain type safety during compile time, as it is type erasure. 

Output:

3. No Runtime Type Information (RTTI)

The instances of a generic type cannot be created or checked at runtime using reflection or instanceof, as the generic type is erased. Let’s take an example: 

Both instanceof T and new T() are not correct, as T is erased during compilation time. Now let’s correct the mistake:

Output:

4. Type Bounds Are Replaced

In case the generic type has an upper bound, it will get replaced with that bound during erasure. 

Explanation along with an example:

public class Calc{
    public int intValue(T n) {
        return n.intValue(); // It is allowed because T is a number
    }
}

Let’s see what happens after type erasure:

public class Calc {
    public int intValue(Number n) { // As T is bounded by number, after compilation it is replaced with number.
        return n.intValue();
    }
}

5. One Class File

Regardless of how many types it is instantiated with, there will be only one .class file generated for a generic class. 

Example:

Explanation: Regardless of how many different types it is used with, only one .class file is generated for a generic class, as generics are implemented through type erasure.

6. Incompatibility with Primitive Types

Generics do not work directly with the primitive types, like int, double, float, etc. Instead, use their wrapper classes, like Integer, Double, etc., because of type erasure.
For example,

Now, let’s see the solution:

Box<Integer> intBox = new Box<>(); // Instead of int, we have used Integer.
intBox.set(42);
System.out.println(intBox.get()); 

Java Generics with Collections

Java generics work seamlessly with the Collections Framework, which enables us to create type-safe, reusable, and readable code when dealing with groups of objects like lists, sets, and maps. Before the introduction of Java 5, elements were stored in collections as raw object types, requiring explicit type casting and risking a ClassCastException. The type of elements a collection can hold can be defined with generics, and the compiler can enforce this at compile time.

Example:

Benefits of Using Generics with Collections:

• Compile-Time Type Checking – the wrong types are prevented from being added.
• Eliminates the Need for Type Casting – Makes the Code Cleaner and More Readable.
• Improves Code Reusability and Flexibility – helps to work with any type.
• Works with Enhanced for-loop – more concise iteration.

Differences Between Raw Types and Generic Types

The difference between Raw Types and Generic Types is:

Java Generics and Type Safety

The main reason for introducing generics to Java is type safety. Generics help to ensure that the code works with the correct types at compile-time and also prevent many common runtime errors and make the programs more robust, readable, and maintainable. Before generics in Java, Java collections like lists or maps could hold any type of object, which means they accepted and returned objects. This required manual type casting, which was error-prone and could cause a ClassCastException at runtime.

Generics fix this problem by allowing us to specify the type of the object a collection or method will work with, which the compiler checks at compile time. 

Let’s take an example along with the explanation of how to solve the problem:

The output will be:

INTELLIPAAT
Exception in thread “main” java.lang.ClassCastException: class java.lang.Integer

With generics, the code becomes type-safe, scalable, and reusable.

Now, this is the code using generics 

It will give the output:

Java Generics vs Templates in C++

The difference between Generics and Templates in C++:

Type Parameter Naming Conventions

Java follows standard naming conventions for type parameters used in generics. These conventions improve code readability and help developers to understand the role of each type parameter, and they are widely used in the Java Collections Framework and other libraries.

Common Type Parameter Names and Their Meanings

Below is the list of Type parameter names along with their meanings:

Advantages of Generics in Java

Some of the advantages are listed below:

1. Type Safety – The type errors are caught during compile time instead of runtime by using the generics. Only the correct type of object passes to a class or method.

2. Elimination of Type Casting – Generics removes the problem of explicit type casting by knowing the object’s type at compile time.

3. Code Reusability – Generics enable us to write generic algorithms and data structures that work with any type of object.

4. Stronger Type Checks at Compile Time – The type constraints for generic codes are checked by the compiler, and it also helps to detect errors early and reduces runtime bugs. 

5. Improved Code Clarity and Readability – The codes are made more self-explanatory by the generics.

6. Support for Generic Algorithms and Utilities – Without rewriting the code, the generic utility methods that work for multiple types can be written.

Limitations of Generics in Java

Java Generics provide powerful features like type safety and code reusability, but there are some limitations and restrictions due to their implementation using type erasure.

1. Generics Work Only with Reference Types

Java generics do not support primitive data types like int, double, float, etc., directly; instead, they allow us to use their wrapper classes such as Integer, Double, Float, etc.

Example:

Output:

2. Generic Types Differ Based on Their Type Arguments

Generic types like List<String> and List<Integer> are treated as different types at compile time but become the same (List) at runtime due to type erasure. Just for saying,

3. No Runtime Type Information (Type Erasure)

At runtime, the type argument cannot be determined or used, as the generic type parameters are erased at runtime. For example,

Output:

4. Cannot Create Instances of Type Parameters

The generic type parameter cannot be instantiated directly, as its type is unknown at runtime.

For example:

public class Box<T>
{
public Box() {  // Cannot instantiate type parameter 'T,' for which a compile-time error occurs.
// T obj = new T();
    }
}

5. Cannot Create Generic Arrays

Due to type safety issues, Java does not allow the creation of arrays with generic types. 

For example,

public class Box<T>{
    T[] array = new T[5]; //  There will be a compile-time error. This problem occurs as Java generics use type erasure.
}

6. Cannot Use Static Fields of Type Parameters

Static members are shared across all instances, regardless of type, due to which generic type parameters cannot be used in static contexts. Let’s take an example:

Common Mistakes with Java Generics

Even though Java generics provide powerful compile-time safety and reusability, developers often make subtle mistakes that can lead to confusing compiler errors, runtime exceptions, or loss of type safety. Listed below are the most common mistakes made when using generics in Java:

1. Using Raw Types – Using a generic class or interface without specifying a type defeats the purpose of the generics and leads to type-safety issues.

2. Assuming Generic Type Exists at Runtime – The actual generic type cannot be checked or used at runtime due to type erasure.

3. Creating Generic Arrays – The creation of generic arrays is not allowed by Java as it can cause heap pollution.

4. Using Static Fields with Type Parameters – As static fields are shared among all instances, we cannot use a generic type parameter in a static context.

5. Unchecked Casts without Suppressing Warnings – Casting with generics can result in unchecked warnings. These should be suppressed only with the understanding.

6. Improper Use of Wildcards – Confusing code or type errors can occur due to misusing wildcards.

7. Overusing or Underusing Generics – Some developers overuse generics even when it is unnecessary, whereas others avoid them entirely, missing out on type safety.

Java Generics Best Practices

By using Java generics effectively, we can greatly improve the code’s type safety, readability, and reusability. However, to avoid pitfalls and to write clean, robust code, it is important to follow some key best practices:

1. Use Generics to Enforce Type Safety – Generics should always be used when working with collections or writing reusable code. This helps to catch the type-related errors at compile time and avoid runtime issues.

2. Avoid Raw Types – Without specifying the type parameters, never use a generic class or interface. Raw types disable type checks and can lead to ClassCastException.

3. Use Bounded Type Parameters When Necessary – When needed, use extends or super to restrict types to a certain range.

4. Prefer List<?> Over Raw List in APIs – To retain type safety, use a wildcard if the method accepts any type.

5. Don’t Create Generic Arrays – The creation of arrays with generic types is not allowed in Java due to type erasure.

6. Use Meaningful Type Parameter Names – Java naming conventions for type parameters to make the code more readable are as follows:

• T for type
• E for element
• K for key
• V for value
• N for number

7. Use Wildcards Wisely – Understand when to use

• ? extends T—for reading
• ? Super T for writing

This follows the PECS principle, which is “Producer Extends, Consumer Super.”

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Conclusion

With this, we have learned about Generics in Java, which are a powerful feature introduced by Java, used to provide compile-time type safety, eliminate the need for manual casting, and promote code reusability. By allowing developers to write flexible, type-agnostic classes and methods, the generics improve the overall readability, maintainability, and robustness of Java programs.

However, to use generics effectively, it is very important to understand their limitations and follow established best practices.

Java Generics – FAQs