Does Using Generics in Java Affect Performance?

Question

Generics have been in Java since version 5. What are the performance implications of using generics in a Java application and can you explain the reasons for their performance impact?

Answer 1

Generics is a compile time feature. It has next to no impact when running your application.

Like most performance questions; it is far more important to write clear and simple code and this is often gives very good performance.

Changing your design for performance reasons is a so often a mistake some people say you should never do it. I think it is worth considering performance before you start but you have to recognise when you have a relatively trivial performance question.

You are better off writing your code and optimising it later when you have a better understanding of how you application behaves. i.e. when you are real use cases and a profiler.

Answer 2

Is it affect the performance of an application and how?

No, it won't affect performance since it's not even there at runtime.

From the official trail:

When a generic type is instantiated, the compiler translates those types by a technique called type erasure — a process where the compiler removes all information related to type parameters and type arguments within a class or method.

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As I've illustrated here both these programs translate to the very same bytecode:

import java.util.*;  class Test {     public static void main(String[] args) {         List<String> l = new ArrayList<String>();         l.add("hello");         System.out.println(l.get(0).length());     } } 

 

import java.util.*;  class Test2 {     public static void main(String[] args) {         List l = new ArrayList();         l.add("hello");         System.out.println(((String) l.get(0)).length());     } }