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Understanding Java Generics Type Erasure: A Dive into What and When it Happens

Java generics are a powerful feature that allows developers to create classes and methods that can work with different types of data. This a...

Java generics are a powerful feature that allows developers to create classes and methods that can work with different types of data. This allows for more flexible and reusable code, as it removes the need for type casting and provides compile-time type checking. However, behind the scenes, Java generics use a process called type erasure, which can sometimes cause confusion and unexpected behavior. In this article, we will take a deep dive into what type erasure is and when it happens.

To understand type erasure, we must first understand what Java generics are and how they work. Generics were introduced in Java 5 to provide a way for classes, interfaces, and methods to be parameterized by one or more types. This means that we can create a class or method that can work with different types of data without having to create multiple versions of it. For example, we can create a generic class called 'Box' that can hold any type of object, such as String, Integer, or even custom objects.

public class Box<T> {

private T content;

public void add(T obj) {

this.content = obj;


public T get() {

return this.content;



In the above code, the 'T' is a type parameter that will be replaced with an actual type when an instance of the 'Box' class is created. This allows us to write generic code that can be used with different types without having to specify the type beforehand.

Now, let's take a look at how the compiler handles this generic code. When a Java program is compiled, the compiler performs a process called type erasure. This means that all generic type parameters are replaced with their upper bound or Object if no upper bound is specified. In the case of our 'Box' class, the compiler will replace 'T' with 'Object'. This is why we can add any type of object to our 'Box' without any errors.

Box<String> stringBox = new Box<>();


String str = stringBox.get(); // no need for type casting

However, there are some situations where type erasure can cause unexpected behavior. This is because the type information is lost during the compilation process. For example, let's say we have a generic class called 'List' that can store a list of objects of any type.

public class List<T> {

private List<T> items;

public void add(T item) {



public T get(int index) {

return this.items.get(index);



Now, if we create a 'List' of Strings and add some values to it, we can retrieve them without any issues. But, if we try to retrieve a specific item and assign it to a String variable, we will get a compiler warning.

List<String> stringList = new List<>();


String str = stringList.get(0); // compiler warning: unchecked conversion

This is because the type information is erased during compilation, and the compiler cannot guarantee that the retrieved value is of type String. This can lead to runtime errors if we try to use the retrieved value as a String.

So, when does type erasure happen? As mentioned earlier, it happens during compilation. This means that it does not happen at runtime, and the JVM has no knowledge of generics. This is why we cannot use generics with Java's reflection API, as the

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