Determine the Class of a Generic Type in Java

1. Overview

Generics, which was released in Java 5, allowed developers to create classes, interfaces, and methods with typed parameters, enabling the writing of type-safe code. Extracting this type of information at runtime allows developers to write more flexible code.

In this tutorial, we’ll learn how to get the class of a generic type.

2. Generics and Type Erasure

Generics were introduced in Java with the major goal of providing compile-time type-safety checks along with flexibility and reusability of code. The introduction of Generics made the Collections framework undergo significant enhancements and improvements. Before generics, Java collections used raw type, which was kind of error-prone, and developers often faced typecast exceptions.

To demonstrate this, let’s consider a simple example where we create a List and add data to it:

void withoutGenerics(){
    List container = new ArrayList();
    container.add(1);
    container.add("2");
    container.add("string");

    for (int i = 0; i < container.size(); i++) {
        int val = (int) container.get(i); //For "string", we get java.lang.ClassCastException: class String cannot be cast to class Integer 
    } 
}

In the above example, the List contains raw data. Hence, we’re able to add Integers and Strings. When we read the list using get(), we’re type-casting it to Integer, but for the String type, we get a type-casting exception.

With generics, we define a type parameter for a collection. If we try to add any other data type than the defined type parameter, then the compiler complains about it.

For example, let’s create a generic List with an Integer type and try adding different types of data to it:

void withGenerics(){
    List<Integer> container = new ArrayList();
    container.add(1);
    container.add("2"); // compiler won't allow this since we cannot add string to list of integer container.
    container.add("string"); // compiler won't allow this since we cannot add string to list of integer container.

    for (int i = 0; i < container.size(); i++) {
        int val = container.get(i); // not casting required since we defined type for List container.
    }
}

In the above code, when we’re trying to add a String data type to the List of integers, the compiler complains about it.

In Generics, the type of information is only available at compile-time. Java compiler erases type information during compilation and it’s not available at runtime. This is called type erasure. 

Due to type erasure, all the type parameter information is replaced with the bound (if the upper bound is defined) or the object type (if the upper bound isn’t defined).

We can confirm this by using the javap utility, which inspects the .class file and helps to examine the bytecode. Let’s compile the code containing the withGenerics() method above and inspect it with the javap utility:

javac CollectionWithAndWithoutGenerics.java // compiling java file

javap -v CollectionWithAndWithoutGenerics // read bytecode using javap tool

// bytecode mnemonics
public static void withGenerics();
    descriptor: ()V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=0
         0: new           #12                 // class java/util/ArrayList
         3: dup
         4: invokespecial #14                 // Method java/util/ArrayList."<init>":()V
         7: astore_0
         8: aload_0
         9: iconst_1
        10: invokestatic  #15                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
        13: invokeinterface #21,  2           // InterfaceMethod java/util/List.add:(Ljava/lang/Object; 

As we can see in bytecode mnemonics, line #13,  List.add method was passed with Object instead of Integer type.

Type erasure was a design choice made by Java designers to support backward compatibility.

3. Getting Class Information

The unavailability of type information at runtime makes it challenging to capture type information at runtime. However, there are certain workarounds to get the type information at runtime.

3.1. Using Class<T> Parameter

In this approach, we explicitly pass the class of generic type T at runtime, and this information is retained so that we can access it at runtime. In the below example, we’re passing Class<T> at runtime to the constructor, which assigns it to the clazz variable. Then, we can access class information using the getClazz() method:

public class ContainerTypeFromTypeParameter<T> {
    private Class<T> clazz;

    public ContainerTypeFromTypeParameter(Class<T> clazz) {
        this.clazz = clazz;
    }

    public Class<T> getClazz() {
        return this.clazz;
    }
}

Our test verifies that we’re successfully storing and retrieving the class information at runtime:

@Test
public void givenContainerClassWithGenericType_whenTypeParameterUsed_thenReturnsClassType(){
    var stringContainer = new ContainerTypeFromTypeParameter<>(String.class);
    Class<String> containerClass = stringContainer.getClazz();

    assertEquals(String.class, containerClass);
}

3.2. Using Reflection

Using a non-generic field with reflection is another workaround that allows us to get generic information at runtime.

Basically, we use reflection to obtain the runtime class of a generic type. In the below example, we use content.getClass(), which gets class information of content at runtime using reflection:

public class ContainerTypeFromReflection<T> {
    private T content;

    public ContainerTypeFromReflection(T content) {
        this.content = content;
    }

    public Class<?> getClazz() {
        return this.content.getClass();
    }
}

Our test verifies that it works for the ContainerTypeFromReflection class and gets the type information:

@Test
public void givenContainerClassWithGenericType_whenReflectionUsed_thenReturnsClassType() {
    var stringContainer = new ContainerTypeFromReflection<>("Hello Java");
    Class<?> stringClazz = stringContainer.getClazz();
    assertEquals(String.class, stringClazz);

    var integerContainer = new ContainerTypeFromReflection<>(1);
    Class<?> integerClazz = integerContainer.getClazz();
    assertEquals(Integer.class, integerClazz);
}

3.3. Using TypeToken

Type tokens are a popular way to capture generic type information at runtime. It was made popular by Joshua Bloch in his book “Effective Java”.

In this approach, we first create an abstract class called TypeToken, where we pass the type information from the client code. Inside the abstract class, we then use the getGenericSuperClass() method to retrieve the passed type argument at runtime:

public abstract class TypeToken<T> {
    private Type type;

    protected TypeToken(){
        Type superClass = getClass().getGenericSuperclass();
        this.type = ((ParameterizedType) superClass).getActualTypeArguments()[0];
    }

    public Type getType() {
        return type;
    }
}

As we can see in the above example, Inside our TokenType abstract class, we are capturing Type info at runtime using getGenericSupperClass(), which we are returning using the getType() method.

Our test verifies that it works for the sample class that extends the abstract TypeToken with String as the type parameter:

@Test
public void giveContainerClassWithGenericType_whenTypeTokenUsed_thenReturnsClassType(){
    class ContainerTypeFromTypeToken extends TypeToken<List<String>> {}

    var container = new ContainerTypeFromTypeToken();
    ParameterizedType type = (ParameterizedType) container.getType();
    Type actualTypeArgument = type.getActualTypeArguments()[0];

    assertEquals(String.class, actualTypeArgument);
}

4. Conclusion

In this article, we discuss generics and type erasure, along with its benefits and limitations. We also explored various workarounds for getting a class of generic type information at runtime, along with code examples.