Java Generics - Type Erasure

Using Java generics you can write generic programs and it also provide tighter type checks at compile time but these generics type remain only at source code level. When the source code is compiled all the generic type parameters are erased and this process is called type erasure in Java Generics.

Table of contents

How does Type erasure work
Type erasure in Generic class
Type erasure in Generic method
Type erasure and bridge methods

How does Type erasure work

Type erasure in Java works as follows-

Replace all type parameters in generic types with their bound type, if no explicit bound type is specified then replace generic type parameter with Object. The produced bytecode, therefore, contains only ordinary classes, interfaces, and methods with all the generic parameters replaced with actual types.
Insert type casts if necessary to preserve type safety.
Generate bridge methods to preserve polymorphism in extended generic types.

Type erasure in Generic class

Consider the following generic class with a generic type parameter T.

public class GenericClass<T> {
  T obj;
  GenericClass(T obj){
    this.obj = obj;
  }
  public T getObj() {
    return obj;
  } 
}

Because the type parameter T is unbounded, the Java compiler replaces it with Object and after compilation class looks like-

public class GenericClass {
  Object obj;
  GenericClass(Object obj){
    this.obj = obj;
  }
  public Object getObj() {
    return obj;
  } 
}

Consider another generic class with a bounded type parameter.

public class GenericClass<T extends String> {
  T obj;
  GenericClass(T obj){
    this.obj = obj;
  }
  public T getObj() {
    return obj;
  } 
}

Because the type parameter T is bounded, the Java compiler replaces it with the bound class String and after compilation class looks like-

public class GenericClass {
  String obj;
  GenericClass(String obj){
    this.obj = obj;
  }
  public String getObj() {
    return obj;
  } 
}

Type erasure in Generic method

The Java compiler also erases type parameters in generic method arguments. Consider the following generic method which counts the number of occurrences of passed element in the passed array.

public static <T> int count(T[] numberArray, T elem) {
  int cnt = 0;
  for (T e : numberArray){
    if (e.equals(elem))
      ++cnt;
  }
  return cnt;
}

Because T is unbounded, the Java compiler replaces it with Object and the compiled method looks like-

public static int count(Object[] numberArray, Object elem) {
  int cnt = 0;
  for (Object e : numberArray){
    if (e.equals(elem))
      ++cnt;
  }
  return cnt;
}

Type erasure and bridge methods

Sometimes, as part of the type erasure process compiler creates a synthetic method, called a bridge method. Consider the following classes to see an example of bridge method in Java.

public class GenClass<T> {
  T obj;
  public GenClass(T obj) { 
    this.obj = obj; 
  }
  public void setObj(T obj) {
    this.obj = obj;
  }  
}

This GenClass is then extended by another class as given below-

public class MyClass extends GenClass {
  public MyClass(Integer data) { 
    super(data); 
  } 
  public void setObj(Integer data) {
    System.out.println("MyClass.setData");
    super.setObj(data);
  } 
}

The intention here is to override the parent class setObj() method in the subclass. After type erasure, the GenClass and MyClass classes become-

public class GenClass {
  Object obj;
  public GenClass(Object obj) { 
    this.obj = obj; 
  }
  public void setObj(Object obj) {
    this.obj = obj;
  }  
}

public class MyClass extends GenClass {
  public MyClass(Integer data) { 
    super(data); 
  } 
  public void setObj(Integer data) {
    System.out.println("MyClass.setData");
    super.setObj(data);
  } 
}

After type erasure, the method signatures do not match. The GenClass method becomes setObj(Object obj) and the MyClass method becomes setObj(Integer data). Therefore, the GenClass setObj method does not override the MyClass setObj method.

To solve this problem and preserve the polymorphism of generic types after type erasure, Java compiler generates a bridge method to ensure that subtyping works as expected. For the MyClass class, the compiler generates the following bridge method for setObj().

public class MyClass extends GenClass {
  public MyClass(Integer data) { 
    super(data); 
  } 
  // Bridge method generated by the compiler
  public void setObj(Object data) {
    setObj((Integer) data);
  }
  public void setObj(Integer data) {
    System.out.println("MyClass.setData");
    super.setObj(data);
  } 
}

As you can see the bridge method has the same method signature as the GenClass’ setObj() method and it delegates to the original setObj() method.

That's all for the topic Java Generics - Type Erasure. If something is missing or you have something to share about the topic please write a comment.

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January 29, 2023