Type Erasure Summary

Type Erasure Concept

• Java's implementation of generics
• Removes type parameters at compile time
• Replaces with Object or bounds
• Maintains backward compatibility
• Example:

  // Before erasure
  class Box<T> {
    private T item;
  }
  
  // After erasure
  class Box {
    private Object item;  // T replaced with Object
  }
  

How Erasure Works

• Replaces type parameters with Object
• Replaces bounded types with first bound
• Adds type casts where necessary
• Example:

  // Before erasure
  Box<String> box = new Box<>();
  String s = box.get();
  
  // After erasure
  Box box = new Box();
  String s = (String) box.get();
  

Erasure with Bounds

• Bounded type replaced with bound
• Example:

  // Before erasure
  class NumberBox<T extends Number> {
    private T number;
  }
  
  // After erasure
  class NumberBox {
    private Number number;  // T replaced with Number
  }
  

Arrays and Generics

• Cannot create arrays of generic types
• Type information lost at runtime
• Can lead to heap pollution
• Example:

  // Not allowed:
  T[] array = new T[10];
  
  // Common workaround:
  T[] array = (T[]) new Object[10];
  

Reifiable Types

• Types with runtime information available
• Includes:
  • Primitive types
  • Non-generic types
  • Raw types
  • Unbounded wildcards
• Example:

  // Reifiable
  String[] strings = new String[10];
  List<?> list = new ArrayList<>();
  
  // Not reifiable
  List<String>[] arrayOfLists;  // compile error
  

Best Practices

• Be aware of type erasure limitations
• Use runtime type tokens when needed
• Avoid mixing arrays and generics
• Document type erasure implications
• Use collections over arrays with generics
• Consider reifiable type alternatives