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Type erasure

Learn post-compile type erasure.

What is type erasure?

Java generics are checked at compile time; at runtime the type info is erased. This is called type erasure. A List<String> and a List<Integer> become the same List class after compilation.

This is why new ArrayList<String>().getClass() and new ArrayList<Integer>().getClass() return the same result. At runtime there are no generic type parameters; only the raw type runs. This was designed for backward compatibility with the JVM.

Erasure limitations

Type erasure blocks some things. At runtime you cannot write (obj instanceof List<String>); only (obj instanceof List<?>) or (obj instanceof List) is valid. Also you cannot create an instance of a generic type with new T().

Generic arrays are not allowed either: new List<String>[10] is forbidden. These follow directly from erasure; because the type parameter is unknown at runtime the JVM cannot do the right check. To work around these limits you pass a Class<T> token or use nested collections like List<List<String>>.

Practice Demonstrate type erasure

In main, create List<String> strings = new ArrayList<>(); and List<Integer> numbers = new ArrayList<>();. Compare the two getClass() calls with == and print the result with System.out.println(strings.getClass() == numbers.getClass());.

Starter code

Main.java
import java.util.ArrayList;
import java.util.List;

// Create two different List<T> instances

// getClass() comparison and println

Sample solution

Main.java
import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> strings = new ArrayList<>();
        List<Integer> numbers = new ArrayList<>();
        System.out.println(strings.getClass() == numbers.getClass());
    }
}

Expected output

true

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the ArrayList instance

Question 
List<String> strings = new Array____<>();
Answer 
List<String> strings = new ArrayList<>();

Explanation: ArrayList is the standard implementation of List.

2 Match the type erasure concepts

Term Meaning
type erasure Removal of type info
getClass() Returns runtime type
<T> Compile-time info
raw type Type without parameter

3 What is forbidden due to type erasure?

  • Declaring List<Integer>
  • Creating a generic instance with new T() Correct answer
  • Calling a generic method
  • Defining a generic class

Explanation: new T() is forbidden because T is unknown at runtime.

All lessons in Intermediate

1 Classes

  1. class
  2. Attributes
  3. Methods
  4. new
  5. toString
  6. Summary - Classes

2 Constructor

  1. Constructor basics
  2. Parameterized constructor
  3. Constructor overloading
  4. this()
  5. Default constructor
  6. Summary - Constructor

3 Inheritance

  1. extends
  2. super
  3. Method overriding
  4. Object class
  5. final class and method
  6. Summary - Inheritance

4 Project: Library

  1. Class design
  2. Book model
  3. Member management
  4. Borrow flow
  5. Putting it together

5 Encapsulation

  1. private fields
  2. Getter
  3. Setter
  4. Package-private
  5. Record
  6. Summary - Encapsulation

6 Static

  1. static fields
  2. static methods
  3. static block
  4. Class variables
  5. instanceof
  6. Summary - Static

7 Constants

  1. static final
  2. Uppercase convention
  3. Constants with enum
  4. Interface constants
  5. Math constants
  6. Summary - Constants

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Interface

  1. interface
  2. implements
  3. default methods
  4. static methods
  5. Multiple interfaces
  6. Summary - Interface

10 Abstract class

  1. abstract class
  2. abstract methods
  3. Abstract and concrete
  4. Abstract vs interface
  5. Sealed class
  6. Summary - Abstract class

11 Polymorphism

  1. Polymorphism basics
  2. Dynamic binding
  3. Override rules
  4. Abstract reference
  5. Pattern matching
  6. Summary - Polymorphism

12 Project: Blog app

  1. Model design
  2. Blog class
  3. Adding comments
  4. Search and filter
  5. Putting it together

13 File I/O

  1. File class
  2. FileReader and FileWriter
  3. BufferedReader
  4. PrintWriter
  5. try-with-resources
  6. Summary - File I/O

14 Serialization

  1. Jackson ObjectMapper
  2. readValue
  3. writeValueAsString
  4. JsonProperty
  5. List and Map serialization
  6. Summary - Serialization

15 Generics

  1. Generic class
  2. Generic method
  3. Bounded type
  4. Wildcard
  5. Type erasure
  6. Summary - Generics

16 Generic collections

  1. List<E>
  2. Map<K,V>
  3. Set<E>
  4. Queue and Deque
  5. Collections class
  6. Summary - Generic collections

17 Iterator

  1. Iterator
  2. remove
  3. ListIterator
  4. Iterable
  5. Spliterator
  6. Summary - Iterator

18 Lambda expressions

  1. Lambda basics
  2. Parameterized lambda
  3. Method reference
  4. forEach with lambda
  5. Comparator with lambda
  6. Summary - Lambda expressions

19 Stream API basics

  1. stream()
  2. filter
  3. map
  4. forEach and count
  5. collect toList
  6. Summary - Stream API basics

20 Intermediate final

  1. Project design
  2. OOP model
  3. Stream layer
  4. File I/O
  5. Putting it together