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Table of Contents
Generics are only valid at compile time
Limitations caused by type erasure
What should I do if I want to retain type information?
Summarize the key points
Home Java javaTutorial Understanding Type Erasure in Java Generics

Understanding Type Erasure in Java Generics

Jul 04, 2025 am 02:56 AM
java generics Type erasure

Java generics provide type checking at compile time, but type erasing is performed at runtime. 1. Type erasure means that List and List are both List types at runtime, resulting in the inability to use generic overload methods; 2. Limitations include not being able to create instances using new T(), being unable to make instanceof judgments, and being unable to declare generic arrays; 3. The solutions include retaining generic information through subclasses, using reflection to obtain generic signatures, or manually passing Class parameters. These mechanisms help understand the limitations and how Java generics are handled.

Understanding Type Erasure in Java Generics

Java generics provide type checking at compile time, but type erases are performed at runtime, which means that all type information related to generics is removed. This is a mechanism used by Java to maintain backward compatibility, but it also brings some limitations and understanding difficulties.

Understanding Type Erasure in Java Generics

Generics are only valid at compile time

Java performs type checks on generic code during the compilation phase and erases the generic information when generating bytecode. For example, both List<string></string> and List<integer></integer> are actually both List types at runtime. You can try printing their class information:

Understanding Type Erasure in Java Generics
 List<String> stringList = new ArrayList<>();
List<Integer> intList = new ArrayList<>();

System.out.println(stringList.getClass() == intList.getClass()); // Output true

This means that the two are exactly the same type at runtime. This is also why you can't overload methods by generics - because they don't make any difference at runtime.

Limitations caused by type erasure

Since the type information is erased, the following situations are not allowed in Java:

Understanding Type Erasure in Java Generics
  • You cannot create generic instances using new T() because the runtime does not know what T is.
  • You cannot use judgments like instanceof List<string></string> because you cannot make run-time judgments for parameterized types.
  • Arrays cannot be declared as generic types, such as new List<string>[10]()</string> , because arrays need to know the exact type at runtime.

These limitations are essentially derived from the fact that generic information does not exist at runtime .

What should I do if I want to retain type information?

If you really need to get the actual type of a generic at runtime, you can consider the following ways:

  • Use subclasses to preserve generic information (such as anonymous internal classes)
  • Get the generic signature of the parent class or field by reflection
  • Manually pass Class<t></t> parameter to save type information

For example, frameworks like Spring or Gson use these techniques to implement parsing and processing of generic types.

Summarize the key points

  • Java generics are compile-time features and will be erased at runtime.
  • Type erasing is for compatibility with older versions of JVMs
  • Causes some syntax restrictions, such as not being able to create instances with generics, not being able to parameterize instances of instanceof, etc.
  • If you need to retain type information, you have to use additional means, such as Class objects or reflections

Basically that's it. Understanding the nature of type erasing can better deal with some of the "strange" restrictions in generic use.

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