Understanding Type Erasure in Java Generics
Jul 04, 2025 am 02:56 AMJava generics provide type checking at compile time, but type erasing is performed at runtime. 1. Type erasure means that List
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.

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:

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:

- You cannot create generic instances using
new T()
because the runtime does not know whatT
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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