Erasure
What is Erasure?
- Erasure is the technique used by Java to implement generics, ensuring backward compatibility with previous versions of the language.
- During compilation, all generic type information is *removed *(or "erased").
2. Motivation for Using Erasure
Compatibility with legacy codes:
- The introduction of generics in Java (JDK 5) should not invalidate older code.
- The design of generics needed to ensure that both generic and non-generic code worked together smoothly.
Appropriate coercions:
- The compiler adds coercions to ensure type compatibility with the given arguments.
- Removing information from generic types:
- Type parameters exist only in the source code, but not at runtime.
4. Example of Erasure in Action
- Generic code:
1 2 3 4 5 6 7 8 9 10 11 |
|
- Code after erasure (how the compiler transforms it):
1 2 3 4 5 6 7 8 9 10 11 |
|
Notes:
- The type parameter is replaced by Object.
- There is no generic information available in the generated code.
5. Limitations of Erasure
Missing type information at runtime:
- Generic types are a source code mechanism and do not exist after compilation.
Not possible:
- Get the generic type of an instance at runtime.
- Creating instances of generic types directly (e.g. new T() is not allowed).
6. Practical Impact
Erasure explains:
- Why generics have no overloading: Methods with signatures that differ only by generic types are not allowed.
- Why is it not possible to use primitive types directly in generics (e.g.: Gen is not valid; you must use Gen).
Compile-time security:
- Although type information is erased, the compiler checks type compatibility during compilation.
7. Conclusion
- Using erasure allowed generics to be added to Java without compromising backwards compatibility.
- Despite some limitations, generics provide type safety at compile time, while the generated code remains compatible with previous versions of the JVM.
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