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:

class Gen<T> {
    T ob;

    Gen(T o) {
        ob = o;
    }

    T getOb() {
        return ob;
    }
}

• Code after erasure (how the compiler transforms it):

class Gen {
    Object ob;

    Gen(Object o) {
        ob = o;
    }

    Object getOb() {
        return ob;
    }
}

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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