Why does using raw types with generic methods lead to type errors in Java?

Combining Raw Types and Generic Methods

When working with generics in Java, it's important to understand the implications of using raw types. A raw type is simply a reference to a generic type without any type parameters specified.

The Problem

Consider the following code:

ArrayList<String> a = new ArrayList<>();
String[] s = a.toArray(new String[0]);

This code compiles successfully because toArray is a generic method that accepts an array of type as its argument. However, if the ArrayList reference is declared as a raw type:

ArrayList a = new ArrayList();
String[] s = a.toArray(new String[0]);

The compiler will generate an error, indicating that a String[] is required but an Object[] was found. This is because the compiler interprets the generic method as returning Object[] despite receiving String[] as its argument.

Understanding Raw Types

To understand this behavior, it's crucial to realize that when you use a raw type, you lose the ability to specify type parameters. The Java Language Specification (JLS) states that for a raw type:

"The type of a constructor, instance method, or non-static field M of a raw type C that is not inherited from its superclasses or superinterfaces is the raw type that corresponds to the erasure of its type in the generic declaration corresponding to C."

In other words, all generic methods and fields within a raw type are treated as if they were declared with raw types themselves.

Implications for Generic Methods

This means that when you use a raw type as the target of a generic method call, the compiler cannot infer the type parameter of the method. As a result, the method will be assumed to be generic only over the type parameter of the argument. In our case, since we passed a String[] argument, the toArray method is treated as . Therefore, the return type will be T[], which is equivalent to String[].

References for Further Reading

• [JLS 4.8: Raw Types](https://docs.oracle.com/javase/specs/jls/se17/html/jls-4.html#jls-4.8)
• [The Java Tutorials: Generics](https://docs.oracle.com/javase/tutorial/java/generics/)

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