Raw Type with List<String> gives compilation error [duplicate]

问题

This question already has answers here:

Combining Raw Types and Generic Methods (5 answers)

Why does this class behave differently when I don't supply a generic type? (2 answers)

Closed 6 years ago.

I have the following Generic class:

import java.util.ArrayList;
import java.util.List;

public class GenericRaw<T> {
    public List<String> get() {
         return new ArrayList<>();
    }
}

Let us consider the case of its usage:

public class Usage {
    public void doSomething() {
        GenericRaw base = new GenericRaw();
        for (String x : base.get()) {   }
    }
}

For this code Idea doesn't give any compilation error but Java compiler itself does:

java: incompatible types required: java.lang.String found: java.lang.Object

Reproducible on JDK 1.6.0_33 as well as on JDK 1.7.0_17.

Can someone help me with the explanation of this issue?

Results of my investigations. The following variants can be successfully compiled:

public void doSomething() {
    GenericRaw<?> base = new GenericRaw();
    for (String x : base.get()) {   }
}

or even:

public void doSomething() {
    GenericRaw base = new GenericRaw();
    List<String> list = base.get();
    for (String x : list) {   }
}

回答1:

Can someone help me with the explanation of this issue?

Sure. It's following the JLS's description of raw types, in section 4.8:

To facilitate interfacing with non-generic legacy code, it is possible to use as a type the erasure (§4.6) of a parameterized type (§4.5) or the erasure of an array type (§10.1) whose element type is a parameterized type. Such a type is called a raw type.

The erasure of GenericRaw<T> is:

public class GenericRaw {
    public List get() { ... }
}

because:

Type erasure also maps the signature (§8.4.2) of a constructor or method to a signature that has no parameterized types or type variables. The erasure of a constructor or method signature s is a signature consisting of the same name as s and the erasures of all the formal parameter types given in s.

The type parameters of a constructor or method (§8.4.4), and the return type (§8.4.5) of a method, also undergo erasure if the constructor or method's signature is erased.

来源：https://stackoverflow.com/questions/15771406/raw-type-with-liststring-gives-compilation-error