Question

Consider the above code example.

Why the compiler complains about “incompatible types” in the for-each-loop and says:

B.java:10: incompatible types
found   : java.lang.Object
required: java.lang.String
            for (String k : a.m.keySet()) {

If I strip the type parameter all compiles fine.

public class A<T> {

    Map<String,Field> m;

    void foo() {
        new B(this).foo();
    }
}

class B {

    A a;

    B(A a) {
        this.a = a;
    }

    void foo() {        
        for (String k : a.m.keySet()) { 
        }
    }
}

Answer 1

No time to find the corresponding JLS part, but it’s simple: If you declare a class with a type parameter and then use it without it, the compiler removes all generic declarations of the class. I.e.

public class A<T> {
    Map<String,Field> m;
}

becomes

public class A {
    Map m;
}

The fix is obvious: Instantiate A with a generic parameter or remove T.