Question

How can I remove the warning from “id = (PK) m.invoke(obj, (Object []) null);“

    private PK getId(T obj) {
    PK id = null;
    for (Field f : type.getDeclaredFields()) {
        if (f.getAnnotation(Id.class) != null) {
            String name = f.getName();
            String method = "get" + name.substring(0, 1).toUpperCase() + name.substring(1);
            try {
                Method m = type.getDeclaredMethod(method, (Class<?>) null);
                id = (PK) m.invoke(obj, (Object []) null);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    return id;
}

Answer 1

I’m assuming PK is a generic type in your class

class MyClass<PK> {

Due to Java type erasure, the type PK is not available at runtime. But if you accept in your constructor a Class<PK> instance, then you can get around this by calling klass.cast(...)

class MyClass<PK> {
    private final Class<PK> keyClass;
    MyClass(Class<PK> keyClass) {
         this.keyClass = keyClass;
    }

    PK getKey() {
        return keyClass.cast(...);
    }
}

If for whatever reason you can’t get a reference to the actual runtime type of PK, then you have little recourse but to suppress the warning.