泛型类中的编译错误

Question

why following code doesn't compile ?

class aa1 <String> {
public void fun(){
String s = ""; // not compiling
}
}
class aa2 <String> {
String s = "";  // not compiling
}
class aa3 <String> {
String s = (String)""; // compiling
}

can some tell or give me link for this thanks.

Answer 1

I think you may have misconstrued the idea behind generics. The point is that your class ( aa , in this case) can be... well, generic. It's not of one fixed type, but rather can be any number of types through polymorphism. The generic type is similar to a variable name, but it represents a class, rather than an instance of a class. You could do something like:

class aa <T> {
    public void fun(T myObject){
        T s = myObject;
    }
}

This is an appropriate use of generics. T just represents "some class". The reason the example you posted didn't compile because you overrode the visibility of String (making it the generic type, rather than java.lang.String ). If you don't want it to be any class, but some subset of classes, you can do something like:

class aa <T extends MyInterface> {
    public void fun(T myObject){
        T s = myObject;
    }
}

In this case, you're guaranteed that instances of class T also extend the interface MyInterface . However, you cannot extend String , since it's final. If all you wanted was for the String object to be set, you do not need generics at all:

class aa {
    public void fun(){
        String s = "";
    }
}

---

For more information about generics, read over the Java tutorials on generics .

Answer 2

The generic parameter "String" hides the java.lang.String in this case.

in the class, if you declare the string s and specify its data type as "java.lang.String" then compiler won't complain.

public class aa<String> {

    java.lang.String s = "" ;

}

Answer 3

Your code makes no sense - the declaration makes no sense - what are you really trying to do here?

The reason you're having issues compiling is that you're making a generics declaration that's hiding the JDK's java.lang.String.

You're also declaring the same class 3 times. Something like this would compile, but still probably doesn't achieve what you want.

class aa<NotString> {
    public void fun() {
        String s = ""; // compiles
    }
}

class aa2<NotString> {
    String s = "";  // compiles
}

class aa3<NotString> {
    String s = (String) ""; // compiling
}

Answer 4

Well, it seems that your generic parameter name ( String ) hides the java.lang.String type.

I would recommend renaming your generic parameter. By the way, which type do you want your s to be: java.lang.String or the same as the parameter? In the latter case, how cuold the compiler assign "" (which is of type java.lang.String ) to the potentially irrelevant type represented by the generic parameter?

Answer 5

In your example you are declaring String as an identifier. You must not use keywords or type names. ( Generics Tutorial ).

Answer 6

Since you're declaring the generic type String , that will override the default java.lang.String type, your code is like typing:

class aa<T> {
    T s = "";
}

T is simply Object after type erasure, but since it can be initialized with anything (for example Integer ), the compiler won't let you assign String s to it.