Everything in java is an OBJECT
String is an object so are the classes
String is a class  but also, you use strings  to declare data types
eg: String name="Baglan"
You make this decleration inside a class
eg:
Public MyClass {
String name="Baglan"
....
...
}
Classes have attributes  (properties) and they have methods  which determine the
behavior of that class
Good luck
If you need further explanation you can refer to
www.java.sun.com

The class  should defined like this
public class MyClass {
String name="Baglan"
....
...
}

Check this URL for more info about UML

www.togethersoft.com/.../index.html

String is a data type of class
Class is an group of entities which can be unifed on some basis.
Object is an instance of class
for example car is class, object is fiat, "car" is an instance