How to use Class<T> in Java?
There's a good discussion of Generics and what they really do behind the scenes over at this question, so we all know that Vector<int[]> is a vector of integer arrays, and HashTable<String, Person> is a table of whose keys are strings and values Persons.
However, what stumps me is the usage of Class<>.
The java class Class is supposed to also take a template name, (or so I'm being told by the yellow underline in eclipse). I don't understand what I should put in there. The whole point of the Class object is when you don't fully have the information about an object, for reflection and such. Why does it make me specify which class the Class object will hold? I clearly don't know, or I wouldn't be using the Class object, I would use the specific one.
All we know is "All instances of a any class shares the same java.lang.Class object of that type of class"
e.g)
Student a = new Student();
Student b = new Student();
Then a.getClass() == b.getClass() is true.
Now assume
Teacher t = new Teacher();
without generics the below is possible.
Class studentClassRef = t.getClass();
But this is wrong now ..?
e.g) public void printStudentClassInfo(Class studentClassRef) {} can be called with Teacher.class
This can be avoided using generics.
Class<Student> studentClassRef = t.getClass(); //Compilation error.
Now what is T ?? T is type parameters (also called type variables); delimited by angle brackets (<>), follows the class name.
T is just a symbol, like a variable name (can be any name) declared during writing of the class file. Later that T will be substituted with
valid Class name during initialization (HashMap<String> map = new HashMap<String>();)
e.g) class name<T1, T2, ..., Tn>
So Class<T> represents a class object of specific class type 'T'.
Assume that your class methods has to work with unknown type parameters like below
/**
* Generic version of the Car class.
* @param <T> the type of the value
*/
public class Car<T> {
// T stands for "Type"
private T t;
public void set(T t) { this.t = t; }
public T get() { return t; }
}
Here T can be used as String type as CarName
OR T can be used as Integer type as modelNumber,
OR T can be used as Object type as valid car instance.
Now here the above is the simple POJO which can be used differently at runtime.
Collections e.g) List, Set, Hashmap are best examples which will work with different objects as per the declaration of T, but once we declared T as String
e.g) HashMap<String> map = new HashMap<String>(); Then it will only accept String Class instance objects.
Generic Methods
Generic methods are methods that introduce their own type parameters. This is similar to declaring a generic type, but the type parameter's scope is limited to the method where it is declared. Static and non-static generic methods are allowed, as well as generic class constructors.
The syntax for a generic method includes a type parameter, inside angle brackets, and appears before the method's return type. For generic methods, the type parameter section must appear before the method's return type.
class Util {
// Generic static method
public static <K, V, Z, Y> boolean compare(Pair<K, V> p1, Pair<Z, Y> p2) {
return p1.getKey().equals(p2.getKey()) &&
p1.getValue().equals(p2.getValue());
}
}
class Pair<K, V> {
private K key;
private V value;
}
Here <K, V, Z, Y> is the declaration of types used in the method arguments which should before the return type which is boolean here.
In the below; type declaration <T> is not required at method level, since it is already declared at class level.
class MyClass<T> {
private T myMethod(T a){
return a;
}
}
But below is wrong as class-level type parameters K, V, Z, and Y cannot be used in a static context (static method here).
class Util <K, V, Z, Y>{
// Generic static method
public static boolean compare(Pair<K, V> p1, Pair<Z, Y> p2) {
return p1.getKey().equals(p2.getKey()) &&
p1.getValue().equals(p2.getValue());
}
}
OTHER VALID SCENARIOS ARE
class MyClass<T> {
//Type declaration <T> already done at class level
private T myMethod(T a){
return a;
}
//<T> is overriding the T declared at Class level;
//So There is no ClassCastException though a is not the type of T declared at MyClass<T>.
private <T> T myMethod1(Object a){
return (T) a;
}
//Runtime ClassCastException will be thrown if a is not the type T (MyClass<T>).
private T myMethod1(Object a){
return (T) a;
}
// No ClassCastException
// MyClass<String> obj= new MyClass<String>();
// obj.myMethod2(Integer.valueOf("1"));
// Since type T is redefined at this method level.
private <T> T myMethod2(T a){
return a;
}
// No ClassCastException for the below
// MyClass<String> o= new MyClass<String>();
// o.myMethod3(Integer.valueOf("1").getClass())
// Since <T> is undefined within this method;
// And MyClass<T> don't have impact here
private <T> T myMethod3(Class a){
return (T) a;
}
// ClassCastException for o.myMethod3(Integer.valueOf("1").getClass())
// Should be o.myMethod3(String.valueOf("1").getClass())
private T myMethod3(Class a){
return (T) a;
}
// Class<T> a :: a is Class object of type T
//<T> is overriding of class level type declaration;
private <T> Class<T> myMethod4(Class<T> a){
return a;
}
}
And finally Static method always needs explicit <T> declaration; It wont derive from class level Class<T>. This is because of Class level T is bound with instance.
Also read Restrictions on Generics
Wildcards and Subtyping
type argument for a generic method
Using the generified version of class Class allows you, among other things, to write things like
Class<? extends Collection> someCollectionClass = someMethod();
and then you can be sure that the Class object you receive extends Collection, and an instance of this class will be (at least) a Collection.
From the Java Documentation:
[...] More surprisingly, class Class has been generified. Class literals now function as type tokens, providing both run-time and compile-time type information. This enables a style of static factories exemplified by the getAnnotation method in the new AnnotatedElement interface:
<T extends Annotation> T getAnnotation(Class<T> annotationType);
This is a generic method. It infers the value of its type parameter T from its argument, and returns an appropriate instance of T, as illustrated by the following snippet:
Author a = Othello.class.getAnnotation(Author.class);
Prior to generics, you would have had to cast the result to Author. Also you would have had no way to make the compiler check that the actual parameter represented a subclass of Annotation. [...]
Well, I never had to use this kind of stuff. Anyone?
I have found class<T> useful when I create service registry lookups. E.g.
<T> T getService(Class<T> serviceClass)
{
...
}
Following on @Kire Haglin's answer, a further example of generics methods can be seen in the documentation for JAXB unmarshalling:
public <T> T unmarshal( Class<T> docClass, InputStream inputStream )
throws JAXBException {
String packageName = docClass.getPackage().getName();
JAXBContext jc = JAXBContext.newInstance( packageName );
Unmarshaller u = jc.createUnmarshaller();
JAXBElement<T> doc = (JAXBElement<T>)u.unmarshal( inputStream );
return doc.getValue();
}
This allows unmarshal to return a document of an arbitrary JAXB content tree type.