How to use Class<T> in Java?

Question

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.

Answer 1

Kanagavelu Sugumar's answer is cool, but I saw different result when running the code snippet, so I'd like to point out due to type erasure, the second myMethod1(also the second myMethod3) would not throw exception when you call with below:

MyClass<Integer> o = new MyClass<>();
o.myMethod1("String");

MyClass<String> o2 = new MyClass<>();
o2.myMethod1(Integer.valueOf("1"));

As from Type Erasure in Java, upon compilation, the compiler replaces the unbound type parameter T with Object, so original

class MyClass<T> {
    
    //Runtime ClassCastException will be thrown if a is not the type T (MyClass<T>).  
    private T myMethod1(Object a){
        return (T) a;
    }
}

becomes

class MyClass {
    
    //No ClassCastException will be thrown  
    private Object myMethod1(Object a){
        return (Object) a;
    }
}

Answer 2

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

Answer 3

In java <T> means Generic class. A Generic Class is a class which can work on any type of data type or in other words we can say it is data type independent.

public class Shape<T> {
    // T stands for "Type"
    private T t;

    public void set(T t) { this.t = t; }
    public T get() { return t; }
}

Where T means type. Now when you create instance of this Shape class you will need to tell the compiler for what data type this will be working on.

Example:

Shape<Integer> s1 = new Shape();
Shape<String> s2 = new Shape();

Integer is a type and String is also a type.

<T> specifically stands for generic type. According to Java Docs - A generic type is a generic class or interface that is parameterized over types.

Answer 4

Just to throw in another example, the generic version of Class (Class<T>) allows one to write generic functions such as the one below.

public static <T extends Enum<T>>Optional<T> optionalFromString(
        @NotNull Class<T> clazz,
        String name
) {
    return Optional<T> opt = Optional.ofNullable(name)
            .map(String::trim)
            .filter(StringUtils::isNotBlank)
            .map(String::toUpperCase)
            .flatMap(n -> {
                try {
                    return Optional.of(Enum.valueOf(clazz, n));
                } catch (Exception e) {
                    return Optional.empty();
                }
            });
}

Answer 5

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.

Answer 6

Just use the beef class:

public <T> T beefmarshal( Class<beef> beefClass, InputBeef inputBeef )
     throws JAXBException {
     String packageName = docClass.getPackage().getBeef();
     JAXBContext beef = JAXBContext.newInstance( packageName );
     Unmarshaller u = beef.createBeef();
     JAXBElement<T> doc = (JAXBElement<T>)u.beefmarshal( inputBeef );
     return doc.getBeef();
}

Answer 7

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.

Answer 8

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?

Answer 9

It is confusing in the beginning. But it helps in the situations below :

class SomeAction implements Action {
}

// Later in the code.
Class<Action> actionClass = Class.forName("SomeAction"); 
Action action = actionClass.newInstance();
// Notice you get an Action instance, there was no need to cast.

Answer 10

I have found class<T> useful when I create service registry lookups. E.g.

<T> T getService(Class<T> serviceClass)
{
    ...
}

Answer 11

As other answers point out, there are many and good reasons why this class was made generic. However there are plenty of times that you don't have any way of knowing the generic type to use with Class<T>. In these cases, you can simply ignore the yellow eclipse warnings or you can use Class<?> ... That's how I do it ;)

Answer 12

You often want to use wildcards with Class. For instance, Class<? extends JComponent>, would allow you to specify that the class is some subclass of JComponent. If you've retrieved the Class instance from Class.forName, then you can use Class.asSubclass to do the cast before attempting to, say, construct an instance.