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 lt int   gt  is a vector of integer arrays  and HashTable lt String  Person gt  is a table of whose keys are strings and values Persons   However  what stumps me is the usage of Class lt  gt    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

User · Answer

Just use the beef class   public  lt T gt  T beefmarshal  Class lt beef gt  beefClass  InputBeef inputBeef        throws JAXBException        String packageName   docClass getPackage   getBeef         JAXBContext beef   JAXBContext newInstance  packageName         Unmarshaller u   beef createBeef         JAXBElement lt T gt  doc    JAXBElement lt T gt  u beefmarshal  inputBeef         return doc getBeef

User · Answer

Using the generified version of class Class allows you  among other things  to write things like  Class lt   extends Collection gt  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

User · Answer

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 lt Student gt  studentClassRef   t getClass      Compilation error    Now what is T    T is type parameters  also called type variables   delimited by angle brackets   lt     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 lt String gt  map   new HashMap lt String gt       e g  class name lt T1  T2       Tn gt   So Class lt T gt  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  lt T gt  the type of the value     public class Car lt T gt           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 lt String gt  map   new HashMap lt String gt     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  lt K  V  Z  Y gt  boolean compare Pair lt K  V gt  p1  Pair lt Z  Y gt  p2            return p1 getKey   equals p2 getKey     amp  amp                 p1 getValue   equals p2 getValue               class Pair lt K  V gt         private K key      private V value      Here  lt K  V  Z  Y gt  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  lt T gt  is not required at method level  since it is already declared at class level   class MyClass lt T gt       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  lt K  V  Z  Y gt          Generic static method     public static  boolean compare Pair lt K  V gt  p1  Pair lt Z  Y gt  p2            return p1 getKey   equals p2 getKey     amp  amp                 p1 getValue   equals p2 getValue               OTHER VALID SCENARIOS ARE  class MyClass lt T gt               Type declaration  lt T gt  already done at class level         private  T myMethod T a               return  a                        lt T gt  is overriding the T declared at Class level            So There is no ClassCastException though a is not the type of T declared at MyClass lt T gt            private  lt T gt  T myMethod1 Object a                   return  T  a                       Runtime ClassCastException will be thrown if a is not the type T  MyClass lt T gt              private T myMethod1 Object a                   return  T  a                               No ClassCastException                    MyClass lt String gt  obj  new MyClass lt String gt                obj myMethod2 Integer valueOf  1                Since type T is redefined at this method level          private  lt T gt  T myMethod2 T a               return  a                        No ClassCastException for the below            MyClass lt String gt  o  new MyClass lt String gt                o myMethod3 Integer valueOf  1   getClass               Since  lt T gt  is undefined within this method              And MyClass lt T gt  don t have impact here         private  lt T gt  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 lt T gt  a    a is Class object of type T            lt T gt  is overriding of class level type declaration           private  lt T gt  Class lt T gt  myMethod4 Class lt T gt  a               return  a                    And finally Static method always needs explicit  lt T gt  declaration  It wont derive from class level Class lt T gt   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

User · Answer

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    lt T extends Annotation gt  T getAnnotation Class lt T gt  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

User · Answer

In java  lt T gt  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 lt T gt           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 lt Integer gt  s1   new Shape    Shape lt String gt  s2   new Shape      Integer is a type and String is also a type    lt T gt  specifically stands for generic type  According to Java Docs - A generic type is a generic class or interface that is parameterized over types

User · Answer

Just use the beef class   public  lt T gt  T beefmarshal  Class lt beef gt  beefClass  InputBeef inputBeef        throws JAXBException        String packageName   docClass getPackage   getBeef         JAXBContext beef   JAXBContext newInstance  packageName         Unmarshaller u   beef createBeef         JAXBElement lt T gt  doc    JAXBElement lt T gt  u beefmarshal  inputBeef         return doc getBeef

User · Answer

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 lt Student gt  studentClassRef   t getClass      Compilation error    Now what is T    T is type parameters  also called type variables   delimited by angle brackets   lt     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 lt String gt  map   new HashMap lt String gt       e g  class name lt T1  T2       Tn gt   So Class lt T gt  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  lt T gt  the type of the value     public class Car lt T gt           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 lt String gt  map   new HashMap lt String gt     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  lt K  V  Z  Y gt  boolean compare Pair lt K  V gt  p1  Pair lt Z  Y gt  p2            return p1 getKey   equals p2 getKey     amp  amp                 p1 getValue   equals p2 getValue               class Pair lt K  V gt         private K key      private V value      Here  lt K  V  Z  Y gt  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  lt T gt  is not required at method level  since it is already declared at class level   class MyClass lt T gt       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  lt K  V  Z  Y gt          Generic static method     public static  boolean compare Pair lt K  V gt  p1  Pair lt Z  Y gt  p2            return p1 getKey   equals p2 getKey     amp  amp                 p1 getValue   equals p2 getValue               OTHER VALID SCENARIOS ARE  class MyClass lt T gt               Type declaration  lt T gt  already done at class level         private  T myMethod T a               return  a                        lt T gt  is overriding the T declared at Class level            So There is no ClassCastException though a is not the type of T declared at MyClass lt T gt            private  lt T gt  T myMethod1 Object a                   return  T  a                       Runtime ClassCastException will be thrown if a is not the type T  MyClass lt T gt              private T myMethod1 Object a                   return  T  a                               No ClassCastException                    MyClass lt String gt  obj  new MyClass lt String gt                obj myMethod2 Integer valueOf  1                Since type T is redefined at this method level          private  lt T gt  T myMethod2 T a               return  a                        No ClassCastException for the below            MyClass lt String gt  o  new MyClass lt String gt                o myMethod3 Integer valueOf  1   getClass               Since  lt T gt  is undefined within this method              And MyClass lt T gt  don t have impact here         private  lt T gt  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 lt T gt  a    a is Class object of type T            lt T gt  is overriding of class level type declaration           private  lt T gt  Class lt T gt  myMethod4 Class lt T gt  a               return  a                    And finally Static method always needs explicit  lt T gt  declaration  It wont derive from class level Class lt T gt   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

User · Answer

I have found class lt T gt  useful when I create service registry lookups  E g    lt T gt  T getService Class lt T gt  serviceClass

User · Answer

You often want to use wildcards with Class  For instance  Class lt   extends JComponent gt   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

User · Answer

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 lt T gt   In these cases  you can simply ignore the yellow eclipse warnings or you can use Class lt   gt      That s how I do it

User · Answer

Just to throw in another example  the generic version of Class  Class lt T gt   allows one to write generic functions such as the one below   public static  lt T extends Enum lt T gt  gt Optional lt T gt  optionalFromString           NotNull Class lt T gt  clazz          String name         return Optional lt T gt  opt   Optional ofNullable name               map String  trim               filter StringUtils  isNotBlank               map String  toUpperCase               flatMap n - gt                    try                       return Optional of Enum valueOf clazz  n                      catch  Exception e                        return Optional empty

User · Answer

Using the generified version of class Class allows you  among other things  to write things like  Class lt   extends Collection gt  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

User · Answer

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 lt T gt   In these cases  you can simply ignore the yellow eclipse warnings or you can use Class lt   gt      That s how I do it

User · Answer

Using the generified version of class Class allows you  among other things  to write things like  Class lt   extends Collection gt  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

User · Answer

I have found class lt T gt  useful when I create service registry lookups  E g    lt T gt  T getService Class lt T gt  serviceClass

User · Answer

You often want to use wildcards with Class  For instance  Class lt   extends JComponent gt   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

User · Answer

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    lt T extends Annotation gt  T getAnnotation Class lt T gt  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

User · Answer

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    lt T extends Annotation gt  T getAnnotation Class lt T gt  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

User · Answer

I have found class lt T gt  useful when I create service registry lookups  E g    lt T gt  T getService Class lt T gt  serviceClass

User · Answer

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    lt T extends Annotation gt  T getAnnotation Class lt T gt  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

User · Answer

Following on  Kire Haglin s answer  a further example of generics methods can be seen in the documentation for JAXB unmarshalling   public  lt T gt  T unmarshal  Class lt T gt  docClass  InputStream inputStream            throws JAXBException     String packageName   docClass getPackage   getName      JAXBContext jc   JAXBContext newInstance  packageName      Unmarshaller u   jc createUnmarshaller      JAXBElement lt T gt  doc    JAXBElement lt T gt  u unmarshal  inputStream      return doc getValue        This allows unmarshal to return a document of an arbitrary JAXB content tree type

User · Answer

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 lt T gt   In these cases  you can simply ignore the yellow eclipse warnings or you can use Class lt   gt      That s how I do it

User · Answer

It is confusing in the beginning  But it helps in the situations below    class SomeAction implements Action         Later in the code  Class lt Action gt  actionClass   Class forName  SomeAction     Action action   actionClass newInstance       Notice you get an Action instance  there was no need to cast

User · Answer

You often want to use wildcards with Class  For instance  Class lt   extends JComponent gt   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

User · Answer

You often want to use wildcards with Class  For instance  Class lt   extends JComponent gt   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

User · Answer

I have found class lt T gt  useful when I create service registry lookups  E g    lt T gt  T getService Class lt T gt  serviceClass

User · Answer

It is confusing in the beginning  But it helps in the situations below    class SomeAction implements Action         Later in the code  Class lt Action gt  actionClass   Class forName  SomeAction     Action action   actionClass newInstance       Notice you get an Action instance  there was no need to cast

User · Answer

In java  lt T gt  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 lt T gt           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 lt Integer gt  s1   new Shape    Shape lt String gt  s2   new Shape      Integer is a type and String is also a type    lt T gt  specifically stands for generic type  According to Java Docs - A generic type is a generic class or interface that is parameterized over types

User · Answer

Following on  Kire Haglin s answer  a further example of generics methods can be seen in the documentation for JAXB unmarshalling   public  lt T gt  T unmarshal  Class lt T gt  docClass  InputStream inputStream            throws JAXBException     String packageName   docClass getPackage   getName      JAXBContext jc   JAXBContext newInstance  packageName      Unmarshaller u   jc createUnmarshaller      JAXBElement lt T gt  doc    JAXBElement lt T gt  u unmarshal  inputStream      return doc getValue        This allows unmarshal to return a document of an arbitrary JAXB content tree type

User · Answer

Just to throw in another example  the generic version of Class  Class lt T gt   allows one to write generic functions such as the one below   public static  lt T extends Enum lt T gt  gt Optional lt T gt  optionalFromString           NotNull Class lt T gt  clazz          String name         return Optional lt T gt  opt   Optional ofNullable name               map String  trim               filter StringUtils  isNotBlank               map String  toUpperCase               flatMap n - gt                    try                       return Optional of Enum valueOf clazz  n                      catch  Exception e                        return Optional empty

User · Answer

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 lt T gt   In these cases  you can simply ignore the yellow eclipse warnings or you can use Class lt   gt      That s how I do it

User · Answer

Using the generified version of class Class allows you  among other things  to write things like  Class lt   extends Collection gt  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

[java] How to use Class<T> in Java?

Examples related to java

Examples related to templates

Examples related to class

Examples related to generics