What does class mean in Java

Question

What does  class mean in Java  For example  if I created a class called Print  What does Print class return

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In generics  an unknown type is represented by the wildcard character      Read here for official example

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I think the key here is understanding the difference between a Class and an Object   An Object is an instance of a Class   But in a fully object-oriented language  a Class is also an Object   So calling  class gets the reference to the Class object of that Class  which can then be manipulated

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class is used when there isn t an instance of the class available    getClass   is used when there is an instance of the class available   object getClass   returns the class of the given object   For example   String string    hello   System out println string getClass   toString       This will output   class java lang String   This is the class of the string object

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It s a generics literal  It means that you don t know the type of class this Class instance is representing  but you are still using the generic version    if you knew the class  you d use Class lt Foo gt   That way you can create a new instance  for example  without casting  Foo foo   clazz newInstance    if you don t use generics at all  you ll get a warning at least  and not using generics is generally discouraged as it may lead to hard-to-detect side effects

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That means a Class with a type of anything  unknown     You should read java generics tutorial to get to understand it better

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If an instance of an object is available  then the simplest way to get its Class is to invoke Object getClass    The  class Syntax  If the type is available but there is no instance then it is possible to obtain a Class by appending  class to the name of the type  This is also the easiest way to obtain the Class for a primitive type   boolean b  Class c   b getClass         compile-time error  Class c   boolean class      correct   See  docs oracle com about class

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It means  the Class reference type can hold any Class object which represents any type  If JVM loads a type  a class object representing that type will be present in JVM  we can get the metadata regarding the type from that class object which is used very much in reflection package  Suppose you have a a class named  quot myPackage MyClass quot   Assuming that is in classpath  the following statements are equivalent  Class lt   gt  myClassObject   MyClass class    compile time check  Class lt   gt  myClassObject   Class forname  quot myPackage MyClass quot      only runtime check  This works in a similar fashion if the Class lt   gt  reference is in method argument as well  Please note that the class  quot Class quot  does not have a public constructor  So you cannot instantiate  quot Class quot  instances with  quot new quot  operator

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A class literal is an expression consisting of the name of a class  interface  array  or primitive type  or the pseudo-type void  followed by a     and the token class  One of the changes in JDK 5 0 is that the class java lang Class is generic  java lang Class Class lt T gt   therefore    Class lt Print gt  p   Print class    References here   https   docs oracle com javase 7 docs api java lang Class html  http   docs oracle com javase tutorial extra generics literals html  http   docs oracle com javase specs jls se7 html jls-15 html jls-15 8 2

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This  lt   gt  is a beast  It often leads to confusion and errors  because  when you see it first  then you start believing   lt   gt  is a wildcard for any java type  Which is    not true   lt   gt  is the unknown type  a slight and nasty difference   It s not a problem when you use it with Class  Both lines work and compile   Class anyType   String class  Class  lt   gt  theUnknownType   String class    But - if we start using it with collections  then we see strange compiletime errors   List lt   gt  list   new ArrayList lt Object gt         ArrayList lt   gt  is not allowed list add  a String                          doesn t compile       Our List lt   gt  is not a collection  that is suitable for just any type of object  It can only store one type  the mystic  unkown type   Which is not a real type  for sure

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It means your Class reference can hold a reference to any Class object   It s basically the same as  Class  but you re showing other people who read your code that you didn t forget about generics  you just want a reference that can hold any Class object   Bruce Eckel  Thinking in Java       In Java SE5  Class lt    is preferred over plain Class  even though they   are equivalent and the plain Class  as you saw  doesn   t produce a   compiler warning  The benefit of Class lt                   is that it   indicates that you aren   t just using a non-specific class reference by   accident  or out of ignorance  You chose the non-specific version

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Just to clarify  this   class  method is not referring to the bytecode file you see after compiling java code nor a confusion between the concepts of Class vs  Object in OOP theory   This   class  method is used in Java for code Reflection  Generally you can gather meta data for your class such as the full qualified class name  list of constants  list of public fields  etc  etc   Check these links  already mentioned above  to get all the details  https   docs oracle com javase tutorial reflect class classNew html https   docs oracle com javase 8 docs api java lang Class html  Normally you don t plan on using Reflection right away when you start building your project  It s something that you know you need after trying to manage already working code  Many times you need it to manage multiple instances of your program  Maybe you want to identify each particular  clone  to determine if something is already defined  or count the number of functions  or just simply log the details of a particular instance of your class

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If there is no instance available then  class syntax is used to get the corresponding Class object for a class otherwise you can use getClass   method to get Class object  Since  there is no instance of primitive data type  we have to use  class syntax for primitive data types       package test       public class Test          public static void main String   args                       there is no instance available for class Test  so use Test class           System out println  Test class getName           Test class getName                   Now create an instance of class Test use getClass             Test testObj   new Test              System out println  testObj getClass   getName           testObj getClass   getName                  For primitive type           System out println  boolean class getName           boolean class getName               System out println  int class getName           int class getName               System out println  char class getName           char class getName               System out println  long class getName           long class getName

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When you write  class after a class name  it references the class literal - java lang Class object that represents information about given class    For example  if your class is Print  then Print class is an object that represents the class Print on runtime  It is the same object that is returned by the getClass   method of any  direct  instance of Print   Print myPrint   new Print    System out println Print class getName     System out println myPrint getClass   getName

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Adding to the above answers  Suppose you have a a class named  quot myPackage MyClass quot   Assuming that is in classpath  the following statements are equivalent    checking class name using string comparison  only Runtime check possible if myInstance getClass   getName   equals Class forName  quot myPackage MyClass quot    getName         checking actual Class object for equality  only Runtime check possible if myInstance getClass   getName      Class forName  quot myPackage MyClass quot          checking actual Class object for equality  but compile time validation   will ensure MyClass is in classpath  Hence this approach is better  according to fail-fast paradigm  if myInstance getClass      MyClass class     Similarly  the following are also equivalent  Class lt   gt  myClassObject   MyClass class    compile time check  Class lt   gt  myClassObject   Class forname  quot myPackage MyClass quot      only runtime check  If JVM loads a type  a class object representing that type will be present in JVM  we can get the metadata regarding the type from that class object which is used very much in reflection package  MyClass class is a shorthand method which actually points to the Class object representing MyClass  As an addendum  some information about Class lt   gt  reference which will be useful to read along with this as most of the time  they are used together  Class lt   gt  reference type can hold any Class object which represents any type  This works in a similar fashion if the Class lt   gt  reference is in method argument as well  Please note that the class  quot Class quot  does not have a public constructor  So you cannot instantiate  quot Class quot  instances with  quot new quot  operator

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Class is a parameterizable class  hence you can use the syntax Class lt T gt  where T is a type  By writing Class lt   gt   you re declaring a Class object which can be of any type    is a wildcard   The Class type is a type that contains meta-information about a class    It s always good practice to refer to a generic type by specifying his specific type  by using Class lt   gt  you re respecting this practice  you re aware of Class to be parameterizable  but you re not restricting your parameter to have a specific type   Reference about Generics and Wildcards  http   docs oracle com javase tutorial java generics wildcards html  Reference about Class object and reflection  the feature of Java language used to introspect itself   https   www oracle com technetwork articles java javareflection-1536171 html

[java] What does .class mean in Java?

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