What is the difference between canonical name simple name and class name in Java Class

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In Java  what is the difference between these   Object o1        o1 getClass   getSimpleName    o1 getClass   getName    o1 getClass   getCanonicalName      I have checked the Javadoc multiple times and yet this never explains it well  I also ran a test and that didn t reflect any real meaning behind the way these methods are called

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public void printReflectionClassNames        StringBuffer buffer   new StringBuffer        Class clazz  buffer getClass        System out println  Reflection on String Buffer Class        System out println  Name    clazz getName         System out println  Simple Name    clazz getSimpleName         System out println  Canonical Name    clazz getCanonicalName         System out println  Type Name    clazz getTypeName        outputs  Reflection on String Buffer Class Name  java lang StringBuffer Simple Name  StringBuffer Canonical Name  java lang StringBuffer Type Name  java lang StringBuffer

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this is best document I found describing getName    getSimpleName    getCanonicalName    https   javahowtodoit wordpress com 2014 09 09 java-lang-class-what-is-the-difference-between-class-getname-class-getcanonicalname-and-class-getsimplename      Primitive type int class getName                - gt  int int class getCanonicalName       - gt  int int class getSimpleName          - gt  int     Standard class Integer class getName                - gt  java lang Integer Integer class getCanonicalName       - gt  java lang Integer Integer class getSimpleName          - gt  Integer     Inner class Map Entry class getName                - gt  java util Map Entry Map Entry class getCanonicalName       - gt  java util Map Entry Map Entry class getSimpleName          - gt  Entry          Anonymous inner class Class lt   gt  anonymousInnerClass   new Cloneable      getClass    anonymousInnerClass getName                - gt  somepackage SomeClass 1 anonymousInnerClass getCanonicalName       - gt  null anonymousInnerClass getSimpleName          - gt     An empty string     Array of primitives Class lt   gt  primitiveArrayClass   new int 0  getClass    primitiveArrayClass getName                - gt   I primitiveArrayClass getCanonicalName       - gt  int   primitiveArrayClass getSimpleName          - gt  int       Array of objects Class lt   gt  objectArrayClass   new Integer 0  getClass    objectArrayClass getName                - gt   Ljava lang Integer  objectArrayClass getCanonicalName       - gt  java lang Integer   objectArrayClass getSimpleName          - gt  Integer

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getName       returns the name of the entity  class  interface  array class  primitive type  or void  represented by this Class object  as a String   getCanonicalName       returns the canonical name of the underlying class as defined by the Java Language Specification   getSimpleName        returns the simple name of the underlying class  that is the name it has been given in the source code   package com practice   public class ClassName   public static void main String   args       ClassName c   new ClassName      Class cls   c getClass          returns the canonical name of the underlying class if it exists   System out println  Class       cls getCanonicalName          Class   com practice ClassName   System out println  Class       cls getName                   Class   com practice ClassName   System out println  Class       cls getSimpleName             Class   ClassName   System out println  Class       Map Entry class getName                    - gt  Class   java util Map Entry   System out println  Class       Map Entry class getCanonicalName           - gt  Class   java util Map Entry   System out println  Class       Map Entry class getSimpleName              - gt  Class   Entry          One difference is that if you use an anonymous class you can get a null value when trying to get the name of the class using the getCanonicalName    Another fact is that getName   method behaves differently than the getCanonicalName   method for inner classes  getName   uses a dollar as the separator between the enclosing class canonical name and the inner class simple name   To know more about retrieving a class name in Java

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I ve been confused by the wide range of different naming schemes as well  and was just about to ask and answer my own question on this when I found this question here  I think my findings fit it well enough  and complement what s already here  My focus is looking for documentation on the various terms  and adding some more related terms that might crop up in other places   Consider the following example   package a b  class C     static class D extends C         D d    D   ds       The simple name of D is D  That s just the part you wrote when declaring the class  Anonymous classes have no simple name  Class getSimpleName   returns this name or the empty string  It is possible for the simple name to contain a   if you write it like this  since   is a valid part of an identifier as per JLS section 3 8  even if it is somewhat discouraged   According to the JLS section 6 7  both a b C D and a b C D D D would be fully qualified names  but only a b C D would be the canonical name of D  So every canonical name is a fully qualified name  but the converse is not always true  Class getCanonicalName   will return the canonical name or null  Class getName   is documented to return the binary name  as specified in JLS section 13 1  In this case it returns a b C D for D and  La b C D  for D    This answer demonstrates that it is possible for two classes loaded by the same class loader to have the same canonical name but distinct binary names  Neither name is sufficient to reliably deduce the other  if you have the canonical name  you don t know which parts of the name are packages and which are containing classes  If you have the binary name  you don t know which   were introduced as separators and which were part of some simple name   The class file stores the binary name of the class itself and its enclosing class  which allows the runtime to make this distinction   Anonymous classes and local classes have no fully qualified names but still have a binary name  The same holds for classes nested inside such classes  Every class has a binary name  Running javap -v -private on a b C class shows that the bytecode refers to the type of d as La b C D  and that of the array ds as  La b C D   These are called descriptors  and they are specified in JVMS section 4 3  The class name a b C D used in both of these descriptors is what you get by replacing   by   in the binary name  The JVM spec apparently calls this the internal form of the binary name  JVMS section 4 2 1 describes it  and states that the difference from the binary name were for historical reasons  The file name of a class in one of the typical filename-based class loaders is what you get if you interpret the   in the internal form of the binary name as a directory separator  and append the file name extension  class to it  It s resolved relative to the class path used by the class loader in question

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It is interesting to note that getCanonicalName   and getSimpleName   can raise InternalError when the class name is malformed  This happens for some non-Java JVM languages  e g   Scala   Consider the following  Scala 2 11 on Java 8    scala gt  case class C   defined class C  scala gt  val c   C   c  C   C    scala gt  c getClass getSimpleName java lang InternalError  Malformed class name   at java lang Class getSimpleName Class java 1330        32 elided  scala gt  c getClass getCanonicalName java lang InternalError  Malformed class name   at java lang Class getSimpleName Class java 1330    at java lang Class getCanonicalName Class java 1399        32 elided  scala gt  c getClass getName res2  String   C   This can be a problem for mixed language environments or environments that dynamically load bytecode  e g   app servers and other platform software

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If you re unsure about something  try writing a test first  I did this  class ClassNameTest       public static void main final String    arguments            printNamesForClass              int class               quot int class  primitive  quot            printNamesForClass              String class               quot String class  ordinary class  quot            printNamesForClass              java util HashMap SimpleEntry class               quot java util HashMap SimpleEntry class  nested class  quot            printNamesForClass              new java io Serializable     getClass                 quot new java io Serializable     getClass    anonymous inner class  quot               private static void printNamesForClass final Class lt   gt  clazz  final String label            System out println label    quot   quot            System out println  quot     getName              quot    clazz getName             System out println  quot     getCanonicalName     quot    clazz getCanonicalName             System out println  quot     getSimpleName        quot    clazz getSimpleName             System out println  quot     getTypeName          quot    clazz getTypeName        added in Java 8         System out println             Prints  int class  primitive       getName             int     getCanonicalName    int     getSimpleName       int     getTypeName         int  String class  ordinary class       getName             java lang String     getCanonicalName    java lang String     getSimpleName       String     getTypeName         java lang String  java util HashMap SimpleEntry class  nested class       getName             java util AbstractMap SimpleEntry     getCanonicalName    java util AbstractMap SimpleEntry     getSimpleName       SimpleEntry     getTypeName         java util AbstractMap SimpleEntry  new java io Serializable     getClass    anonymous inner class       getName             ClassNameTest 1     getCanonicalName    null     getSimpleName            getTypeName         ClassNameTest 1  There s an empty entry in the last block where getSimpleName returns an empty string  The upshot looking at this is   the name is the name that you d use to dynamically load the class with  for example  a call to Class forName with the default ClassLoader  Within the scope of a certain ClassLoader  all classes have unique names  the canonical name is the name that would be used in an import statement  It might be useful during toString or logging operations  When the javac compiler has complete view of a classpath  it enforces uniqueness of canonical names within it by clashing fully qualified class and package names at compile time  However JVMs must accept such name clashes  and thus canonical names do not uniquely identify classes within a ClassLoader   In hindsight  a better name for this getter would have been getJavaName  but this method dates from a time when the JVM was used solely to run Java programs   the simple name loosely identifies the class  again might be useful during toString or logging operations but is not guaranteed to be unique  the type name returns  quot an informative string for the name of this type quot    quot It s like toString  it s purely informative and has no contract value quot    as written by sir4ur0n

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Adding local classes  lambdas and the toString   method to complete the previous two answers  Further  I add arrays of lambdas and arrays of anonymous classes  which do not make any sense in practice though    package com example   public final class TestClassNames       private static void showClass Class lt   gt  c            System out println  getName                 c getName             System out println  getCanonicalName        c getCanonicalName             System out println  getSimpleName           c getSimpleName             System out println  toString                c toString             System out println               private static void x Runnable r            showClass r getClass             showClass java lang reflect Array newInstance r getClass    1  getClass        Obtains an array class of a lambda base type             public static class NestedClass         public class InnerClass         public static void main String   args            class LocalClass            showClass void class           showClass int class           showClass String class           showClass Runnable class           showClass SomeEnum class           showClass SomeAnnotation class           showClass int   class           showClass String   class           showClass NestedClass class           showClass InnerClass class           showClass LocalClass class           showClass LocalClass   class           Object anonymous   new java io Serializable               showClass anonymous getClass             showClass java lang reflect Array newInstance anonymous getClass    1  getClass        Obtains an array class of an anonymous base type          x    - gt                enum SomeEnum      BLUE  YELLOW  RED      interface SomeAnnotation      This is the full output   getName             void getCanonicalName    void getSimpleName       void toString            void  getName             int getCanonicalName    int getSimpleName       int toString            int  getName             java lang String getCanonicalName    java lang String getSimpleName       String toString            class java lang String  getName             java lang Runnable getCanonicalName    java lang Runnable getSimpleName       Runnable toString            interface java lang Runnable  getName             com example SomeEnum getCanonicalName    com example SomeEnum getSimpleName       SomeEnum toString            class com example SomeEnum  getName             com example SomeAnnotation getCanonicalName    com example SomeAnnotation getSimpleName       SomeAnnotation toString            interface com example SomeAnnotation  getName              I getCanonicalName    int   getSimpleName       int   toString            class  I  getName              Ljava lang String  getCanonicalName    java lang String   getSimpleName       String   toString            class  Ljava lang String   getName             com example TestClassNames NestedClass getCanonicalName    com example TestClassNames NestedClass getSimpleName       NestedClass toString            class com example TestClassNames NestedClass  getName             com example TestClassNames InnerClass getCanonicalName    com example TestClassNames InnerClass getSimpleName       InnerClass toString            class com example TestClassNames InnerClass  getName             com example TestClassNames 1LocalClass getCanonicalName    null getSimpleName       LocalClass toString            class com example TestClassNames 1LocalClass  getName              Lcom example TestClassNames 1LocalClass  getCanonicalName    null getSimpleName       LocalClass   toString            class  Lcom example TestClassNames 1LocalClass   getName             com example TestClassNames 1 getCanonicalName    null getSimpleName        toString            class com example TestClassNames 1  getName              Lcom example TestClassNames 1  getCanonicalName    null getSimpleName          toString            class  Lcom example TestClassNames 1   getName             com example TestClassNames  Lambda 1 1175962212 getCanonicalName    com example TestClassNames  Lambda 1 1175962212 getSimpleName       TestClassNames  Lambda 1 1175962212 toString            class com example TestClassNames  Lambda 1 1175962212  getName              Lcom example TestClassNames  Lambda 1  getCanonicalName    com example TestClassNames  Lambda 1 1175962212   getSimpleName       TestClassNames  Lambda 1 1175962212   toString            class  Lcom example TestClassNames  Lambda 1    So  here are the rules  First  lets start with primitive types and void    If the class object represents a primitive type or void  all the four methods simply returns its name    Now the rules for the getName   method    Every non-lambda and non-array class or interface  i e  top-level  nested  inner  local and anonymous  has a name  which is returned by getName    that is the package name followed by a dot  if there is a package   followed by the name of its class-file as generated by the compiler  whithout the suffix  class   If there is no package  it is simply the name of the class-file  If the class is an inner  nested  local or anonymous class  the compiler should generate at least one   in its class-file name  Note that for anonymous classes  the class name would end with a dollar-sign followed by a number  Lambda class names are generally unpredictable  and you shouldn t care about they anyway  Exactly  their name is the name of the enclosing class  followed by   Lambda   followed by a number  followed by a slash  followed by another number  The class descriptor of the primitives are Z for boolean  B for byte  S for short  C for char  I for int  J for long  F for float and D for double  For non-array classes and interfaces the class descriptor is L followed by what is given by getName   followed by    For array classes  the class descriptor is   followed by the class descriptor of the component type  which may be itself another array class   For array classes  the getName   method returns its class descriptor  This rule seems to fail only for array classes whose the component type is a lambda  which possibly is a bug   but hopefully this should not matter anyway because there is no point even on the existence of array classes whose component type is a lambda    Now  the toString   method    If the class instance represents an interface  or an annotation  which is a special type of interface   the toString   returns  interface     getName    If it is a primitive  it returns simply getName    If it is something else  a class type  even if it is a pretty weird one   it returns  class     getName      The getCanonicalName   method    For top-level classes and interfaces  the getCanonicalName   method returns just what the getName   method returns  The getCanonicalName   method returns null for anonymous or local classes and for array classes of those  For inner and nested classes and interfaces  the getCanonicalName   method returns what the getName   method would replacing the compiler-introduced dollar-signs by dots  For array classes  the getCanonicalName   method returns null if the canonical name of the component type is null  Otherwise  it returns the canonical name of the component type followed by       The getSimpleName   method    For top-level  nested  inner and local classes  the getSimpleName   returns the name of the class as written in the source file  For anonymous classes the getSimpleName   returns an empty String  For lambda classes the getSimpleName   just returns what the getName   would return without the package name  This do not makes much sense and looks like a bug for me  but there is no point in calling getSimpleName   on a lambda class to start with  For array classes the getSimpleName   method returns the simple name of the component class followed by     This have the funny weird side-effect that array classes whose component type is an anonymous class have just    as their simple names

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In addition to Nick Holt s observations  I ran a few cases for Array data type     primitive Array int demo     new int 5   Class lt   extends int   gt  clzz   demo getClass    System out println clzz getName     System out println clzz getCanonicalName     System out println clzz getSimpleName             System out println        Object Array Integer demo     new Integer 5    Class lt   extends Integer   gt  clzz   demo getClass    System out println clzz getName     System out println clzz getCanonicalName     System out println clzz getSimpleName       Above code snippet prints    I int   int     Ljava lang Integer  java lang Integer   Integer

[java] What is the difference between canonical name, simple name and class name in Java Class?

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