Determining if an Object is of primitive type

Question

I have an Object   array  and I am trying to find the ones that are primitives  I ve tried to use Class isPrimitive    but it seems I m doing something wrong   int i   3  Object o   i   System out println o getClass   getName                               o getClass   isPrimitive       prints java lang Integer  false   Is there a right way or some alternative

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Google s Guava library has a Primitives utility that check if a class is a wrapper type for a primitive  Primitives isWrapperType class    Class isPrimitive   works for primitives

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I think this happens due to auto-boxing   int i   3  Object o   i  o getClass   getName       prints Integer   You may implement a utility method that matches these specific boxing classes and gives you if a certain class is primitive   public static boolean isWrapperType Class lt   gt  clazz        return clazz equals Boolean class              clazz equals Integer class             clazz equals Character class             clazz equals Byte class             clazz equals Short class             clazz equals Double class             clazz equals Long class             clazz equals Float class

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The primitve wrapper types will not respond to this value  This is for class representation of primitives  though aside from reflection I can t think of too many uses for it offhand  So  for example  System out println Integer class isPrimitive       prints  false   but  public static void main  String args    throws Exception       Method m   Junk class getMethod   a  null       System out println  m getReturnType   isPrimitive        public static int a         return 1      prints  true

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public class CheckPrimitve       public static void main String   args            int i   3          Object o   i          System out println o getClass   getSimpleName   equals  Integer             Field   fields   o getClass   getFields            for Field field fields                System out println field getType                          Output  true int int class java lang Class int

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Just so you can see that is is possible for isPrimitive to return true  since you have enough answers showing you why it is false    public class Main       public static void main final String   argv                final Class clazz           clazz   int class          System out println clazz isPrimitive               This matters in reflection when a method takes in  int  rather than an  Integer    This code works   import java lang reflect Method   public class Main       public static void main final String   argv          throws Exception               final Method method           method   Main class getDeclaredMethod  foo   int class              public static void foo final int x                  This code fails  cannot find the method    import java lang reflect Method   public class Main       public static void main final String   argv          throws Exception               final Method method           method   Main class getDeclaredMethod  foo   Integer class              public static void foo final int x

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Integer is not a primitive  Class isPrimitive   is not lying

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As several people have already said  this is due to autoboxing   You could create a utility method to check whether the object s class is Integer  Double  etc  But there is no way to know whether an object was created by autoboxing a primitive  once it s boxed  it looks just like an object created explicitly   So unless you know for sure that your array will never contain a wrapper class without autoboxing  there is no real solution

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For those who like terse code   private static final Set lt Class gt  WRAPPER TYPES   new HashSet Arrays asList      Boolean class  Character class  Byte class  Short class  Integer class  Long class  Float class  Double class  Void class    public static boolean isWrapperType Class clazz        return WRAPPER TYPES contains clazz

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Get ahold of BeanUtils from Spring     http   static springsource org spring docs 3 0 x javadoc-api   Probably the Apache variation  commons beans  has similar functionality

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The types in an Object   will never really be primitive - because you ve got references  Here the type of i is int whereas the type of the object referenced by o is Integer  due to auto-boxing    It sounds like you need to find out whether the type is a  wrapper for primitive   I don t think there s anything built into the standard libraries for this  but it s easy to code up   import java util     public class Test       public static void main String   args                        System out println isWrapperType String class            System out println isWrapperType Integer class               private static final Set lt Class lt   gt  gt  WRAPPER TYPES   getWrapperTypes         public static boolean isWrapperType Class lt   gt  clazz                return WRAPPER TYPES contains clazz              private static Set lt Class lt   gt  gt  getWrapperTypes                 Set lt Class lt   gt  gt  ret   new HashSet lt Class lt   gt  gt             ret add Boolean class           ret add Character class           ret add Byte class           ret add Short class           ret add Integer class           ret add Long class           ret add Float class           ret add Double class           ret add Void class           return ret

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I m late to the show  but if you re testing a field  you can use getGenericType   import static org junit Assert     import java lang reflect Field  import java lang reflect Type  import java util Arrays  import java util Collection  import java util HashSet   import org junit Test   public class PrimitiveVsObjectTest        private static final Collection lt String gt  PRIMITIVE TYPES                new HashSet lt  gt  Arrays asList  byte    short    int    long    float    double    boolean    char          private static boolean isPrimitive Type type            return PRIMITIVE TYPES contains type getTypeName                public int i1   34      public Integer i2   34        Test     public void primitive type   throws NoSuchFieldException  SecurityException           Field i1Field   PrimitiveVsObjectTest class getField  i1            Type genericType1   i1Field getGenericType            assertEquals  int   genericType1 getTypeName             assertNotEquals  java lang Integer   genericType1 getTypeName             assertTrue isPrimitive genericType1                Test     public void object type   throws NoSuchFieldException  SecurityException           Field i2Field   PrimitiveVsObjectTest class getField  i2            Type genericType2   i2Field getGenericType            assertEquals  java lang Integer   genericType2 getTypeName             assertNotEquals  int   genericType2 getTypeName             assertFalse isPrimitive genericType2              The Oracle docs list the 8 primitive types

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For users of javapoet  there s also this way   private boolean isBoxedPrimitive Class lt   gt  type        return TypeName get type  isBoxedPrimitive

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Starting in Java 1 5 and up  there is a new feature called auto-boxing  The compiler does this itself  When it sees an opportunity  it converts a primitive type into its appropriate wrapper class   What is probably happening here is when you declare  Object o   i    The compiler will compile this statement as saying  Object o   Integer valueOf i     This is auto-boxing  This would explain the output you are receiving  This page from the Java 1 5 spec explains auto-boxing more in detail

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you could determine if an object is wrapper type by beneath statements      objClass isAssignableFrom Number class        and you could also determine a primitive object by using the isPrimitive   method

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public static boolean isValidType Class lt   gt  retType        if  retType isPrimitive    amp  amp  retType    void class  return true      if  Number class isAssignableFrom retType   return true      if  AbstractCode class isAssignableFrom retType   return true      if  Boolean class    retType  return true      if  Character class    retType  return true      if  String class    retType  return true      if  Date class isAssignableFrom retType   return true      if  byte   class isAssignableFrom retType   return true      if  Enum class isAssignableFrom retType   return true      return false

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commons-lang ClassUtils has relevant methods    The new version has   boolean isPrimitiveOrWrapped        ClassUtils isPrimitiveOrWrapper object getClass       The old versions have wrapperToPrimitive clazz  method  which will return the primitive correspondence    boolean isPrimitiveOrWrapped        clazz isPrimitive      ClassUtils wrapperToPrimitive clazz     null

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This is the simplest way I could think of  The wrapper classes are present only in java lang package  And apart from the wrapper classes  no other class in java lang has field named TYPE  You could use that to check whether a class is Wrapper class or not   public static boolean isBoxingClass Class lt   gt  clazz        String pack   clazz getPackage   getName        if   java lang  equals pack            return false      try                clazz getField  TYPE               catch  NoSuchFieldException e                 return false                       return true

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You have to deal with the auto-boxing of java  Let s take the codepublic class test       public static void main String     args                int i   3          Object o   i          return         You get the class test class and javap -c test let s you inspect the generated bytecode Compiled from  test java  public class test extends java lang Object  public test      Code     0    aload 0    1    invokespecial    1    Method java lang Object      V    4    return  public static void main java lang String       Code     0    iconst 3    1    istore 1    2    iload 1    3    invokestatic     2    Method java lang Integer valueOf  I Ljava lang Integer     6    astore 2    7    return   As you can see the java compiler added invokestatic     2    Method java lang Integer valueOf  I Ljava lang Integer  to create a new Integer from your int and then stores that new Object in o via astore 2

[java] Determining if an Object is of primitive type

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