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?

Answer 1

Integer is not a primitive, Class.isPrimitive() is not lying.

Answer 2

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<String> PRIMITIVE_TYPES = 
            new HashSet<>(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.

Answer 3

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 "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<Class<?>> WRAPPER_TYPES = getWrapperTypes();

    public static boolean isWrapperType(Class<?> clazz)
    {
        return WRAPPER_TYPES.contains(clazz);
    }

    private static Set<Class<?>> getWrapperTypes()
    {
        Set<Class<?>> ret = new HashSet<Class<?>>();
        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;
    }
}

Answer 4

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. So you can do:

boolean isPrimitiveOrWrapped = 
    clazz.isPrimitive() || ClassUtils.wrapperToPrimitive(clazz) != null;

Answer 5

You have to deal with the auto-boxing of java.
Let's take the code

public 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

Answer 6

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"