Determining if an Object is of primitive type

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?

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<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;
    }
}

commons-lang http://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/ClassUtils.html">`ClassUtils`</a> 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;

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

For those who like terse code.

private static final Set<Class> 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);
}

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.

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<?> 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);
}

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

public static boolean isValidType(Class<?> retType)
{
	if (retType.isPrimitive() && 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;
}

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

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)
    {
    }
}

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.

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"

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.

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.

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<?> clazz)
{
	String pack = clazz.getPackage().getName();
	if(!"java.lang".equals(pack)) 
		return false;
	try 
	{
		clazz.getField("TYPE");
	} 
	catch (NoSuchFieldException e) 
	{
		return false;
	}			
	return true;		
}

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

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

For users of javapoet, there's also this way:

private boolean isBoxedPrimitive(Class<?> type) {
    return TypeName.get(type).isBoxedPrimitive();
}