Java reflection & isInstance on primitives

Question

I must be missing something here, but I seem to be having some trouble doing some basic reflection. I thought that due to things like boxing that I would receive true for each of the two prints below. Here is the simple Main class:

package com.reflection;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class TestingReflection {

    public static void main(String[] args) throws SecurityException,
                                                  NoSuchMethodException,
                                                  IllegalArgumentException, 
                                                  IllegalAccessException,
                                                  InvocationTargetException 
    {
        final Class c = Reflection.class;
        Reflection p = new Reflection();
        p.setIntObj(new Integer(1));
        p.setIntPrim(1);
        for (Field field : c.getDeclaredFields()) {
            char first = Character.toUpperCase(field.getName().charAt(0));
            String capitalized = first + field.getName().substring(1);
            Method getField = 
                  c.getDeclaredMethod("get" + capitalized, new Class [] {});
            Class fieldClass = getField.getReturnType();
            Method setField = 
                  c.getDeclaredMethod("set" + capitalized,
                                      new Class [] { fieldClass });
            Object value = getField.invoke(p, new Object [] {});
            if (value != null) {
                System.out.println("Field Class: " 
                                    + fieldClass.getName() 
                                    + " instanceOf: " 
                                    + fieldClass.isInstance(value) 
                                    + " Value Class: " 
                                    + value.getClass().getName());
            }
        }
    }
}

Here is the class I am running it against:

package com.reflection;

public class Reflection {

    private int intPrim;
    private Integer intObj;
    public int getIntPrim() { return intPrim; }
    public void setIntPrim(int intPrim) { this.intPrim = intPrim; }
    public Integer getIntObj() { return intObj; }
    public void setIntObj(Integer intObj) { this.intObj = intObj; }
}

Here is the output I receive:

Field Class: int instanceOf: false Value Class: java.lang.Integer
Field Class: java.lang.Integer instanceOf: true Value Class: java.lang.Integer

Is there a different method I should be using to determine this? isAssignableFrom also returns false for the primitive.

Peter Lawrey · Accepted Answer · 2011-06-15 12:40:33Z

5

The simplest way to get the primitive type for int is int.class You can also use Integer.TYPE but I believe this is there for backward compatibility.

The Java doc Class.isIntanceof(Object) says

If this Class object represents a primitive type, this method returns false.

This is perhaps not a useful definition, but it is the way it works. I haven't found a simple way to get the wrapper class for a primitive type so I use a predefined HashMap (it has 9 classes, including void)

answered Jun 15, 2011 at 12:40

Peter Lawrey

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Costi Ciudatu · Accepted Answer · 2011-06-15 12:11:23Z

3

Have a look at Integer.TYPE

answered Jun 15, 2011 at 12:11

Costi Ciudatu

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Comments

Paul Bellora · Accepted Answer · 2011-08-16 19:58:54Z

1

To build on Peter's answer, the reason that Class.isInstance() can't report on primitive types is because any primitive passed in as an argument must be boxed in order to be passed as an Object. This essentially makes the method useless for the Class objects representing the primitive types.

Of course in your case, the int's value has already been boxed with your call:

Object value = getField.invoke(p, new Object [] {});

answered Aug 16, 2011 at 19:58

Paul Bellora

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