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.

Answer 1

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)