Java Reflection ~ Set Inner Object Value Of A Primitive Type

Question

I have an object that is either type of int, short, byte or long and I need to give it a new value. Would that be possible in Java ? And if yes how ?

public static void set(Object obj, int value) throws Exception
{
    Class<?> c = obj.getClass();
    if (c.equals(Integer.class))
    {
        // ???
    }
}

Answer 1

Integer is immutable. You cannot set a value to an Integer instance.

Similarly, other wrapper classes for primitive types are also immutable.

Answer 2

Yes, so long as you know what primitive type you're dealing with.

Class clazz = Class.forName("TheClass");
Field f = clazz.getDeclaredField("ThePrimitiveField");
Object obj;
f.setBoolean(obj, true);

This will change the "ThePrimitiveField" field of obj. If you don't know the type...

Field f;
Object obj;
try {
    f.setBoolean(obj, true);
} catch (IllegalArgumentException ex) {
    try {
        f.setByte(obj, 16);
    } catch (IllegalArgumentException ex) {
        try {
            f.setChar(obj, 'a');
            // etc
        }
    }
}

Answer 3

If you know the type do this:

public class Main 
{
    public static void main(String[] args) 
        throws NoSuchFieldException, 
               IllegalArgumentException, 
               IllegalAccessException 
    {
        Foo            fooA;
        Foo            fooB;
        final Class<?> clazz;
        final Field    field;

        fooA = new Foo();
        fooB = new Foo();
        clazz = fooA.getClass();
        field = clazz.getDeclaredField("bar");

        System.out.println(fooA.getBar());
        System.out.println(fooB.getBar());
        field.setAccessible(true);  // have to do this since bar is private
        field.set(fooA, 42);
        System.out.println(fooA.getBar());
        System.out.println(fooB.getBar());
    }
}

class Foo
{
    private int bar;

    public int getBar()
    {
        return (bar);
    }
}

If you do not know the type you can do something like this:

public class Main 
{
    public static void main(String[] args) 
        throws NoSuchFieldException, 
               IllegalArgumentException, 
               IllegalAccessException 
    {
        Foo            fooA;
        Foo            fooB;
        final Class<?> clazz;
        final Class<?> type;
        final Field    field;

        fooA = new Foo();
        fooB = new Foo();
        clazz = fooA.getClass();
        field = clazz.getDeclaredField("bar");

        System.out.println(fooA.getBar());
        System.out.println(fooB.getBar());
        field.setAccessible(true);  // have to do this since bar is private        
        type = field.getType();

        if(type.equals(int.class))
        {
            field.set(fooA, 42);
        }
        else if(type.equals(byte.class))
        {
            field.set(fooA, (byte)1);
        }
        else if(type.equals(char.class))
        {
            field.set(fooA, 'A');
        }

        System.out.println(fooA.getBar());
        System.out.println(fooB.getBar());
    }
}

class Foo
{
    private char bar;

    public char getBar()
    {
        return (bar);
    }
}

And, if you want to use wrapper classes (Integer, Character, etc..) you can add this:

else if(type.equals(Integer.class))
{
    field.set(fooA, new Integer(43));
}