Question

I am trying to obtain a field’s value via reflection. The problem is I don’t know the field’s type and have to decide it while getting the value.

This code results with this exception:

Can not set java.lang.String field com….fieldName to java.lang.String

Field field = object.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
        
Class<?> targetType = field.getType();
Object objectValue = targetType.newInstance();

Object value = field.get(objectValue);

I tried to cast, but I get compilation errors:

field.get((targetType)objectValue)

or

targetType objectValue = targetType.newInstance();

How can I do this?

Answer 1

Like answered before, you should use:

Object value = field.get(objectInstance);

Another way, which is sometimes prefered, is calling the getter dynamically. example code:

public static Object runGetter(Field field, BaseValidationObject o)
{
    // MZ: Find the correct method
    for (Method method : o.getMethods())
    {
        if ((method.getName().startsWith("get")) && (method.getName().length() == (field.getName().length() + 3)))
        {
            if (method.getName().toLowerCase().endsWith(field.getName().toLowerCase()))
            {
                // MZ: Method found, run it
                try
                {
                    return method.invoke(o);
                }
                catch (IllegalAccessException e)
                {
                    Logger.fatal("Could not determine method: " + method.getName());
                }
                catch (InvocationTargetException e)
                {
                    Logger.fatal("Could not determine method: " + method.getName());
                }

            }
        }
    }


    return null;
}

Also be aware that when your class inherits from another class, you need to recursively determine the Field. for instance, to fetch all Fields of a given class;

    for (Class<?> c = someClass; c != null; c = c.getSuperclass())
    {
        Field[] fields = c.getDeclaredFields();
        for (Field classField : fields)
        {
            result.add(classField);
        }
    }