Copy all values from fields in one class to another through reflection

I solved the above problem in Kotlin that works fine for me for my Android Apps Development:

 object FieldMapper {

fun <T:Any> copy(to: T, from: T) {
    try {
        val fromClass = from.javaClass

        val fromFields = getAllFields(fromClass)

        fromFields?.let {
            for (field in fromFields) {
                try {
                    field.isAccessible = true
                    field.set(to, field.get(from))
                } catch (e: IllegalAccessException) {
                    e.printStackTrace()
                }

            }
        }
    } catch (e: Exception) {
        e.printStackTrace()
    }

}

private fun getAllFields(paramClass: Class<*>): List<Field> {

    var theClass:Class<*>? = paramClass
    val fields = ArrayList<Field>()
    try {
        while (theClass != null) {
            Collections.addAll(fields, *theClass?.declaredFields)
            theClass = theClass?.superclass
        }
    }catch (e:Exception){
        e.printStackTrace()
    }

    return fields
}

}

public static <T> void copyAvalableFields(@NotNull T source, @NotNull T target) throws IllegalAccessException {
    Field[] fields = source.getClass().getDeclaredFields();
    for (Field field : fields) {
        if (!Modifier.isStatic(field.getModifiers())
                && !Modifier.isFinal(field.getModifiers())) {
            field.set(target, field.get(source));
        }
    }
}

We read all the fields of the class. Filter non-static and non-final fields from the result. But there may be an error accessing non-public fields. For example, if this function is in the same class, and the class being copied does not contain public fields, an access error will occur. The solution may be to place this function in the same package or change access to public or in this code inside the loop call field.setAccessible (true); what will make the fields available

Mladen's basic idea worked (thanks), but needed a few changes to be robust, so I contributed them here.

The only place where this type of solution should be used is if you want to clone the object, but can't because it is a managed object. If you are lucky enough to have objects that all have 100% side-effect free setters for all fields, you should definitely use the BeanUtils option instead.

Here, I use lang3's utility methods to simplify the code, so if you paste it, you must first import Apache's lang3 library.

Copy code

static public <X extends Object> X copy(X object, String... skipFields) {
        Constructor constructorToUse = null;
        for (Constructor constructor : object.getClass().getConstructors()) {
            if (constructor.getParameterTypes().length == 0) {
                constructorToUse = constructor;
                constructorToUse.setAccessible(true);
                break;
            }
        }
        if (constructorToUse == null) {
            throw new IllegalStateException(object + " must have a zero arg constructor in order to be copied");
        }
        X copy;
        try {
            copy = (X) constructorToUse.newInstance();

            for (Field field : FieldUtils.getAllFields(object.getClass())) {
                if (Modifier.isStatic(field.getModifiers())) {
                    continue;
                }

                //Avoid the fields that you don't want to copy. Note, if you pass in "id", it will skip any field with "id" in it. So be careful.
                if (StringUtils.containsAny(field.getName(), skipFields)) {
                    continue;
                }

                field.setAccessible(true);

                Object valueToCopy = field.get(object);
                //TODO add here other special types of fields, like Maps, Lists, etc.
                field.set(copy, valueToCopy);

            }

        } catch (InstantiationException | IllegalAccessException | IllegalArgumentException
                | InvocationTargetException e) {
            throw new IllegalStateException("Could not copy " + object, e);
        }
        return copy;
}

    public <T1 extends Object, T2 extends Object> void copy(T1 origEntity, T2 destEntity) {
        DozerBeanMapper mapper = new DozerBeanMapper();
        mapper.map(origEntity,destEntity);
    }

 <dependency>
            <groupId>net.sf.dozer</groupId>
            <artifactId>dozer</artifactId>
            <version>5.4.0</version>
        </dependency>

Here is my solution, will cover the child class case:

/**
 * This methods transfer the attributes from one class to another class if it
 * has null values.
 * 
 * @param fromClass from class
 * @param toClass   to class
 */
private void loadProperties(Object fromClass, Object toClass) {
    if (Objects.isNull(fromClass) || Objects.isNull(toClass))
        return;
    
    Field[] fields = toClass.getClass().getDeclaredFields();
    Field[] fieldsSuperClass = toClass.getClass().getSuperclass().getDeclaredFields();
    Field[] fieldsFinal = new Field[fields.length + fieldsSuperClass.length];

    Arrays.setAll(fieldsFinal, i -> (i < fields.length ? fields[i] : fieldsSuperClass[i - fields.length]));

    for (Field field : fieldsFinal) {
        field.setAccessible(true);
        try {
            String propertyKey = field.getName();
            if (field.get(toClass) == null) {
                Field defaultPropertyField = fromClass.getClass().getDeclaredField(propertyKey);
                defaultPropertyField.setAccessible(true);
                Object propertyValue = defaultPropertyField.get(fromClass);
                if (propertyValue != null)
                    field.set(toClass, propertyValue);
            }
        } catch (IllegalAccessException e) {
            logger.error(() -> "Error while loading properties from " + fromClass.getClass() +" and to " +toClass.getClass(), e);
        } catch (NoSuchFieldException e) {
            logger.error(() -> "Exception occurred while loading properties from " + fromClass.getClass()+" and to " +toClass.getClass(), e);
        }
    }
}