how to compare two objects and find the fields/properties changed?

Question

I need to write some generic solution to find out what properties in two objects have changed and return the changed properties (not the value).

class Student {
    public String name;
    public Address address;
    public int age;
}

class Address {
    public String hno;
    public String street;
    public int pin;
}

public class Test {
    public static void main(String... arg) {

        Student s1 = new Student();
        s1.name = "Krishna";
        s1.age = 30;
        s1.address = new Address();
        s1.address.hno = "2-2-22";
        s1.address.street = "somewhere";
        s1.address.pin = 123;

        Student s2 = new Student();
        s2.name = "Krishna";
        s2.age = 20;
        s2.address = new Address();
        s2.address.hno = "1-1-11";
        s2.address.street = "nowhere";
        s2.address.pin = 123;

        List<String> = difference(s1, s2);
        // expected result
        // Student.age
        // Student.address.hno
        // Student.address.street
    }
}

Can anyone please suggest some solution?

PS

Overriding equals/hashcode is not an option for me.

I have written the below code but I am not sure how to identify if a type is complex (for example Address)

private static List<String> difference(Student s1, Student s2) throws IllegalArgumentException, IllegalAccessException {

        for(Field field: Student.class.getDeclaredFields()) {
            System.out.println(field.getName() + " " +field.get(s1).equals(field.get(s2)));
        }
        return null;
    }

Answer 1

This is an enhancement of Dumbo's difference method, using recursion to check all nested complex fields.

    private static void difference(Object s1, Object s2, List<String> changedProperties, String parent) throws IllegalAccessException {
        for (Field field : s1.getClass().getDeclaredFields()) {
            if (parent == null) {
                parent = s1.getClass().getSimpleName();
            }
            field.setAccessible(true);
            Object value1 = field.get(s1);
            Object value2 = field.get(s2);
            if (value1 == null && value2 == null) {
                continue;
            }
            if (value1 == null || value2 == null) {
                changedProperties.add(parent + "." + field.getName());
            } else {
                if (isBaseType(value1.getClass())) {
                    if (!Objects.equals(value1, value2)) {
                        changedProperties.add(parent + "." + field.getName());
                    }
                } else {
                    difference(value1, value2, changedProperties, parent + "." + field.getName());
                }
            }
        }
    }

    private static final Set<Class> BASE_TYPES = new HashSet(Arrays.asList(
            String.class, Boolean.class, Character.class, Byte.class, Short.class, Integer.class, Long.class, Float.class, Double.class, Void.class));
    public static boolean isBaseType(Class clazz) {
        return BASE_TYPES.contains(clazz);
    }

Sample usage (assumes the referenced model objects have getters/setters):

    public static void main(String[] args) throws IllegalAccessException {
        Student s1 = new Student();
        s1.setName("Krishna");
        s1.setAge(30);
        Address address = new Address();
        s1.setAddress(address);
        s1.getAddress().setHno("2-2-22");
        s1.getAddress().setStreet("somewhere");
        s1.getAddress().setPin(123);

        Student s2 = new Student();
        s2.setName("Krishna");
        s2.setAge(20);
        address = new Address();
        s2.setAddress(address);
        s2.getAddress().setHno("1-1-11");
        s2.getAddress().setStreet("nowhere");
        s2.getAddress().setPin(123);
        List<String> changedProperties = new ArrayList<>();

        difference(s1, s2, changedProperties, null);
        System.out.println("changedProperties = " + changedProperties);
        // expected result
        // Student.age
        // Student.address.hno
        // Student.address.street
    }

Result:

changedProperties = [Student.address.hno, Student.address.street, Student.age]

Primitive adapting checking adapted from https://stackoverflow.com/a/711226/1527469