Merging two objects in Java

Question

I have two objects of same type.

Class A {
  String a;
  List b;
  int c;
}

A obj1 = new A();
A obj2 = new A();

obj1 => {a = "hello"; b = null; c = 10}
obj2 => {a = null; b = new ArrayList(); c = default value}

Can you please let me know what is the best way to combine this objects into single object?

obj3 = {a = "hello"; b = (same arraylist from obj2); c = 10}

Answer 1

This works as long as you have POJOs with their own getters and setters. The method updates obj with non-null values from update . It calls setParameter() on obj with the return value of getParameter() on update :

public void merge(Object obj, Object update){
    if(!obj.getClass().isAssignableFrom(update.getClass())){
        return;
    }

    Method[] methods = obj.getClass().getMethods();

    for(Method fromMethod: methods){
        if(fromMethod.getDeclaringClass().equals(obj.getClass())
                && fromMethod.getName().startsWith("get")){

            String fromName = fromMethod.getName();
            String toName = fromName.replace("get", "set");

            try {
                Method toMetod = obj.getClass().getMethod(toName, fromMethod.getReturnType());
                Object value = fromMethod.invoke(update, (Object[])null);
                if(value != null){
                    toMetod.invoke(obj, value);
                }
            } catch (Exception e) {
                e.printStackTrace();
            } 
        }
    }
}

Answer 2

Maybe something like

class A {
    String a;
    List<..> b;
    int c;

    public void merge(A other) {
        this.a = other.a == null ? this.a : other.a;
        this.b.addAll(other.b);
        this.c = other.c == 0 ? this.c : other.c;
    }
}

A a1 = new A();
A a2 = new A();

a1.a = "a prop";
a2.c = 34;

a1.merge(a2);

A.merge might return a new A object instead of modifing current.

Answer 3

I am using Spring Framework. I was facing the same issue on a project.
To solve it i used the class BeanUtils and the above method,

public static void copyProperties(Object source, Object target)

This is an example,

public class Model1 {
    private String propertyA;
    private String propertyB;

    public Model1() {
        this.propertyA = "";
        this.propertyB = "";
    }

    public String getPropertyA() {
        return this.propertyA;
    }

    public void setPropertyA(String propertyA) {
        this.propertyA = propertyA;
    }

    public String getPropertyB() {
        return this.propertyB;
    }

    public void setPropertyB(String propertyB) {
        this.propertyB = propertyB;
    }
}

public class Model2 {
    private String propertyA;

    public Model2() {
        this.propertyA = "";
    }

    public String getPropertyA() {
        return this.propertyA;
    }

    public void setPropertyA(String propertyA) {
        this.propertyA = propertyA;
    }
}

public class JustATest {

    public void makeATest() {
        // Initalize one model per class.
        Model1 model1 = new Model1();
        model1.setPropertyA("1a");
        model1.setPropertyB("1b");

        Model2 model2 = new Model2();
        model2.setPropertyA("2a");

        // Merge properties using BeanUtils class.
        BeanUtils.copyProperties(model2, model1);

        // The output.
        System.out.println("Model1.propertyA:" + model1.getPropertyA(); //=> 2a
        System.out.println("Model1.propertyB:" + model1.getPropertyB(); //=> 1b
    }
}

Answer 4

Just accommodating boolean sync. and case sensitive(camel notation)

public boolean merge(Object obj){

    if(this.equals(obj)){
        return false;
    }

    if(!obj.getClass().isAssignableFrom(this.getClass())){
        return false;
    }

    Method[] methods = obj.getClass().getMethods();

    for(Method fromMethod: methods){
        if(fromMethod.getDeclaringClass().equals(obj.getClass())
                && (fromMethod.getName().matches("^get[A-Z].*$")||fromMethod.getName().matches("^is[A-Z].*$"))){

            String fromName = fromMethod.getName();
            String toName ;
            if(fromName.matches("^get[A-Z].*")){
                toName = fromName.replace("get", "set");
            }else{
                toName = fromName.replace("is", "set");
            }

            try {
                Method toMetod = obj.getClass().getMethod(toName, fromMethod.getReturnType());
                Object value = fromMethod.invoke(this, (Object[])null);
                if(value != null){
                    toMetod.invoke(obj, value);
                }
            } catch (Exception e) {
                e.printStackTrace();
            } 
        }
    }

    return true;
}

Answer 5

If you create getters and setters for the attributes, you can use the copyProperties method from Commons BeanUtils.

Answer 6

In your very special case it looks like you want a new object that takes the real values from both instances. Here is an implementation that will do that. The method should be add to class A so that it can access the fields.

 public A specialMergeWith(A other) {
   A result = new A();

   result.a = (a == null ? other.a : a);
   result.b = (b == null ? other.b : b);
   result.c = (c == DEFAULT_VALUE ? other.c : c);

   return result;
 }

Answer 7

Add this method to your POJO , then use it like myObject.merge(newObject) . It uses generics to loop through your POJO's fields, so you don't mention any field names :

/**
 * Fill current object fields with new object values, ignoring new NULLs. Old values are overwritten.
 *
 * @param newObject Same type object with new values.
 */
public void merge(Object newObject) {

  assert this.getClass().getName().equals(newObject.getClass().getName());

  for (Field field : this.getClass().getDeclaredFields()) {

    for (Field newField : newObject.getClass().getDeclaredFields()) {

      if (field.getName().equals(newField.getName())) {

        try {

          field.set(
              this,
              newField.get(newObject) == null
                  ? field.get(this)
                  : newField.get(newObject));

        } catch (IllegalAccessException ignore) {
          // Field update exception on final modifier and other cases.
        }
      }
    }
  }
}

Answer 8

There is a dynamic solution to merge any two objects which require Reflection and Recursion.

public <T> T merge(T local, T remote, ArrayList<String> listOfClass)
        throws IllegalAccessException, InstantiationException {
    Class<?> clazz = local.getClass();
    Object merged = clazz.newInstance();
    for (Field field : clazz.getDeclaredFields()) {
        field.setAccessible(true);
        Object localValue = field.get(local);
        Object remoteValue = field.get(remote);
        if (localValue != null) {
            if (listOfClass.contains(localValue.getClass().getSimpleName())) {
                field.set(merged, this.merge(localValue, remoteValue, listOfClass));
            } else {
                field.set(merged, (remoteValue != null) ? remoteValue : localValue);
            }
        } else if (remoteValue != null) {
            field.set(merged, remoteValue);
        }
    }
    return (T) merged;
}

Variable Description:

• local: The object on to which the other will be merged
• remote: The object which will be merged to the local object
• listOfClass: The ArrayList of custom classes in the given object

The function returns a merged object which is good to go.

Kudos: :)

Answer 9

public static Object mergeObjects(Object source, Object target) throws Exception {
        Field[] allFields = source.getClass().getDeclaredFields();
        for (Field field : allFields) {
            if(Modifier.isStatic(field.getModifiers()) || Modifier.isFinal(field.getModifiers())){
                continue;
            }

            if (!field.isAccessible() && Modifier.isPrivate(field.getModifiers()))
                field.setAccessible(true);
            if (field.get(source) != null) {
                field.set(target, field.get(source));
            }
        }

        return target;
    }

Using java reflection, support only for the same class.