Merging two objects in Java

I have two开发者_StackOverflow社区 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}

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();
            } 
        }
    }
}

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
    }
}

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.

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;
}

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

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.
        }
      }
    }
  }
}

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! :)

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;
 }

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.