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

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
    }
}