How to copy objects in java

Question

You can make an object copy in Java?

MinhaClasse mc = new MinhaClasse();

MinhaClasse mc2 = mc;

But I wanted to make changes in mc2 unaffected mc.

Answer 1

Just overwrite the method Object#clone, but your class needs to implement the interface Cloneable:

By convention, the classes implementing this interface must replace Object.clone (which is protected) by a public method.

Using the example of MinhaClasse used in the question, this would look like this:

public class MinhaClasse implements Cloneable {

    // Atributos e métodos 

    @Override
    public MinhaClasse clone() throws CloneNotSupportedException {
        return (MinhaClasse) super.clone();
    }
}

In this way it would be enough to call the method clone in this way:

MinhaClasse meuObjeto = new MinhaClasse();
MinhaClasse meuOutroObjeto = meuObjeto.clone();

Answer 2

The most correct to create copies, duplicate or clone objects is by implementing the method clone() and the interface Clonable. This allows you to define exactly the values that will be copied and even clone object contained in the main object, if applicable.

However, depending on the context, there are other alternatives that may be feasible. For example, if the idea is to just copy a Java Bean with simple attributes, getters and setters, then it is possible the method cloneBean() library Apache Commons Beansutils. There is also the method copyProperties() that copies the attributes of one object to another.

Another option, if the class is serializable, is to run the serialization of the same followed by the deserialization. Ready, you have an exact copy.

Of course it is not recommended to use the two methods mentioned above for such a simple case, but they can be useful for developing frameworks or very specific functionalities.

One of the scenarios where I’ve used this is in a system that literally had hundreds of constantly changing Java Beans. It was not feasible to create and keep up to date the method clone() in all classes.

Answer 3

Yes, just define the Constructor method

class MinhaClasse {
  private String str;

  public MinhaClasse (MinhaClasse outro) {
    this.str= outro.str; // tens acesso directo
  }
}

All Objects have a clone method that by default allows copying objects. Good practice dictates that you should define your copying method.

After setting this method it is simple to copy objects. You can simply do the following:

MinhaClasse mC = new MinhaClasse();
....(fazer algo com o objecto)
mc.Update();

MinhaClasse mC2(mc);
....(fazer algo com o objecto)
mC2.Update()  <- esta alteração apenas irá afectar mC2!

In response to your comment. If your Minhaclasse class has as members other classes than primitive type (int, string, ...) then those classes must also have defined copy constructors. For example.

class ClasseA {

   private ClasseB b;
   private String str;

   ClasseA (ClasseA outro) {
      this.b(outro.b) (aqui tens que garantir que a ClasseB possui um constructor de copia ou método =)  
      this.str= outro.str; // tens acesso directo     
      }
}

EDIT:

To all those who are suggesting to implement the Cloneable interface and define the clone() method I suggest reading the following articles. Especially the reading of Effective Java:

• "Copy Constructor versus Cloning"
• "Effective Java Effective Cloning"

I leave just one more quote from Josh Bloch

"The Cloneable interface was intended as a mixin interface for Objects to advertise that they Permit Cloning. Unfortunately it fails to serve this purpose ... This is a highly atypical use of interfaces and not one to be Emulated ... In order for implementing the interface to have any Effect on a class, it and all of its superclasses must Obey a fairly Complex, unenforceable and largely undocumented Protocol"