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How do I copy an object in Java?

Posted by: admin November 2, 2017 Leave a comment

Questions:

Consider the below code:

DummyBean dum = new DummyBean();
dum.setDummy("foo");
System.out.println(dum.getDummy()); // prints 'foo'

DummyBean dumtwo = dum;
System.out.println(dumtwo.getDummy()); // prints 'foo'

dum.setDummy("bar");
System.out.println(dumtwo.getDummy()); // prints 'bar' but it should print 'foo'

So, I want to copy the ‘dum’ to ‘dumtwo’ and I want to change ‘dum’ without affecting the ‘dumtwo’. But the above code is not doing that. When I change something in ‘dum’, the same change is happening in ‘dumtwo’ also.

I guess, when I say dumtwo = dum, Java copies the reference only. So, is there any way to create a fresh copy of ‘dum’ and assign it to ‘dumtwo’?

Answers:

Create a copy constructor:

class DummyBean {
  private String dummy;

  public DummyBean(DummyBean another) {
    this.dummy = another.dummy; // you can access  
  }
}

Every object has also a clone method which can be used to copy the object, but don’t use it. It’s way too easy to create a class and do improper clone method. If you are going to do that, read at least what Joshua Bloch has to say about it in Effective Java.

Questions:
Answers:

Basic: Object Copying in Java.

Let us Assume an object- obj1, that contains two objects, containedObj1 and containedObj2.
enter image description here

shallow copying:
shallow copying creates a new instance of the same class and copies all the fields to the new instance and returns it. Object class provides a clone method and provides support for the shallow copying.

enter image description here

Deep copying:
A deep copy occurs when an object is copied along with the objects to which it refers. Below image shows obj1 after a deep copy has been performed on it. Not only has obj1 been copied, but the objects contained within it have been copied as well. We can use Java Object Serialization to make a deep copy. Unfortunately, this approach has some problems too(detailed examples).
enter image description here

Possible Problems:
clone is tricky to implement correctly.
It’s better to use Defensive copying, copy constructors(as @egaga reply) or static factory methods.

  1. If you have an object, that you know has a public clone() method, but you don’t know the type of the object at compile time, then you have problem. Java has an interface called Cloneable. In practice, we should implement this interface if we want to make an object Cloneable. Object.clone is protected, so we must override it with a public method in order for it to be accessible.
  2. Another problem arises when we try deep copying of a complex object. Assume that the clone() method of all member object variables also does deep copy, this is too risky of an assumption. You must control the code in all classes.

For example org.apache.commons.lang.SerializationUtils will have method for Deep clone using serialization(Source). If we need to clone Bean then there are couple of utility methods in org.apache.commons.beanutils (Source).

  • cloneBean will Clone a bean based on the available property getters and setters, even if the bean class itself does not implement Cloneable.
  • copyProperties will Copy property values from the origin bean to the destination bean for all cases where the property names are the same.
Questions:
Answers:

Just follow as below:

public class Deletable implements Cloneable{

    private String str;
    public Deletable(){
    }
    public void setStr(String str){
        this.str = str;
    }
    public void display(){
        System.out.println("The String is "+str);
    }
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

and wherever you want to get another object, simple perform cloning.
e.g:

Deletable del = new Deletable();
Deletable delTemp = (Deletable ) del.clone(); // this line will return you an independent
                                 // object, the changes made to this object will
                                 // not be reflected to other object

Questions:
Answers:

In the package import org.apache.commons.lang.SerializationUtils; there is a method:

SerializationUtils.clone(Object);

Example:

this.myObjectCloned = SerializationUtils.clone(this.object);

Questions:
Answers:

Why is there no answer for using Reflection API?

private static Object cloneObject(Object obj){
        try{
            Object clone = obj.getClass().newInstance();
            for (Field field : obj.getClass().getDeclaredFields()) {
                field.setAccessible(true);
                field.set(clone, field.get(obj));
            }
            return clone;
        }catch(Exception e){
            return null;
        }
    }

It’s really simple.

EDIT: Include child object via recursion

private static Object cloneObject(Object obj){
        try{
            Object clone = obj.getClass().newInstance();
            for (Field field : obj.getClass().getDeclaredFields()) {
                field.setAccessible(true);
                if(field.get(obj) == null || Modifier.isFinal(field.getModifiers())){
                    continue;
                }
                if(field.getType().isPrimitive() || field.getType().equals(String.class)
                        || field.getType().getSuperclass().equals(Number.class)
                        || field.getType().equals(Boolean.class)){
                    field.set(clone, field.get(obj));
                }else{
                    Object childObj = field.get(obj);
                    if(childObj == obj){
                        field.set(clone, clone);
                    }else{
                        field.set(clone, cloneObject(field.get(obj)));
                    }
                }
            }
            return clone;
        }catch(Exception e){
            return null;
        }
    }

Questions:
Answers:

Yes, you are just making a reference to the object. You can clone the object if it implements Cloneable.

Check out this wiki article about copying objects.

Refer here: Object copying

Questions:
Answers:

I use Google’s JSON library to serialize it then create a new instance of the serialized object. It does deep copy with a few restrictions:

  • there can’t be any recursive references

  • it won’t copy arrays of disparate types

  • arrays and lists should be typed or it won’t find the class to instantiate

  • you may need to encapsulate strings in a class you declare yourself

I also use this class to save user preferences, windows and whatnot to be reloaded at runtime. It is very easy to use and effective.

import com.google.gson.*;

public class SerialUtils {

//___________________________________________________________________________________

public static String serializeObject(Object o) {
    Gson gson = new Gson();
    String serializedObject = gson.toJson(o);
    return serializedObject;
}
//___________________________________________________________________________________

public static Object unserializeObject(String s, Object o){
    Gson gson = new Gson();
    Object object = gson.fromJson(s, o.getClass());
    return object;
}
       //___________________________________________________________________________________
public static Object cloneObject(Object o){
    String s = serializeObject(o);
    Object object = unserializeObject(s,o);
    return object;
}
}

Questions:
Answers:

Yes. You need to Deep Copy your object.

Questions:
Answers:

Add Cloneable and below code to your class

public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

Use this clonedObject = (YourClass) yourClassObject.clone();

Questions:
Answers:

To do that you have to clone the object in some way. Although Java has a cloning mechanism, don’t use it if you don’t have to. Create a copy method that does the copy work for you, and then do:

dumtwo = dum.copy();

Here is some more advice on different techniques for accomplishing a copy.

Questions:
Answers:

Here’s a decent explanation of clone() if you end up needing it…

Here: clone (Java method)

Questions:
Answers:

Use a deep cloning utility:

SomeObjectType copy = new Cloner().deepClone(someObject);

This will deep copy any java object, check it out at https://github.com/kostaskougios/cloning

Questions:
Answers:

Other than explicitly copying, another approach is to make the object immutable (no set or other mutator methods). In this way the question never arises. Immutability becomes more difficult with larger objects, but that other side of that is that it pushes you in the direction of splitting into coherent small objects and composites.

Questions:
Answers:

Deep Cloning is your answer, which requires implementing the Cloneable interface and overriding the clone() method.

public class DummyBean implements Cloneable {

   private String dummy;

   public void setDummy(String dummy) {
      this.dummy = dummy;
   }

   public String getDummy() {
      return dummy;
   }

   @Override
   public Object clone() throws CloneNotSupportedException {
      DummyBean cloned = (DummyBean)super.clone();
      cloned.setDummy(cloned.getDummy());
      // the above is applicable in case of primitive member types, 
      // however, in case of non primitive types
      // cloned.setNonPrimitiveType(cloned.getNonPrimitiveType().clone());
      return cloned;
   }
}

You will call it like this
DummyBean dumtwo = dum.clone();

Questions:
Answers:

This works too. Assuming model

class UserAccount{
   public int id;
   public String name;
}

First add
compile 'com.google.code.gson:gson:2.8.1' to your app>gradle & sync. Then

Gson gson = new Gson();
updateUser = gson.fromJson(gson.toJson(mUser),UserAccount.class);

You can exclude using a field by using transient keyword after access modifier.

Note: This is bad practice. Also don’t recommend to use Cloneable or JavaSerialization It’s slow and broken. Write copy constructor for best performance ref.

Something like

class UserAccount{
        public int id;
        public String name;
        //empty constructor
        public UserAccount(){}
        //parameterize constructor
        public UserAccount(int id, String name) {
            this.id = id;
            this.name = name;
        }

        //copy constructor
        public UserAccount(UserAccount in){
            this(in.id,in.name);
        }
    }

Test stats of 90000 iteration:
Line UserAccount clone = gson.fromJson(gson.toJson(aO), UserAccount.class); takes 808ms

Line UserAccount clone = new UserAccount(aO); takes less than 1ms

Conclusion: Use gson if your boss is crazy and you prefer speed. Use second copy constructor if you prefer quality.

You can also use copy constructor code generator plugin in Android Studio.

Questions:
Answers:
class DB {
  private String dummy;

  public DB(DB one) {
    this.dummy = one.dummy; 
  }
}

Questions:
Answers:

You can deep copy automatically with XStream, from http://x-stream.github.io/:

XStream is a simple library to serialize objects to XML and back
again.

Add it to your project (if using maven)

<dependency>
    <groupId>com.thoughtworks.xstream</groupId>
    <artifactId>xstream</artifactId>
    <version>1.3.1</version>                
</dependency>

Then

DummyBean dum = new DummyBean();
dum.setDummy("foo");
DummyBean dumCopy = (DummyBean) XSTREAM.fromXML(XSTREAM.toXML(dum));

With this you have a copy without the need to implement any cloning interface.

Questions:
Answers:

Pass the object whcih you wants to copy and get the object which you wants ,

private Object copyObject(Object objSource) {
        try {
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(bos);
            oos.writeObject(objSource);
            oos.flush();
            oos.close();
            bos.close();
            byte[] byteData = bos.toByteArray();
            ByteArrayInputStream bais = new ByteArrayInputStream(byteData);
            try {
                objDest = new ObjectInputStream(bais).readObject();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return objDest;

    }

Now parse the objDest to desigered object.

Happy Coding

Questions:
Answers:

You can try to implement Cloneable and use the clone() method; however, if you use the clone method you should – by standard – ALWAYS override Object‘s public Object clone() method.

Questions:
Answers:

If you can add an annotation to the source file, an annotation processor or code generator like this one can be used.

import net.zerobuilder.BeanBuilder

@BeanBuilder
public class DummyBean { 
  // bean stuff
}

A class DummyBeanBuilders will be generates, which has a static method dummyBeanUpdater to create shallow copies, the same way as you would do it manually.

DummyBean bean = new DummyBean();
// Call some setters ...
// Now make a copy
DummyBean copy = DummyBeanBuilders.dummyBeanUpdater(bean).done();