Java cloning abstract objects

问题:

I'm wondering if there is any way to do the following. I have an abstract class, Shape, and all its different subclasses and I want to override the clone method. All I want to do in the method is create a new Shape from the toString() of the current one. Obviously I can't do the following because Shape is abstract. Is there another way to do this because overriding clone in every subclass just for a simple name change seems useless.

public abstract class Shape {

    public Shape(String str) {
        // Create object from string representation
    }

    public Shape clone() {
        // Need new way to do this
        return new Shape(this.toString());   
    }

    public String toString() {
        // Correctly overriden toString()
    }
}

回答1:

You can try to use reflection:

public abstract class AClonable implements Cloneable{

private String val;

public AClonable(){

}

public AClonable(String s){
    val=s;
}

public String toString(){
    return val;
}

@Override
public AClonable clone(){
    try {
        System.out.println(getClass().getCanonicalName());
        AClonable b= getClass().getDeclaredConstructor(String.class).newInstance(val);

        return b;
    } catch (InstantiationException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalArgumentException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (SecurityException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (NoSuchMethodException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    return null;
}

}

in the clone() method you call getClass(). Because the ACloneble ist abstract, there call will allways go to the concrete class.

   public class ClonebaleOne extends AClonable{

public ClonebaleOne(){
    super();
}

public ClonebaleOne(String s) {
    super(s);
    // TODO Auto-generated constructor stub
}

}

and

  public class ClonebaleTwo extends AClonable{

public ClonebaleTwo(){
    super();
}

public ClonebaleTwo(String s) {
    super(s);
    // TODO Auto-generated constructor stub
}

}

and finally

   public static void main(String[] args){
    AClonable one = new ClonebaleOne("One");
    AClonable tow= new ClonebaleTwo("Two");
    AClonable clone = one.clone();
    System.out.println(clone.toString());
    clone = tow.clone();
    System.out.println(clone.toString());

}

Output:

  ClonebaleOne
  One
  ClonebaleTwo
  Two

But it's more a hack than a solution

[EDIT] my two clones were faster than ;)

[EDIT] To be complete. Another implentation of clone() can be

 @Override
public AClonable clone(){
    try {
        ByteArrayOutputStream outByte = new ByteArrayOutputStream();
        ObjectOutputStream outObj = new ObjectOutputStream(outByte);
        ByteArrayInputStream inByte;
        ObjectInputStream inObject;
        outObj.writeObject(this);
        outObj.close();
        byte[] buffer = outByte.toByteArray();
        inByte = new ByteArrayInputStream(buffer);
        inObject = new ObjectInputStream(inByte);
        @SuppressWarnings("unchecked")
        Object deepcopy =  inObject.readObject();
        inObject.close();
        return (AClonable) deepcopy;
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

when your abstract class implements Serialazable. There you write your object to disc and create a copy with the value from the disc.

回答2:

You can't create deep clone of abstract class because they can't be instantiated. All you can do is shallow cloning by using Object.clone() or returning this

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

or

@Override
public Object clone() throws CloneNotSupportedException {
    return this;
}

An abstract class can act as a reference, and it cannot have an instance so shallow cloning works in this case

OR

As a better approach, you can declare clone() as abstract and ask child class to define it, something like this

abstract class Shape {

    private String str;

    public Shape(String str) {
        this.str = str;
    }

    public abstract Shape clone();

    public String toString() {
        return str;
    }
}

class Circle extends Shape {

    public Circle(String str) {
        super(str);
    }

    @Override
    public Shape clone() {
        return new Circle("circle");
    }

}

回答3:

Although I doubt it is a good idea, you could use reflection:

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Test {

    public static void main(String[] args) {        
        Square s1 = new Square("test");
        Square s2 = (Square) s1.clone();

        // show that s2 contains the same data  
        System.out.println(s2);
        // show that s1 and s2 are really different objects
        System.out.println(s1 == s2);
    }

    public static abstract class Shape {
        private String str;

        public Shape(String str) {
            this.str = str;
        }

        public Shape clone() {          
            try {
                Class<?> cl = this.getClass();
                Constructor<?> cons = cl.getConstructor(String.class);
                return (Shape) cons.newInstance(this.toString());           
            } catch (NoSuchMethodException | SecurityException |
                     InstantiationException | IllegalAccessException |
                     IllegalArgumentException | InvocationTargetException e) {  
                e.printStackTrace();
            }           

            return null;
        }

        @Override
        public String toString() {
            return str;
        }
    }

    public static class Square extends Shape {
        public Square(String str) {
            super(str);
        }
    }   
}

回答4:

You can resolve with reflection:

public abstract class Shape {

    private String str;

    public Shape()  {

    }

    protected Shape(String str) {
        this.str = str;
    }

    public Shape clone() throws CloneNotSupportedException
    {
        try {
            return (Shape)getClass().getDeclaredConstructor(String.class).newInstance(this.toString());
        } catch (Exception e) {
            throw new CloneNotSupportedException();
        }
    }

    public String toString() {
        return "shape";
    }

public class Round extends Shape
{
    public Round()
    {
        super();
    }
    protected Round(String str) {
        super(str);
    }

    @Override
    public String toString() {
        return "round";
    }
}

main(){
  Shape round = new Round();        
  Shape clone = round.clone();
  System.out.println(round);
  System.out.println(clone);
}

but - IMO - is a poor implementation and error-prone with a lot of pits; the best use of Cloneable and Object.clone() is to not use them! You have a lot of way to do the same thing (like serialization for deep-clone) and shallow-clone that allow your a better control of flow.