Reputation: 177
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()
}
}
Upvotes: 14
Views: 16707
Reputation: 540
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.
Upvotes: 6
Reputation: 18423
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.
Upvotes: 1
Reputation: 13187
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);
}
}
}
Upvotes: 2
Reputation: 17622
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");
}
}
Upvotes: 2