I want to create a list/array of object with the same parent class which then I will use it for reference. but i dont know how to clone those object to make a new object.
here is the example
BigFoo a;
SmallFoo b;
ChickenFoo c;
List<Foo> foos;
foos.add(a);
foos.add(b);
foos.add(c);
Foo foo = foos.get(1).clone();
but in Java i found no clone function in the default function. I wonder how this is accomplished?
The general suggestion: use a copy constructor. In fact, only a class itself knows how to create a clone of itself. No class can clone an instance of another class. The idea goes like this:
public class Foo {
public List<Bar> bars = new ArrayList<Bar>();
private String secret;
// Copy constructor
public Foo(Foo that) {
// new List
this.bars = new ArrayList<Bar>();
// add a clone of each bar (as an example, if you need "deep cloning")
for (Bar bar:that.bars) {
this.bars.add(new Bar(bar));
}
// clone the secret value
this.secret = new String(that.secret);
}
// ...
}
So if we want to clone a foo, we simply create a new one based on foo:
Foo clonedFoo = new Foo(foo);
That's the recommended way to clone an instance.
copy constructor works well with inheritance. Consider a subclass
public ChildFoo extends Foo {
private int key;
public ChildFoo(ChildFoo that) {
super(that);
this.key = that.key;
}
}
Foo has a copy constructor and ChildFoo simply calls it from it's own copy constructor.
Your example is possible but not advisable. What will happen:
Foo a = new Foo();
ChildFoo b = new ChildFoo(a);
This would require a constructor on ChildFoo like:
public ChildFoo(Foo that) {
// call the copy constructor of Foo -> no problem
super(that);
// but how to initialize this.key? A Foo instance has no key value!
// Maybe use a default value?
this.key = 0;
}
Technically not a challenge but b is not a clone of a because the objects don't have the same type. So this (your example) is not cloning.
List, which class constructor are going to use (ArrayList, LinkedLink, etc)?
List interface does not have a clone method, so how would you do that? We can't clone interfaces.One easy way would be to use a json mapper (Jackson or Gson) and write out the object as a string and then creating the clone object by using the string.
