How to copy elements from an ArrayList to another one NOT by reference?

问题

I'm trying to copy each element from one ArrayList (av) to another one (copia). The thing is that they're copied by reference, so whenever I make any changes in the original one, the copy is modified as well. Of course, this behavior is not wanted. How should I write this method?

public void copiarArrayList(ArrayList<Articulo_Venta> copia, ArrayList<Articulo_Venta> av){
    copia.clear();
    for (int i = 0; i < av.size(); i++) {
        copia.add(av.get(i));
    }
}

Articulo_Venta has these fields:

int codigo;
String nombre;
float cantidad;

PS: I also tried the next:

copia = new ArrayList<Articulo_Venta>(av);

but it still has its elements pointing to the original ArrayList.

回答1:

What you want is the deep copy. If your object contains only primitive you could use clone(), otherwise best way is to do manually:-

Make a constructor in your Articulo_Venta class which takes another Articulo_Venta object and initializes member variables.

Then change the code as:-

public void copiarArrayList(ArrayList<Articulo_Venta> copia, ArrayList<Articulo_Venta> av){
        copia.clear();
        for (int i = 0; i < av.size(); i++) {
            copia.add(new Articulo_Venta(av.get(i)));
        }

Also read here - how-do-you-make-a-deep-copy-of-an-object-in-java

回答2:

Cloning the objects before adding them. For example, instead of newList.addAll(oldList);

for(Articulo_Venta av : oldList) {
    newList.add(av.clone());
}

clone should be correctly overriden in Articulo_Venta.

This is how you do it.

public class Articulo_Venta {

String a;  //assuming you have these fields, then
Date d;
...

public Articulo_Venta clone(){
    Articulo_Venta  av = new Articulo_Venta();
    av.a = this.a.clone();
    av.d = this.d.clone();
    ...
    return av;
}
}

回答3:

Create a new constructor in your class Articulo_Venta.

public Articulo_Venta(int codigo, String number, float candidad)
{
  this.codigo = codigo;
  this.number = number;
  this.candidad = candidad;
}

public void copiarArrayList(List<Articulo_Venta> copia, List<Articulo_Venta> av)
{
   av.stream().forEach(t -> {
     Articulo_Venta newObj = new Articulo_Venta(t.getCodigo(), t.getNumber(), t.getCandidad());
     copia.add(newObj);
   });
}

来源：https://stackoverflow.com/questions/41391667/how-to-copy-elements-from-an-arraylist-to-another-one-not-by-reference

标签

java

arraylist

constructor

pass-by-value