Question

I have an object that contains several fields (ints, Strings, etc) but also a HashMap and an ArrayList. This objects holds parameters used to later build a database query but there are times when I need to re-use all of the exact same parameters for an additional query, except for different items in the ArrayList.

I’ve noticed that when I change what’s in the array list, it always changes the original object. I’ve figured out how to make a shallow copy by overriding the clone() method but the array list always remains shared by any copy of the object. Before diving into deep copies and the like I think I need advice on whether that’s the best route.

Here’s an example of the object I need to find a way to duplicate.

public class QueryParameters implements Cloneable {

    protected HashMap<String,String> foundArgs = new HashMap<String,String>();
    protected ArrayList<ActionType> action_types = new ArrayList<ActionType>();

    protected String lookup_type = "lookup";
    protected Location loc;
    protected Vector player_location;
    protected int id = 0;
    protected int radius;
    protected boolean allow_no_radius = false;
    protected String player;
    protected String world;
    protected String time;
    protected String entity;
    protected String block;
    // ... lots of getters/setters
}

I could always make a new instance and use getters/setters to transfer the data but that feels too verbose to me – there are about 15 fields I’d have to copy over and if I ever add new ones I’d need to remember to add them here.

What’s the best way for me to obtain a new instance/clone so that changes to action_types doesn’t affect the original object

Answer 1

I suggest writing a clone method that uses super.clone() to copy most of the fields, and then replaces the ArrayList in the clone. Something like this:

import java.util.ArrayList;

public class QueryParameters implements Cloneable {
  @Override
  public String toString() {
    return "QueryParameters [action_types=" + action_types + ", otherData="
        + otherData + "]";
  }

  protected ArrayList<String> action_types = new ArrayList<String>();
  protected String otherData;

  @Override
  public QueryParameters clone() throws CloneNotSupportedException {
    QueryParameters cloned = (QueryParameters) super.clone();
    cloned.action_types = new ArrayList<String>(action_types);
    return cloned;
  }

  public static void main(String[] args) throws CloneNotSupportedException {
    QueryParameters test = new QueryParameters();
    test.otherData = "a field";
    test.action_types.add("xyzzy");
    test.action_types.add("abcde");
    System.out.println("Original: " + test);
    QueryParameters clone = test.clone();
    clone.action_types.add("A new action type");
    System.out.println("Modified clone: " + clone);
    System.out.println("Original after clone modification: " + test);
  }
}