Question

I am totally confused with ArrayList behavior. Wrote really long post, then realized no one is going to analyse huge code, so just core of the problem. Numbers are for convenience, but in my app these 0 and 24 are dynamic values.

ArrayList<VoipBlock> sortedBlocks = new ArrayList<VoipBlock>();

VoipBlock vb3 =new VoipBlock();

vb3=sortedBlocks.get(0);

vb3.setPacketNumber(24); 

Essentially my final aim is to: modify and add back to arrayList as new value. However when I do that the guy at position 0 in ArrayList -> unsortedBlocks.get(0); replicates all the changes done to vb3 which of course is not what I want. I want vb3 acquire same values as VoipBlock inside of ArrayList, but I want it to be detached.

This is yet another case of passing by reference. I hate technical explanations – Java passes everything by value, BUT in some cases it passes references by values – this is same as saying not-oily oil. Please help.

It reminds me my start of learning JavaScript – I hated the language – until I watched proper materials at lynda.com – JavaScript Good Practices? – Diagrams killed me. It is the lazy description that turns us-youth away from brilliant technology, not the technology itself.

Please don’t let it bother my stress and don’t be in any way offended by me, it is just general complaining, maybe someone will look at it and make life better 🙂

Thanks for Your time,
Desperately awaiting for help 🙂

Answer 1

To achieve your objective you can use clone method. you have to override this method in VoipBlock class

Lets say VoipBlock is as follows

public class VoipBlock {

     private int packetNumber;
     private String type;

     public int getPacketNumber() {
      return packetNumber;
     }

     public String getType() {
      return type;
     }

     public void setPacketNumber(int value) {
       packetNumber = value;
     } 

     public void setType(String value) {
       type = value
     }

     public VoipBlock clone() {
        VoipBlock clone = VoipBlock();
        clone.setType(this.getType());
        clone.setPacketNumber(this.getPacketNumber());
        return clone;
     }

}

So, using the same code you can do like as follows

ArrayList<VoipBlock> sortedBlocks = new ArrayList<VoipBlock>();

VoipBlock vb3 =new VoipBlock();

sortedBlocks.add(vb3);

vb3=sortedBlocks.get(0).clone();

vb3.setPacketNumber(24);

Note that upon calling clone method in above code segment, vb3 get assigned with a new VoipBlock instance. And already inserted VoipBlock to the array remains unchanged.

if you are looking to have kind of sample instances of VoipBlock instances which you later wanted to use in creating similar instances like them. check on immutability/mutability aspect of the code. check “Effective Java” by Joshua Blouch