如何迭代对象列表以从中创建新Map？

Question

I am trying to iterate a List of an Object such that after iterating I can make a Map of String and List<String> from it.

Below is my Machine class which has dataCenter, hostName and hostId.

public class Machine { 

    private String dataCenter;
    private String hostName;
    private String hostId;

    // getters here

    public Machine(String dataCenter, String hostName, String hostId) {
        // some code here

    }
}

If you see my below code, I have a List of machines in which I have 6 machine object. And then iterate machines object and make a new Map out of it.

    Map<String, List<String>> holder = new LinkedHashMap<String, List<String>>();

    List<Machine> machines = new ArrayList<Machine>();

    Machine machine1 = new Machine("DC1", "machineA", "hostA");
    Machine machine2 = new Machine("DC1", "machineB", "hostB");
    Machine machine3 = new Machine("DC2", "machineC", "hostC");
    Machine machine4 = new Machine("DC2", "machineD", "hostD");
    Machine machine5 = new Machine("DC3", "machineE", "hostE");
    Machine machine6 = new Machine("DC3", "machineF", "hostF");

    machines.add(machine1);
    machines.add(machine2);
    machines.add(machine3);
    machines.add(machine4);
    machines.add(machine5);
    machines.add(machine6);

    // some other code here

    // iterate machines object
    for (Machine machine : machines) {
        LinkedList<String> ppMachine = new LinkedList<String>();

        // below code doesn't work as it shows only last machine for that Datacenter
        ppMachine.add(machine.getHostId());
        holder.put(machine.getDatacenter(), ppMachine);
    }

    System.out.println(holder);

The new map should be something like this -

DC1 - hostA,hostB
DC2 - hostC,hostD
DC3 - hostE,hostF

Here DC1 is the key of the new map and hostA,hostB is the list of string values for that map. Similarly for others.

But somehow in my holder map, I always see latest value of machine for that Datacenter, not all the machines for that Datacenter.

I am doing very silly mistake somehow.

Answer 1

    LinkedList<String> ppMachine = new LinkedList<String>();

    // below code doesn't work as it shows only last machine for that Datacenter
    ppMachine.add(machine.getHostId());
    holder.put(machine.getDatacenter(), ppMachine);

You're replacing the entire list associated with machine.getColo() , not adding a new entry.

Probably the simplest fix would be

    LinkedList<String> dynMachine = holder.get(machine.getDatacenter());
    if (dynMachine == null) {
      dynMachine = new LinkedList<String>();
      holder.put(machine.getDatacenter(), dynMachine);
    }
    dynMachine.add(machine.getDatacenter());

Answer 2

Thats because your are creating a new list on each iteration, this is how you can achieve it:

    // iterate machines object
    for (Machine machine : machines) {
        LinkedList<String> dynMachine = holder.get(machine.getColo());
        if (null == dynMachine) {
           dynMachine = new LinkedList<String>();
           holder.put(machine.getColo(), dynMachine);
        }
        dynMachine.add(machine.getHostId());
    }