Java/Weka - Need to create unique instances

问题

I wonder if there is a method that checks whether my new instance has been already created and added to my instances in Java... Like ;

Instance instance = new Instance(i, vals);
if(instances.contains(instance) == false) { //or hasInstance maybe
    instances.add(instance);
}

回答1:

I understand, that you want for two objects x1 and x2 with x1.equals(x2) that x1 is the same instance (x1 == x2).

One needs to do something like this:

private Map<Instance, Instance> identityMap = new HashMap<>();

public Instance unique(Instance instance) {
    Instance first = identityMap.get(instance);
    if (first == null) {
        first = instance;
        identityMap.put(instance, instance);
    }
    return first;
}

Instance instance = new Instance(i, vals);
instance = unique(instance);

The reason is, that you want to maintain the first instance, to be used by all.

---

By the way - for other purposes:

Set<Instance> instances = ...;

Instead of

if (!instances.contains(instance)) { // if not instances contains

one could use code like

if (instances.add(instance)) {
    // Added, hence new...
} else {
    // Not added, hence already existing...
}

来源：https://stackoverflow.com/questions/23877776/java-weka-need-to-create-unique-instances