Retrieving a specific state in corda

In My Corda project, I need to retrieve a specific state based on the some specific field of the state class, which I have set Unique for each state. How to do that in Corda? The implementation of state is as follows:

@BelongsToContract(OfferContract.class)
public class OfferState implements LinearState {
    private final UniqueIdentifier linearID;
    private AnonymousParty sender;
    private AnonymousParty receiver;
    private final String policyID;
    private final double faceValue;
    private double offeredAmount;

    private boolean isActive;

    public OfferState(UniqueIdentifier linearID, AnonymousParty sender, AnonymousParty receiver, String policyID, double faceValue, double offeredAmount, boolean isActive) {
        this.linearID = linearID;
        this.sender = sender;
        this.receiver = receiver;
        this.policyID = policyID;
        this.faceValue = faceValue;
        this.offeredAmount = offeredAmount;
        this.isActive = isActive;
    }

    public UniqueIdentifier getLinearID() {
        return linearID;
    }

    public AnonymousParty getSender() {
        return sender;
    }

    public AnonymousParty getReceiver() {
        return receiver;
    }

    public String getPolicyID() {
        return policyID;
    }

    public double getFaceValue() {
        return faceValue;
    }

    public double getOfferedAmount() {
        return offeredAmount;
    }

    public boolean isActive() {
        return isActive;
    }

    @NotNull
    @Override
    public UniqueIdentifier getLinearId() {
        return linearID;
    }

    @NotNull
    @Override
    public List<AbstractParty> getParticipants() {
        return Arrays.asList(sender,receiver);
    }

    @Override
    public String toString() {
        return super.toString();
    }
}

I am querying in the API with the following code But, that is showing empty brackets like [] in the postman output.

@GetMapping(value = "/getOfferWithID",headers = "Content-Type=application/x-www-form-urlencoded")
    private List<StateAndRef<OfferState>> getOfferWithID(HttpServletRequest request) throws IllegalArgumentException {
        String s  = request.getParameter("PolicyID");
        return proxy.vaultQuery(OfferState.class).getStates().stream().filter(it->it.getState().getData().getPolicyID().equalsIgnoreCase(s)).collect(Collectors.toList());
    }

Can you please tell me, How can I filter states while querying, What is going wrong here? is there any direct query method so that I can filter it in rpc call itself or Do I really need to wirte a flow for it?