I recently started using Builder pattern in one of my projects and I am trying to add some sort of validations on my Builder class.

Below is my builder class -

    public final class DataKey {
    
    	private final long userId;
    	private final String uuid;
    	private final String deviceId;
    	private final int clientId;
    	private final FlowEnum flow;
    
    	private DataKey(Builder builder) {
    		this.userId = builder.userId;
    		this.uuid = builder.uuid;
    		this.deviceId = builder.deviceId;
    		this.clientId = builder.clientId;
			this.flow = (userId == 0) ? FlowEnum.DEVICEFLOW : FlowEnum.USERFLOW;
    	}
    
    	public static class Builder {
    		protected final int clientId;
			protected long userId;
    		protected String uuid;
    		protected String deviceId;
			protected boolean userIdFlag = false;
			protected boolean uuidFlag = false;
			protected boolean deviceIdFlag = false;			
    
    		public Builder(int clientId) {
    			this.clientId = clientId;
    		}
    
    		public Builder setUserId(long userId) {
    			this.userId = userId;
				this.userIdFlag = true;
    			return this;
    		}
    
    		public Builder setUuid(String uuid) {
    			this.uuid = uuid;
				 this.uuidFlag = true;
    			return this;
    		}
    
    		public Builder setDeviceId(String deviceId) {
    			this.deviceId = deviceId;
				this.deviceIdFlag = true;
    			return this;
   		}
    
			public DataKey build(){
            if (userIdFlag) {
				if (userId <= 0) {
					throw new IllegalArgumentException(" USERID cannot be negative or zero");
				}		                
            } else if (uuidFlag && !userIdFlag) {
				if (uuid == null || uuid.isEmpty()) {
					throw new IllegalArgumentException("UUID cannot be null OR empty");
				}	                
            } else if (deviceIdFlag && !userIdFlag && !uuidFlag) {
				if (deviceId == null || deviceId.isEmpty()) {
					throw new IllegalArgumentException("DEVICEID cannot be null OR empty");
				}	                
            } else if (!userIdFlag && !uuidFlag && !deviceIdFlag) {
                throw new IllegalStateException("You have to pass at least one"
                        + " of the following: userId, cguid or udid");
            }			

				return DataKey(this);
			}
    	}
    
    	public long getUserId() {
    		return userId;
    	}
    
    	public String getUuid() {
    		return uuid;
    	}
    
    	public String getDeviceId() {
    		return deviceId;
    	}
    
    	public FlowEnum getFlow() {
    		return flow;
    	}
    
    	public int getClientId() {
    		return clientId;
    	}
    }

I also have an enum which determines whether we are using `userId` or `uuid`/`deviceId`. If `userId` is present then flow type is `USERFLOW`, otherwise flow type is `DEVICEFLOW`.

	public enum FlowEnum {
		USERFLOW, DEVICEFLOW
	}
	
While making keys using the above Builder  class - I will pass `clientId` always, and I will pass atleast one "ID key" from these - `userId`, `uuid` and `deviceId`. If by any chance, all the  ID keys are missing then I need to throw an illegal state exception. 

**Problem Statement:**

As I mentioned above I have three kind of "ID's" - `userId, uuid and deviceId`. `userId` takes the priority first, then `uuid` and then `deviceId`. Now I need to do validations on all these three ID's with below rules -

- Suppose if we are passing all the three ID's, then I need to use `userId` since `userId` takes the priority so I will check whether `userId` is negative or zero, if it is, then throw an `IllegalArgumentException` that `userId` cannot be negative or zero. It is fine, if somebody is passing `uuid` and `deviceid` as null string or empty string in this case.
- Now suppose if we are passing `uuid` and `deviceid` only, then I need to use `uuid` since `uuid` takes the priority now as `userId` is not present so I will check whether `uuid` is null or empty, if it is, then throw an `IllegalArgumentException` that `uuid` cannot be null or empty. It is fine, if somebody is passing `deviceid`  as null string or empty string in this case.
- Now suppose if we are passing `deviceid` only, then I need to use `deviceid` since `deviceid` takes the priority now as `userId` and `uuid` are not present so I will check whether `deviceid` is null or empty, if it is, then throw an `IllegalArgumentException` that `deviceid` cannot be null or empty.

I'm opting for code review to see whether we can simplify or improve the above code basis on the above logic if possible. Is there any improvements which I can do? Do I need boolean flag to make this work?

I can wrap null/empty string and long check in a method in my Utilities class but apart from that, is there anything I can improve?

This is one way of making the keys -

    DataKey keys = new DataKey.Builder(100).setUserId(1234L).setUuid(null).setDeviceId("").build();

or

    DataKey keys = new DataKey.Builder(100).setUserId(1234L).build();