Object Key combining multiple variables for using a key

UserProfileKey.java

package com.daeheehan.cache;

public class UserProfileKey implements Comparable<UserProfileKey>{
	private String userId;
	private String deviceId;
	
	public UserProfileKey(String userId, String deviceId) {
		this.userId = userId;
		this.deviceId = deviceId;
	}

	public String getUserId() { 
		return userId;
	}
	public String getDeviceId() { 
		return deviceId;
	}

	public boolean isEmpty() {
		if( userId == null || deviceId == null) {
			return true;
		}
		
		return false; 
	}
	@Override
	public int hashCode() {
	return userId.hashCode()+31 * deviceId.hashCode();
	}
	
	@Override
	public boolean equals(Object o) {
		if(o==this) return true;
		
		if(o==null || !(o instanceof UserProfileKey)) return false;
		
		UserProfileKey cp= UserProfileKey.class.cast(o);
		
		return userId.equals(cp.userId) && deviceId.equals(cp.deviceId);
	}

	public int compareTo(UserProfileKey cp) {
	     if(cp==this) return 0;
	     
	     int i= userId.compareTo(cp.userId);
	     
	     if(i!=0) return i;
	     
	     return deviceId.compareTo(cp.deviceId);
	}
	
	@Override
    public String toString() {
       return "(" + userId +";" + deviceId + ")";
	}
}