Creating different POJO child objects depending on criteria but share common fields

Mustafa Source

I am implementing a log management system and want the types of logs to be extendible. We get a base object parsed from JSON (from Filebeat) such as:

class LogLine {
   String message
   Date timestamp
   String type
   String source

Given this LogLine object, I want to be able to create different objects, which will also extend this LogLine.

class SomeLog extends LogLine {
    int myfield
    String username

class SomeOtherLog extends LogLine {
   Date startTime
   Date endTime
   String username
   String transactionID

So, in my current non-ideal implementation:

void parse(String s){
   LogLine logLine = .....parseFromString(s)
   if ( logline.type.equals('def') ){
      SomeLog someLog = new
   } else if ( logline.message.containts('abc') ){
      SomeOtherLog someotherLog = new

However, as you can imagine the builders in subclasses copies the superclass LogLine object, is there anyway I can do that without copying the values as they are already subclasses? Is there a design pattern to achieve this? I would not like to rely on reflection like BeanUtils.copyProperperties



answered 5 days ago ADS #1

When you create a new object based on another it's a good idea to make a copy of all field. It's a best practice called defensive copying.

Since you parse a string, a defensive copy doesn't needed. Also I suppose you'll want to parse some specific fields from input string like myfield for SomeLog and startDate for SomeOtherLog. You could re-factor object creation like

LogLine result = null;
if (s.contains('type=def') { 
  result = SomeLog.parse(s);
} else if (trickyRegexp.mathces(s)) {
   result = SomeOtherLog.parse(s);
} else {
   result = LogLine.parse(s);

If you have many subclasses of LogLine then probably you'll want to move creation logic to a LogFactory which manages all the stuff regarding parsing string to specific object.

answered 5 days ago René Link #2

Introduce a factory interface for creating LogLine objects.

public interface LogLineFactory {
    public LogLine createLog(LogLine logLine);

and use a Map for the lookup.

private Map<String, LogLineFactory > logLineFactories = new HashMap<>();

    logLineFactories .put("def", new SomeLogFactory());
    logLineFactories .put("abc", new SomeOtherLogFactory());

You can then ommit the if else branches using the map looup.

LogLine logLine = parseFromString(s);
LogFactory logFactory = logLineFactories.get(logLine.type);

if(logFactory != null) {
    LogLine wrappedLogLine = logFactory.createLog(logLine);

Maybe you will need more information to create the LogLines and you have to change the interface.

public interface LogLineFactory {
    public LogLine createLog(LogLine logLine, String s);

PS: with Java 8 you might want to use method references.

logLineFactories.put("def", SomeLog::new);
logLineFactories.put("abc", SomeOtherLog::new);    

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