Question

Assume:
A simple modular java library has only two module:

package com.mycorp.lib.logger;
public class LogcatLogger implements Logger {

    @Override
    public void out(LogcatLogger.LogMessage message){
        Log.i("TAG", message.status);
    }

    ...
}

package com.mycorp.lib.webservice;
public class JsonWebservice implements Webservice {

    @Override
    public void onErrorFound(String status) {
        LogcatLogger.LogMessage message = new LogcatLogger.LogMessage();
        message.status = status;
        LogcatLogger.out(message);
    }

    ...
}

Question:
How to make com.mycorp.lib.webservice and com.mycorp.lib.logger independent like this? ( client application can invoke methods too )

Notes:
I try Command Pattern before:
Result: api has many methods and many command class must create

Update1:
Added LogMessage as nested static class of LogcatLogger for more challenging situation

Update2:
Target Design image added.

Any help appreciated.

Answer 1

Use the Strategy Pattern. Basically, make LogcatLogger implement a custom interface, and reference that interface in the service.

package com.mycorp.lib.invoker;
public interface Logger {
    void out(Loggable message);
}

package com.mycorp.lib.invoker;
public interface Loggable {
    void setStatus(String status);
    String getStatus();
}

package com.mycorp.lib.invoker;
public class LoggableFactory {
    public static Loggable createDatabaseLogMessage() { return new DatabaseLogMessage(); }
    public static Loggable createWebserviceLogMessage() { return new WebserviceLogMessage(); }
}

package com.mycorp.lib.logger;
public class WebserviceLogMessage implements Logger {

    @Override public void out(Loggable message){
        Log.i("TAG", message.getStatus());
    }

    ...
}

package com.mycorp.lib.logger;
public class DatabaseLogMessage implements Logger {
    @Override public void out(Loggable message) {
        Log.i("JDBC",message.getStatus());
        otherFancyStuff(); // whatever ? :)
    }
}

package com.mycorp.lib.webservice;
public class JsonWebservice implements Webservice {
    Logger out;

    JsonWebservice(Logger out) { this.out = out; }

    @Override
    public void onErrorFound(String status) {
        Loggable message = LoggableFactory.createWebserviceLogMessage();
        message.setStatus(status);
        out.out(message);
    }

    ...
}