README.md

Adapter Pattern

Let's say we have a simple interface like a Logger

interface Logger {
  void log(String message);
}

and we have an existing API that takes a Logger

public static void sayHello(Logger logger) {
  logger.log("hello");
}

Now, we introduce another logging API named Logger2, perhaps from another library

enum Level { WARNING, ERROR }

interface Logger2 {
  void log(Level level, String message);
}

and we want to use that new interface Logger2 with the method sayHello(). Obviously, we can not directly call sayHello with a Logger2, we need to do adaptation work.

Adapter when we do not control Logger2

A Logger2 is a Logger with a supplementary Level so adapting a Logger2 to be a Logger necessitate a level.

public static Logger adapt(Logger2 logger2, Level level) {
  return msg -> logger2.log(level, msg);
}

And we can use the method adapt() like this

Logger logger = adapt(logger2, Level.WARNING);
logger.log("abort abort !");

An adapter acts a vue that transfers the data from one method of the new interface to a method call to the old interface.

Adapter when we control Logger2

If we control Logger2, the method adapt() can be an instance method of Logger2

interface Logger2 {
  void log(Level level, String message);

  default Logger adapt(Level level) {
    return msg -> log(level, msg);
  }
}

In terms of UML diagram, the lambda inside adapt acts as an implementation of Logger that delegates its implementation to Logger2

classDiagram
class Logger {
  <<interface>>
  log(String message);
}
class LoggerLambda {
  <<lambda>>
  log(String message);
}
class Logger2 {
  <<interface>>
  log(Level level, String message)
}
LoggerLambda <|.. Logger
LoggerLambda --> Logger2 : delegates

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and we can call adapt() directly on an instance of Logger2

Logger logger = logger2.adapt(Level.WARNING);
logger.log("abort abort !");