templatemethod

Template Method Pattern

The idea of the template method pattern is to decompose a problem in two parts, a generic reusable part and a specific part.

Using inheritance

Let's say our generic part add hearts to a String and the specific part returns the string to be decorated.

Using an abstract class DecoratedString, the abstract method plainString()returns the string to be decorated and the methodloveString()` the generic part that adds a heart before and after the string.

abstract class DecoratedString {
  protected abstract String plainString();

  public String loveString() {
    return "❤️ " + plainString() + " ❤️";
  }
}

and the specific part use inheritance (here with an anonymous class) to specify the string to be decorated.

static void main(String[] args) {
  var decoratedString = new DecoratedString() {
    @Override
    protected String plainString() {
      return "hello";
    }
  };
  System.out.println(decoratedString.loveString());
}

Using inheritance here as usual is not the best solution because the API of DecoratedString contains both methods even if it may not make sense. In particular, inside the method plainString(), one can call the method loveString() leading to a recursive loop that will blow the stack.

Using delegation

A better design is to use delegation instead of inheritance. The class DecoratedString takes a Supplier at construction and call it in loveString().

class DecoratedString {
  private final Supplier<String> supplier;

  public DecoratedString(Supplier<String> supplier) {
    this.supplier = supplier;
  }

  public String loveString() {
    return "❤️ " + supplier.get() + " ❤️";
  }
}

A use site, the supplier can be specified as a lambda

static void main(String[] args) {
  var decoratedString = new DecoratedString(() -> "hello");
  System.out.println(decoratedString.loveString());
}

The Memoizer Pattern is a more advanced example of template method pattern.