Push Down

Push Down relocates a method or operator from a superclass into the subclasses you choose: removed from the superclass, copied into each one picked. Here fly() is declared on Bird, but a Penguin cannot fly - only Eagle should keep it. To play it in the IDE: Help > Lessons > Refactoring…, or ek9 -ide -lesson push-down.

Start from

The code the lesson starts from: copy it into a file of your own and follow the steps.

#!ek9
defines module lesson.birds

  defines class

    Bird as open

      fly() as pure
        <- rtn as String: "soaring through the sky"

    Eagle extends Bird

    Penguin extends Bird

  defines program

    Birds()
      stdout <- Stdout()
      eagle <- Eagle()
      stdout.println(eagle.fly())

//EOF

Step 1: Where it is

Put the caret on fly, on line 8.

Step 2: Push Down

Right-click it - or choose Refactor > Refactor This… - and choose Push 'fly' Down…

Tick Eagle only - Penguin stays without fly() - and choose Preview.

Step 3: Preview and apply

The preview shows every change the refactoring makes; then the IDE formats each file it changed. Choose Apply.

The result

That is Push Down: fly() is gone from Bird and lives on Eagle alone; Penguin has none at all. A caller that reached the member through the superclass type, or a subclass left out that still needed it, surfaces in the preview instead of a silent wrong move. It is on the right-click menu wherever the caret is on a method or operator of a class with direct subclasses; AI assistants use the ek9_push_down tool.

Where this lesson comes from

• For AI assistants - the MCP tool ek9_push_down.