Definition: The interpreter pattern builds an interpreter for a language.

Usage: Use the interpreter pattern to convert a language into another form. It can converted into another language, a set of objects within the program, or even into a program itself. It also can be executed one line at a time.

Example

enum AiAction {JUMP, MOVE_LEFT, MOVE_RIGHT}

class Interpreter {
	static ArrayList interpret(String text) {
		String[] aTextLine = split text up at each newline
		ArrayList aAiAction = new ArrayList();
		for(int iTextLine = 0; iTextLine < aTextLine.length; iTextLine++) {
			String line = aTextLine[iTextLine]
			if(line.equalsIgnoreCase("jump"))
				aAiAction.add(AiAction.JUMP);
			else if(line.equalsIgnoreCase("move left"))
				aAiAction.add(AiAction.MOVE_LEFT);
			else if(line.equalsIgnoreCase("move right"))
				aAiAction.add(AiAction.MOVE_RIGHT);
			else
				output an error
		}

		return aAiAction;
	}
}

The purpose of this interpreter is to cause a computer-controlled sprite to take various actions from a list of actions.

This is a simple and very ugly example of the interpreter design pattern. A more realistic example would span several classes and be much too complex. It is important to read a book about compilers and interpreters if you want to program a professional quality interpreter.