Distinction between Rule and Indicator

[sgflt]

I am a little bit confused between purposes of Rule and Indicator.

How to decide which interface I should implement.

For example DownTrendIndicator as Indicator or DownTrendRule and isStatisfiedAt(index)?

[TheCookieLab]

Let the return types guide your thinking. An Indicator is the closest 1:1 mapping with what you'd typically think of for a technical indicator, and it typically produces some numerical result. A Rule uses Indicators to determine whether or not some condition is satisfied. It can be ambiguous at times, but for your example DownTrendRule is more appropriate.

[sgflt]

So Rules can be reused inside indicators?

For example Hammer candle indicator have to know whether it is in downtrend.

ta4j/ta4j-core/src/main/java/org/ta4j/core/indicators/candles/HammerIndicator.java

Line 72 in 9a88ee3

&& this.trendIndicator.getValue(index);

After some hours of flipping between indicator and rule I gave higher weight to Rules javadoc and implemented trend inidcator instead.

ta4j/ta4j-core/src/main/java/org/ta4j/core/Rule.java

Lines 32 to 33 in 9a88ee3

	* A rule (also called "trading rule") used to build a {@link Strategy trading
	* strategy}. A trading rule can consist of a combination of other rules.