#590 add explanation for Observer
#590 add explanation for Observer

javaseeds/java-design-patterns,javaseeds/java-design-patterns,mookkiah/java-design-patterns,mookkiah/java-design-patterns,javaseeds/java-design-patterns,javaseeds/java-design-patterns,mookkiah/java-design-patterns,mookkiah/java-design-patterns