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+package design_patterns.observer_design_pattern;%0Aimport design_patterns.observer_design_pattern.Subject;%0A%0A/*%0A%09Interaface observer - To be implemented by all observers%0A*/%0A%0Ainterface Observer%7B%0A%0A%09// update %0A%09void update(String mode);%0A%0A%09// subscribe to a subject%0A%09void subscribe(Subject sub);%0A%0A%09// unsubscribe from subject%0A%09void unsubscribe(Subject sub);%0A%0A%7D

Add observer interface