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CMSC 433 – Programming Language
Technologies and Paradigms
Spring 2007
Observer Pattern
Apr. 17, 2007
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What is it? And an Example
• The observer pattern (someti mes known as
publish/subscribe) is a design pattern used to observe the
state of an object.
• For example, we often like to display data in more than
one form at the same time and have all of the displays
reflect any changes in that data.
– For example, you might repre sent stock price changes both as a
graph and as a table or list box.
– Each time the price changes, we’d expect both representations to
change at once without any ac tion on our part.
– In Java we can easily make use of the Observer Design Pattern to
cause our program to behave in th is way.
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“The Observer Pattern Assumption”
• The Observer pattern
assumes that the object
containing the data is
separate from the object s
that display the data, and
that these display objects
observe changes in that
data.
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The Observer Philosophy
• We usually refer to the data as the Subject and
each of the displays as Observers.
• Each of these observers registers its interest in the
data by calling a public method in the Subject.
• Then, each observer has a known interface that the
subject calls when the data change.
