What is meant by Composite Design Pattern?

COMPOSITE:
           Build a complex object out of elemental objects and itself like a tree structure.

 

Where to use & benefits

• Want to represent a part-whole relationship like tree folder system
• Group components to form larger components, which in turn can be grouped to form still larger components.
• Related patterns include 

Example

A component has many elements and itself which has many elements and itself, etc. A file system is a typical example. Directory is a composite pattern. When you deal with Directory object, if isFile() returns true, work on file, if isDirectory() returns true, work on Directory object.

class Directory {
   Directory dir;
   File[] f;
   ...
   
   boolean isDirectory() {
       return f == null;
   }
   boolean isFile() {
       return f != null;
   }
   File getFile(int i) {
      if (isFile())
         return f[i];
      return null'
   }
   Directory getDirectory() {
      if (isDirectory())
          return dir;
      return null;
   }
   ....
}

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