JAVA EXAMPLE PROGRAMS

JAVA EXAMPLE PROGRAMS

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Composite Pattern in java


The composite pattern is a partitioning design pattern. The composite pattern describes that a group of objects are to be treated in the same way as a single instance of an object. The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.

When dealing with Tree-structured data, programmers often have to discriminate between a leaf-node and a branch. This makes code more complex, and therefore, error prone. The solution is an interface that allows treating complex and primitive objects uniformly. In object-oriented programming, a composite is an object designed as a composition of one-or-more similar objects, all exhibiting similar functionality. This is known as a "has-a" relationship between objects. The key concept is that you can manipulate a single instance of the object just as you would manipulate a group of them. The operations you can perform on all the composite objects often have a least common denominator relationship. For example, if defining a system to portray grouped shapes on a screen, it would be useful to define resizing a group of shapes to have the same effect (in some sense) as resizing a single shape.

Composite should be used when clients should ignore the difference between compositions of objects and individual objects. If programmers find that they are using multiple objects in the same way, and often have nearly identical code to handle each of them, then composite is a good choice; it is less complex in this situation to treat primitives and composites as homogeneous. Below is the UML structure of composite pattern.

Class representing Component:

package com.java2novice.dp.composite.pattern;

public interface Employee {

	public void showEmployeeDetails();
}

Class representing Leaf

package com.java2novice.dp.composite.pattern;

public class Engineer implements Employee {

	private String name;
	private long empId;
	private String department;

	public Engineer(long empId, String name, String department){
		this.empId = empId;
		this.name = name;
		this.department = department;
	}
	
	@Override
	public void showEmployeeDetails() {
		System.out.println(empId+" ***** "+name+" ***** "+department);
	}
}

Class representing Leaf

package com.java2novice.dp.composite.pattern;

public class Accountant implements Employee {

	private String name;
	private long empId;
	private String department;

	public Accountant(long empId, String name, String department){
		this.empId = empId;
		this.name = name;
		this.department = department;
	}
	
	@Override
	public void showEmployeeDetails() {
		System.out.println(empId+" ***** "+name+" ***** "+department);
	}
}

Class representing Composite

package com.java2novice.dp.composite.pattern;

import java.util.ArrayList;
import java.util.List;

public class CompanyDirectory implements Employee {

	private List<Employee> employeeList = new ArrayList<Employee>();
	
	@Override
	public void showEmployeeDetails() {
		for(Employee emp:employeeList){
			emp.showEmployeeDetails();
		}
	}
	
	public void addEmployee(Employee emp){
		employeeList.add(emp);
	}
	
	public void removeEmployee(Employee emp){
		employeeList.remove(emp);
	}
}

Class representing Client

package com.java2novice.dp.composite.pattern;

public class Company {

	public static void main(String a[]){
		
		Engineer eng1 = new Engineer(100, "Nataraj", "Engineering");
		Engineer eng2 = new Engineer(101, "Ravi", "Engineering");
		CompanyDirectory engDirectory = new CompanyDirectory();
		engDirectory.addEmployee(eng1);
		engDirectory.addEmployee(eng2);
		
		Accountant acc1 = new Accountant(200, "Gopi", "Accounts");
		Accountant acc2 = new Accountant(201, "RamGopal", "Accounts");
		CompanyDirectory accDirectory = new CompanyDirectory();
		accDirectory.addEmployee(acc1);
		accDirectory.addEmployee(acc2);
	
		CompanyDirectory directory = new CompanyDirectory();
		directory.addEmployee(engDirectory);
		directory.addEmployee(accDirectory);
		directory.showEmployeeDetails();
	}
}

Output:
100 ***** Nataraj ***** Engineering
101 ***** Ravi ***** Engineering
200 ***** Gopi ***** Accounts
201 ***** RamGopal ***** Accounts
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Java design pattern examples

  1. Factory Pattern
  2. Abstract Factory Pattern
  3. Builder Design Pattern
  4. Prototype Pattern
  5. Adapter Pattern
  6. Composite Pattern
  7. Proxy Pattern
Knowledge Centre
Java class can be private?
We can not declare top level class as private. Java allows only public and default modifier for top level classes in java. Inner classes can be private.
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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java is registered trademark of Oracle.
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