Headline

Object composition in Java

Characteristics

Basic style of OO programming is applied. A simple object model is provided for Feature:Hierarchical company. The object model leverages object composition to compose companies from (nested) departments and employees. The operations for Feature:Total and Feature:Cut are implemented as instance methods on the classes for companies, departments, and employees. Closed serialization is enabled for all the classes with the help of Java's Technology:Object Streams. To this end, the marker interface Serializable is applied to the classes of the object model for companies. This form of serialization counts as closed serialization because the serialization format is definitely Java-specific.

Illustration

The data model is implemented as POJOs. For example Feature:Company looks like this:

/**
 * A company has a name and consists of (possibly nested) departments.
 */
public class Company implements Serializable {

	private static final long serialVersionUID = -200889592677165250L;
	
	private String name;
	private List<Department> depts = new LinkedList<Department>();

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public List<Department> getDepts() {
		return depts;
	}
	
	public Double total() {
		double total = 0;
		for (Department d : getDepts())
			total += d.total();
		return total;
	}	
	
	public void cut() {
		for (Department d : getDepts())
			d.cut();
	}	
}

Feature:Closed serialization is implemented using Language:Java Technology:Object Streams:

    /*
     * Read (say, deserialize) a company.
     */
    public static Company deserializeCompany(String filename) {

        Object o = null;

        try {
            FileInputStream fis = new FileInputStream(filename);
            ObjectInputStream in = new ObjectInputStream(fis);
            o = in.readObject();
            in.close();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return (Company) o;
    }
    /*
     * Write (say, serialize) a company.
     */
    public static boolean serializeCompany(Company c, String filename) {

        FileOutputStream fos = null;
        ObjectOutputStream out = null;

        try {
            fos = new FileOutputStream(filename);
            out = new ObjectOutputStream(fos);
            out.writeObject(c);
            out.close();
            return true;
        } catch (IOException ex) {
            ex.printStackTrace();
            return false;
        }
    }

Feature:Total and Feature:Cut are implemented as calls of POJO methods:

    public static double total(Company c) {
        // Total is implemented in the Company class
        return c.total();
    }

    public static void cut(Company c) {
        // Cut is implemented in the Company class
        c.cut();
    }

Test cases are implemented for all Namespace:Features.

Relationships

For basic OO without inheritance see Contribution:javaComposition.

For basic OO with inheritance see Contribution:javaInheritance.

For modular OO programming with static methods see Contribution:javaStatic.

For use of Java reflection see Contribution:javaReflection (data processing) and Contribution:javaSyb (SYB-style generic programming).

For design patterns see Contribution:javaTemplate (template design pattern), Contribution:javaVisitor (visitor design pattern) and Contribution:javaExorcism (excessive illustration of design patterns).

Architecture

The contribution follows a standardized structure:

  • inputs contains input files for tests
  • src/main/java contains the following packages:
  • src/test/java contains the following packages:

Usage

This contribution uses Technology:Gradle for building. Technology:Eclipse is supported.

See https://github.com/101companies/101simplejava/blob/master/README.md

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