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copied!<p>First, lets define a Printer class.</p> <pre><code>package com.ggl.modeltest; public class Printer { private String companyName; private String printerName; public Printer(String companyName, String printerName) { this.companyName = companyName; this.printerName = printerName; } public String getCompanyName() { return companyName; } public String getPrinterName() { return printerName; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((companyName == null) ? 0 : companyName.hashCode()); result = prime * result + ((printerName == null) ? 0 : printerName.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Printer other = (Printer) obj; if (companyName == null) { if (other.companyName != null) return false; } else if (!companyName.equals(other.companyName)) return false; if (printerName == null) { if (other.printerName != null) return false; } else if (!printerName.equals(other.printerName)) return false; return true; } public Printer copy() { return new Printer(companyName, printerName); } } </code></pre> <p>This class has a constructor to define the values, and two getter methods to retrieve the values. Once you create an instance of this class, you can't change it. </p> <p>These types of non modifiable classes make debugging simpler.</p> <p>The hashCode and equals methods look pretty scary. I had Eclipse generate those methods for me. The reason that we override these methods is that we're going to use them in the <code>Computer</code> class.</p> <p>So now, we define the <code>Computer</code> class.</p> <pre><code>package com.ggl.modeltest; import java.util.ArrayList; import java.util.List; public class Computer { private List<Printer> printerNames; private String computerName; public Computer(String computerName) { this.computerName = computerName; this.printerNames = new ArrayList<Printer>(); } public void addPrinter(Printer printerName) { this.printerNames.add(printerName); } public void removePrinter(Printer printerName) { for (int i = printerNames.size() - 1; i >= 0; i--) { if (printerNames.get(i).equals(printerName)) { printerNames.remove(i); } } } public List<Printer> getPrinterNames() { return printerNames; } public String getComputerName() { return computerName; } } </code></pre> <p>The constructor takes the name of the computer. Once an instance is constructed, there's no way to change the name of the computer.</p> <p>Printers are added to and removed from a computer. Two methods have been defined to add a printer to the computer, and remove a printer from a computer.</p> <p>The remove method removes all instances of the printer from the <code>List</code>, in case more than one was entered.</p> <p>The method that gets the printer list returns an instance of the <code>List</code>. The calling program can change the contents of this List. If you want to make sure that the calling program can't change the contents of the list, you make a deep copy of the <code>List</code>.</p> <p>To make a deep copy, you write a copy method for the <code>Printer</code> class.</p> <pre><code>public Printer copy() { return new Printer(companyName, printerName); } </code></pre> <p>Since strings are unchangeable, we don't have to make a copy of the strings. If we had changeable values, we would have to make a copy of them as well. That's why this is called a deep copy. It can be difficult to get everything copied correctly the first time. That's what testing is for.</p> <p>Then you call this copy method in a loop in the <code>getPrinterNames</code> method of the <code>Computer</code> class.</p> <pre><code>public List<Printer> getPrinterNames() { List<Printer> list = new ArrayList<Printer>(); for (Printer printer : printerNames) { list.add(printer.copy()); } return list; } </code></pre>

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