Ref - See Guideline 6-5: Do not trust identity equality when overridable on input reference objects
http://jroller.com/tackline/entry/user_supplied_objects_may_not
Noncompliant Code Example
How many items are there in layouts in the end?
public class Widget {
private int noOfComponents;
public Widget(int noOfComponents) {
this.noOfComponents = noOfComponents;
}
public int getNoOfComponents() {
return noOfComponents;
}
public void setNoOfComponents(int noOfComponents) {
this.noOfComponents = noOfComponents;
}
// Also overrides hashcode() (code is omitted) ...
public boolean equals(Object o) {
if (o == null || !(o instanceof Widget)) {
return false;
}
// check for negative components
if (noOfComponents < 0) {
return false;
}
Widget widget = (Widget) o;
return this.noOfComponents == ((Widget) o).getNoOfComponents();
}
}
public class Navigator extends Widget {
public Navigator(int noOfComponents) {
super(noOfComponents);
}
@Override
public boolean equals(Object o) {
if (o == null) {
return false;
}
return true;
}
}
public class LayoutManager {
private Set<Widget> layouts = new HashSet<Widget>();
public void addWidget(Widget widget) {
if (!layouts.contains(widget)) {
layouts.add(widget);
}
}
public int getLayoutSize() {
return layouts.size();
}
}
Client code
Widget nav = new Navigator(-1);
Widget widget = new Widget(10);
LayoutManager manager = new LayoutManager();
manager.addWidget(nav);
manager.addWidget(widget);
System.out.println(manager.getLayoutSize()); // prints 2
Noncompliant Code Example
What gets printed - main or child / both / either ?
public class Trusted implements Runnable {
Trusted() { }
public void startThread(String name) {
new Thread(this, name).start();
}
@Override
public void run() {
System.out.println("Child");
}
}
public class Untrusted extends Trusted {
// Note untrusted code may start a new thread even during construction
Untrusted(String name) {
super.startThread(name);
}
@Override
public void run() {
System.out.println("Main");
}
}
Client code:
Trusted a = new Untrusted("Main");
a.run();