Imagine a bean containing a field.

class Car {
private final List tires = new ArrayList();

public void setTires( List tires ) {
this.tires.clear();
this.tires.addAll( tires );
}

public void addTire( Tire tires ) {
this.tires.add( tires );
}

public List getTires() {
return Collections.unmodifiableList( tires );
}
}

What is the output of that code?

Car car = new Car();
car.addTire( new Tire() );
car.addTire( new Tire() );

List carTires = car.getTires();
System.out.println( "before: " + carTires.size() );
car.setTires( carTires );
System.out.println( "after1: " + car.getTires().size() );
System.out.println( "after2: " + carTires.size() );

The first part is easy:

before: 2

But the rest?

before: 2
after1: 0
after2: 0

We have managed to empty the collection accidentally – even if it is unmodifiable.
The problem lies in the setter and how Collections.unmodifiableList() is implemented:

Collections.unmodifiableList returns a list that is backed by the original list. Every change to the original collections is reflected.
Therefore the “clear” call empties also the unmodifiable list.

public void setTires( List tires ) {
this.tires.clear(); //the list is cleared! But since the method parameter is a view of this.tires, it is emptied, too.
this.tires.addAll( tires ); //since tires is empty now, addAll doesn't do anything...
}

So be careful with collections. They might change a really surprising moments…

They have now released another project that can be very, very helpful. It is called Google Collections and provides some small but helpful classes.

A series describing the details can be found here:
http://publicobject.com/2007/09/coding-in-small-with-google-collections_08.html