List Behavior that seems a bit non intuitive

listBehavior

/*
  * in the simple test below I am a bit uncomfortable with how 
  * the private property pathSet is updated w/o a setter
  * is this exploiting a Java bug or is it intentional and expected behavior
  * that will be supported for a long time?
*/

public class SimpleClass {
    private List<String> pathSet;
    public List<String> getPathSet() {
        if(this.pathSet == null) {
            this.pathSet = new ArrayList();
        }
        return this.pathSet;
    }
}

public class JavaScratchsheet{           
	SimpleClass ss = new SimpleClass();
    ss.getPathSet().add("foo");
    System.out.println("pathSet = " + ss.getPathSet()); // >> pathSet = [foo]
    ss.getPathSet().add("bar");
    System.out.println("pathSet = " + ss.getPathSet()); // >> pathSet = [foo, bar]
}