The reason is that the string concatenation operation is doing a String.valueOf() on the argument, and that delegates to toString() that promises a human readab

The reason is that the string concatenation operation is doing a String.valueOf() on the argument, and that delegates to toString() that promises a human readable string that is relevant. For some reason someone decided to check for null instead of the stricter option of throwing a NullPointerException. Maybe to help debugging or to follow the gist of toString(). I guess it was too late to change even in Java 0.9... Autoboxing, iterators and other newer features are more strict,so I suppose it's just one of the few irregularities left from prehistoric times...