Item 10: Always override toString

While java.lang.Object provides an implementation of the toString method, the string that it returns is generally not what the user of your class wants to see. It consists of the class name followed by an “at” sign (@) and the unsigned hexadecimal representation of the hash code, for example, “PhoneNumber@163b91.” The general contract for toString says that the returned string should be “a concise but informative representation that is easy for a person to read” [JavaSE6]. While it could be argued that “PhoneNumber@163b91” is concise and easy to read, it isn’t very informative when compared to “(707) 867-5309.” The toString contract goes on to say, “It is recommended that all subclasses override this method.” Good advice, indeed!

While it isn’t as important as obeying the equals and hashCode contracts (Item 8, Item 9), providing a good toString implementation makes your class much more pleasant to use. The toString method is automatically invoked when an object is passed to println, printf, the string concatenation operator, or assert, or printed by a debugger. (The printf method was added to the platform in release 1.5, as were related methods including String.format, which is roughly equivalent to C’s sprintf.)