If your class makes a data structure, then indented output showing any tree structures will greatly ease debugging. Be careful about circular references (A writes out B which writes out A). In a tree, a child node should print out the parentId instead of the full dump of the parent (which would include the child and loop infinitely).

Use the the methods in StringUtils to implement this and pretty-print Indented data structures for joyful debugging.

You'll typically then want to implement .toString() by calling indentedStr(0). If your class's reason for existence is to generate a string, you should probably use that string for debugging instead of implementing this interface.