This technical question asks you to define a concept within Java. When interviewing at Facebook or any other organizaiton, you should carefully listen to the questions you are being asked. Do not start to formulate your answer until the interviewer has completed asking the question. This will ensure that you understand what they are asking and that your answer corresponds to what they are looking for. When answering this question, the mistake you can make is providing too much information since you are only being asked to define the concept.