Item 7: Eliminate obsolete object references(排除过时的对象引用)
Ifyouswitchedfromalanguagewithmanualmemorymanagement,suchasCorC++,toagarbage-collectedlanguagesuchasJava,yourjobasaprogrammerwasmademucheasierbythefactthatyourobjectsareautomaticallyreclaimedwhenyou’r