1.GC of java is Runtime, while ARC is compile time.（GC是java运行时,而ARC是编译时。） 2.GC has reference to the objects at runtime and check for the dependencies of object runtime. While ARC appends the release, retain, autorelease calls at compiletime.（GC引用的对象在运行时,检查运行时对象的依赖项。而ARC在编译时间 添加调用release, retain, autorelease）