HashMap和IdentityHashMap的区别in an IdentityHashMap, two keys k1 and k2 are considered equal if and
HashMap和IdentityHashMap的区别
in an IdentityHashMap, two keys k1 and k2 are considered equal if and only if (k1==k2). (In normal Map implementations (like HashMap) two keys k1 and k2 are considered equal if and only if (k1==null ? k2==null : k1.equals(k2)).)
--From Java docs