单项选择题

Which statement is true？（）

A.Compilation fails because the hashCode method is not overridden.
B.A HashSet could contain multiple Person objects with the same name.
C.All Person objects will have the same hash code because the hashCode method is not overridden.
D.If a HashSet contains more than one Person object with name="Fred"， then removing another Person， also with name="Fred"， will remove them all.

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单项选择题What is the result？（）

A.The code runs with no output.
B.An exception is thrown at runtime.
C.Compilation fails because of an error in line 20.
D.Compilation fails because of an error in line 21.
E.Compilation fails because of an error in line 23.
F.Compilation fails because of an error in line 25.

单项选择题What is the result？（）

A.3， 2， 1
B.1， 2， 3
C.Compilation fails.
D.The code runs with no output.
E.An exception is thrown at runtime.

单项选择题What is the result？（）

A.A,B,C
B.B,C,A
C.Compilation fails.
D.The code runs with no output
E.An exception is thrown at runtime.

单项选择题What inserted at line 39， will sort the keys in the props HashMap？（）

A.Arrays.sort（s）；
B.s = new TreeSet（s）；
C.Collections.sort（s）；
D.s = new SortedSet（s）；

单项选择题What is the result？（）

A.2，1，0
B.2，1，1
C.3，2，1
D.3，2，2
E.Compilation fails.
F.An exception is thrown at runtime.