Java written test face questions

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Record some Java written exams, and interview with less certain questions:

The difference between 1,string,stringbuffer,stringbuilder

2,servlet is a single instance or multi-instance

3,hashmap<integer,string>, please write out a possible output result

4,hibernate, the three object states of the session

5,javascript closure, JS get form <input id= "test" type= "name"/>, please describe how to take

There are also sessions in 6,web, where the SessionID is allocated, the session expires when the browser is closed, and SessionID also exists in the client's cookie,

When it shuts down, the client will know that the SessionID has failed, (Expiration time)

7, integer array, know the maximum value is 20, please arrange from small to large, the algorithm complexity of 1

Can I change the value of a 8,final modified variable?

9, where is the memory allocated by the object, heap, stack?

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Java written test face questions