General Java Questions I

iteration with well defined semantics. 

Method names have been improved. 

The first point is important: There was no safe way to remove elements from a

collection while traversing it with an Enumeration. The semantics of this operation

were ill defined, and differed from implementation to implementation. 

The Iterator interface is shown below: 

public interface Iterator {

   boolean hasNext();

   Object next();

   void remove(); // Optional

}

The hasNext method is identical in function to Enumeration.hasMoreElements, and

the next method is identical in function to Enumeration.nextElement. The remove

method removes from the underlying Collection the last element that was returned by

next. The remove method may be called only once per call to next, and throws an

exception if this condition is violated. Note that Iterator.remove is the only safe way to

modify a collection during iteration; the behavior is unspecified if the underlying

collection is modified in any other way while the iteration is in progress. 

The following snippet shows you how to use an Iterator to filter a Collection, that is, to

traverse the collection, removing every element that does not satisfy some condition: 

static void filter(Collection c) {

   for (Iterator i = c.iterator(); i.hasNext(); )

       if (!cond(i.next()))

       i.remove();

}

Two things should be kept in mind when looking at this simple piece of code: 

The code is polymorphic: it works for any Collection that supports element removal,

regardless of implementation. That s how easy it is to write a polymorphic algorithm

under the collections framework! 

It would have been impossible to write this using Enumeration instead of Iterator,

because there s no safe way to remove an element from a collection while traversing

it with an Enumeration.

How can I find the first dimension length of the 2 dimenstions array? I have use

the array[].length but it does not work, how can I solve this problem? 

Answer: Java doesn t really have "multidimensional arrays", only arrays of arrays. So

try: array[0].length and you will get this dimension.

I guess what I m asking is "Is java.util.Hashtable thread safe?"

Q: It s been a while since I ve used hashtables for anything significant, but I seem to

recall the get() and put() methods being synchronized. 

The JavaDocs don t reflect this. They simply say that the class Hashtable is

synchronized. What can I assume? If several threads access the hashtable at the

same time (assuming they are not modifying the same entry), the operations will

succeed, right? I guess what I m asking is "Is java.util.Hashtable thread safe?"

Answer: That is right! It is recommendable, if you have questions like these, always

look at source for the API, it s freely available.

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