General Java Questions II

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.

Q: I try to copy an object of my own using the clone() method from

java.lang.Object, but this is a protected method so I can t use it. Is there some other

way to get my objective of duplicating an arbitrary object?

Answer: If you want to clone your object, you need to make it cloneable. To achieve

this, you need to do two things:

1. implement the interface Cloneable

2. override the method clone(), so that it 

a. becomes public

b. calls super.clone()

c. if necessary, clones any members, or

d. if a member can t be cloned, creates a new instance.

Simple example:

public MyClass implements Cloneable {

   int someNumber;

   String someString;

   public Object clone() {

   // primitives and Strings are no

   // problem

      return super.clone();

  }

}

In this case the method clone() of the class MyClass returns a new instance of

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