Solving Common Collections Problems

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Trail: Collections

Lesson: Solving Common Collections Problems

Problem: When I try to use Arrays.sort() or Collections.sort() I get a ClassCastException.
The problem is that you're using the 1.1 backport. To sort objects without providing an explicit comparator, the objects must be "mutually comparable." In 1.2, String (and many other JDK classes) have been retrofitted to implement Comparable. In 1.1, you have to provide an explicit String comparator. Since this is your lucky day, I'll provide you with one:
    static final Comparator stringCmp = new Comparator() {
        public int compare(Object o1, Object o2) {
            String s1 = (String)o1;
            String s2 = (String)o2;
            int len1 = s1.length();
            int len2 = s2.length();
                for (int i=0, n=Math.min(len1, len2); i<n; i++) {
                    char c1 = s1.charAt(i);
                    char c2 = s2.charAt(i);
                    if (c1 != c2)
                        return c1 - c2;
                }
             return len1 - len2;
        }
    };  
With this in your program, merely replace the line:
        Collections.sort(l);
With:
        Collections.sort(l, stringCmp);
and you're in business!
Note: A better solution might be to download the Java 2 Platform (formerly known as JDK 1.2) to take advantage of the implementation of Comparable.

Problem: What collection interface behaves like the legacy Vector class? The feature I'm most interested in is a Vector's ability to grow dynamically in size.
All of the new general-purpose collection implementations have the ability to grow dynamically in size. The new interface that models Vector's behavior is List. The two general purpose implementations of List are ArrayList and LinkedList. The one whose performance properties are similar to Vector's is ArrayList. All other things being equal, ArrayList is the preferred List implementation. So, to recap, the Java 2 replacement for the legacy:
    Vector a = new Vector();
is:
    List a = new ArrayList();
Problem: Are Hashtables synchronized in Java 2 (formerly known as JDK 1.2)?

Yes, Hashtables are still synchronized in Java 2. The fact that the new implementations in Java 2 are unsynchronized represents a break with the past: Vector and Hashtable were synchronized in versions of the JDK prior to 1.2. In other words, legacy collections (like Vector and Hashtable) are synchronized, whereas new collections (like ArrayList and HashMap) are unsynchronized, and must be "wrapped" via Collections.SynchronizedList or Collections.synchronizedMap if synchronization is desired.

Problem: How do the Java collections compare to STL in C++?

While the new Java collections is somewhat similar to STL, it's also markedly different: Iterators and iterator-pairs play a central role in STL which they do not play in our framework; collection interfaces play a central role in our framework which they don't play in STL.
We designed Java collections from a "clean sheet of paper" and chose names for clarity, brevity, and similarity with preexisting Java APIs. Happily, most STL users have had little difficulty adapting to the framework's nomenclature.

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