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Issue 0532002-07-23 Category: Performance Java version:

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Charting unknown waters in JDK 1.4 Part I

by Dr. Heinz M. Kabutz

Welcome to the 53rd edition of The Java(tm) Specialists' Newsletter sent to over 4100 Java Specialists in 85 countries. Our friendly archive host Carl Smotricz has been experiencing some problems with his service provider, so our archive was offline for a bit. In the meantime, I have added my own archive to my website, so in future please look at our website for old issues.

Ahh, the pressures of life. This month has been hectically busy. The first week I gave some Java training at the University of Stellenbosch IT department to teach their Natural programmers Java. Very enjoyable week. The second week was spent presenting a Java 2 Standard Edition course and last week we moved in to our new house. I'm now sitting in my new office at home overlooking False Bay, and wondering what gems I can dig up in the source code of the JDK 1.4 ;-) Please arrange to visit me if you ever come to the Helderberg, a village about 50km outside of Cape Town.

NEW: Please see our new "Extreme Java" course, combining concurrency, a little bit of performance and Java 8. Extreme Java - Concurrency & Performance for Java 8.

Charting unknown waters in JDK 1.4 Part I

As I said in my opener to this newsletter, I started by just looking at the java.util.* package, without even the subpackages, and that kept me busy for most of the afternoon. I have not finished yet, but I already have enough material for several newsletters.

toString() was broken - did you know that?

In the collection classes all hell breaks loose when a collection contains itself and you try to call toString() on it. For example, consider the following code:

import java.util.Hashtable;

public class HashtableTest {
  public static void main(String[] args) {
    Hashtable ht = new Hashtable();
    ht.put("Heinz", "Kabutz");
    ht.put(ht, "all");
    ht.put("all", ht);
    ht.put(ht, ht);
    try {
      System.out.println(ht);
    } catch (StackOverflowError e) {
      System.out.println("Caused Stack Overflow Error!");
    }
  }
}

Under the JDK 1.3 and before, when you ran this program, a Stack Overflow was generated, however, in JDK 1.4, the output is:

{Heinz=Kabutz, all=(this Map), (this Map)=all, (this Map)=(this Map)}

This same principle applies to the other collections. Is it a good idea to use a Hashtable as a key? Remember what happens when the hash code of a key changes during its life? I discussed that at length in Issue 031 of The Java(tm) Specialists' Newsletter. Each time a Hashtable changes, its hash code changes as well, so you should not use it as a key. However, there is no limit to the amount of stupid code that gets written every day, and the chaps at Sun didn't expect people to add a collection to itself.

RandomAccess

A highly paid software developer once wrote something along these lines:

/** @author Mr M.O.Nument */
import java.util.*;
public class ListSearching {
  private List names = new LinkedList();
  public void f() {
    for (int i=0; i<size(); i++) {
      if (names.get(i) == "Heinz")
        System.out.println("Found it");
    }
  }
  private int size() {
    int result = 0;
    Iterator it = names.iterator();
    while(it.hasNext()) {
      result++;
      it.next();
    }
    return result;
  }
}

Take a moment to read through the code and try to understand it. In defense of the programmer, firstly he had not been on my Java course ;-), secondly there was a lot of code inbetween f() and size() so the problem was not as obvious as we can now see and thirdly the list was always very short so performance was not an issue either.

In JDK 1.4, a new tag interface (like java.io.Serializable) was introduced into the java.util package, called RandomAccess. This allows classes such as Collections to optimize their algorithms in the case where:

for (int i=0, n=list.size(); i < n; i++)
  list.get(i);

runs faster than this loop:

for (Iterator i=list.iterator(); i.hasNext(); )
  i.next();

What I found interesting was that the algorithms always treat the collections as RandomAccess unless they are bigger than some threshold. This makes a lot of sense to me, because I have often found algorithms that we thought to be faster for linked lists actually being slower, as I discussed in Issue 024 of The Java(tm) Specialists' Newsletter. The values of the thresholds are of course not configurable and look like empirical thumbsuck that would work well with the LinkedList.

RuntimeException Specifications - Admission of Guilt?

All over the JDK 1.4 java.util.* package, I have seen that the RuntimeExceptions are now also specified in the @throws clause of the JavaDocs. Bruce Eckel and I spent quite a few emails debating the current exception model, and we ended up wondering whether it was not perhaps fundamentally flawed. For example, java.io.IOException has over 30 subclasses that could be the actual exception being thrown. What good is that? When you see that a method throws IOException, what is actually being thrown, and how do you deal with it?

I think that checked exceptions should be scrapped. They don't work. They cause bad code, such as:

while(true) {
  try {
    Thread.sleep(1000);
    // do some other work
  } catch (Exception e) {}
}

Checked exceptions are responsible for more sloppy code and bugs than any other construct in the Java language. C# does not have checked exceptions, neither does C++. Why does Java have to be encumbered by them?

I think that declaring as comments what runtime exceptions could possibly be thrown by a method is a step in the right direction.

I've discovered some other interesting little snippets, which I will share with you in my next newsletter.

Until the next newsletter...

Heinz

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