Java Compile Time Constants

This tutorials is all about Java Compile Time Constants.

2005-09-16 The Java Specialists' Newsletter [Issue 114] - Compile-time String Constant Quiz

Author: Dr. Heinz M. Kabutz

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Welcome to the 114th edition of The Java(tm) Specialists' Newsletter. I learned last Tuesday that I had been nominated as a Java Champion. The nomination was approved this Monday. So now I am one of the world's first elected Java Champions. I do not fully understand yet what that means, but I am overwhelmed that I had been noticed, considering how small our subscription base is :)

I am going on a world tour with my family next week, to France, Germany, Crete, USA, Greece and then back to South Africa. In the USA, on the 5th of October, I am speaking at the Java In Action conference, and will demonstrate how dynamic proxies can be put to use to reduce the amount of code that you need to write. Please email me if you are attending, so that we can make an effort to meet. This is a new type of conference that promises to be more than the usual vendor fest. The name should give a hint: Java in Action. Kirk Pepperdine is doing a live performance tuning demonstration, which will be very educational to watch. (Kirk went from being a Cray Supercomputer whizz to Java performance fundi and has some great insight into how to make the code run really fast.)

Compile-time String Constant Quiz

This week's newsletter is based on a quiz sent to me by Clark Updike from the John Hopkins University Applied Physics Laboratory. Thanks Clark :)

Consider the following interface (remember that all fields in an interface are automatically public static final):

public interface StaticFinalTest {
  String LITERAL = "Literal";
  String LITERAL_PLUS = "Literal" + "Plus";
  String LITERAL_NEW = new String("LiteralNew");
  String LITERAL_CONCAT = "LiteralConcat".concat("");
}
  

And we can use this as follows:

public class StaticFinalTestClient {
  public static void main(String[] args) {
    System.out.println(StaticFinalTest.LITERAL);
    System.out.println(StaticFinalTest.LITERAL_PLUS);
    System.out.println(StaticFinalTest.LITERAL_NEW);
    System.out.println(StaticFinalTest.LITERAL_CONCAT);
  }
}
  

When we run the program, we see:

Literal
LiteralPlus
LiteralNew
LiteralConcat
  

Now Change StaticFinalTest, compile it, but do not compile StaticFinalTestClient. If you in an IDE, you will have to compile it from the command line.

public interface StaticFinalTest {
  String LITERAL = "LiteralXXX";
  String LITERAL_PLUS = "Literal" + "PlusXXX";
  String LITERAL_NEW = new String("LiteralNewXXX");
  String LITERAL_CONCAT = "LiteralConcat".concat("XXX");
}
  

Here is the quiz: What is the output? (scroll down for the answer)

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Literal
LiteralPlus
LiteralNewXXX
LiteralConcatXXX
  

At compile time, all final fields that have a constant value, and are either primitive or String, get inlined by the compiler. This includes of course the Strings in our StaticFinalTest class. Since the NEW and CONCAT values are not compile time literals they are not inlined. So, when you change libraries, you have to recompile all your code. This limits how you change between libraries. You cannot simply swap out libraries at runtime, because if you as soon as you use constants, they are inlined and require a full recompile of your code.

Whilst I would think this is really widely understood, I have met a number of Java programmers who did not realise this.

Unit Testing

Be careful when writing unit tests. If you look at the following (incorrect) code, we can write unit tests that make it appear correct:

public class Car {
  private final String registrationNumber;
  public Car(String registrationNumber) {
    this.registrationNumber = registrationNumber;
  }
  public boolean equals(Object o) {
    if (!(o instanceof Car)) return false;
    return registrationNumber == ((Car) o).registrationNumber;
  }
  public int hashCode() {
    return registrationNumber.hashCode();
  }
}
  

The code is obviously incorrect, because it compares Strings using the == operator, instead of equals(). But look at this unit test, which one would you write?

import junit.framework.TestCase;

public class CarTest extends TestCase {
  public void testIncorrect() {
    assertEquals(
        new Car("CET192233"),
        new Car("CET192233"));
  }
  public void testCorrect() {
    assertEquals(
        new Car(new String("CET192233")),
        new Car("CET192233"));
  }
}
  

The first test is incorrect, since it compares the Car objects with identical strings, so the equals() method appears correct.

In JDK 1.1 and 1.0, final methods were inlined as well if you compiled with the -O option. Since Java 2, final methods are only inlined at runtime by the hotspot compiler (if necessary).

Hope to see you in Orlando at Java in Action in three weeks time :)

Kind regards

Heinz

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