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Looking at Varargs in J2SE 1.5

  • March 11, 2004
  • By Jeff Langr
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Java 1.5 is slated to be released by Sun in late summer 2004. It contains many significant new language features and will dramatically alter the look and feel of Java code. The new varargs language feature was introduced to allow more flexibility and simplicity in coding.

Varargs

The varargs, or variable arguments, feature allows a developer to declare that a method can take a variable number of parameters for a given argument. The vararg must be the last argument in the formal argument list.

You might use varargs to allow adding one to many objects to a collection that you have defined by using a single add method call. Suppose you need a method that writes any number of records to the console. Using varargs, you can make any of the following method calls:

write("line 1");
write("line 2", "line 3");
write("line 4", "line 5", "line 6", "line 7", "line 8", "line 9",
      "line 10");

Before 1.5, you would need to define overloaded versions of the write method to accomplish this. This isn't an effective solution if you want to be able to support any number of records being passed in.

The write method introduces a couple new bits of syntax:

public void write(String... records) {
   for (String record: records)
      System.out.println(record);
}

First, the records argument is defined as type String.... This indicates to the compiler that calling code can pass a variable number of String parameters. For all other intents and purposes, however, String... equates to a String array (String[]).

The Enhanced For Loop

Second, the code uses the new enhanced for loop syntax that is being introduced in Java 1.5. You read the for loop in the example above as "for each String record in records." Or more explicitly, "for each object stored in the String array records, assign the object to a reference named record of type String."

The enhanced for loop, also known as foreach, is a nice feature that can also be used with any of the collection classes in Java 1.5. In addition, Java supplies hooks that allow you to make your own class iterable, so that it can be referenced in a foreach loop.

Behind the Scenes

There isn't any real magic going on with respect to varargs. Behind the scenes, Java is converting the new syntax into equivalent code that would work under Java 1.4 and older versions. The example above is equivalent to coding the following:

write(new String[] { "line 1" });
write(new String[] { "line 2", "line 3" });
write(new String[] { "line 4", "line 5", "line 6"});

// ... 

public void write(String[] records) {
   for (String record: records)
      write(record);
}

One could view varargs as pure syntactic sugar. It does help remove some of the cruft from your code.

Java 1.5 includes a Formatter class that depends upon the varargs feature to be effective. This new class allows you to use C-like format strings so that you can easily produce formatted output. I'll discuss this class in a follow-up article.

Trying it Out

You can try out the above code by downloading the current beta version of J2SE 1.5 from Sun's Web site. The download page is located at http://java.sun.com/j2se/1.5.0/download.jsp.

About the Author

Jeff Langr is a freelance author and the owner of Langr Software Solutions. He is working on a second book on Java and test-driven development entitled Agile Java, due out from Prentice Hall in fall 2004.






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