The new lang.Closeable interface and ARM blocks

at 2010-12-31 in Language, Examples, 5.8-SERIES by friebe (0 comments)

The new lang.Closeable interface and the io.streams classes being retrofitted to implement it may not seem very useful on the first glance, although they serve the purpose of supporting the so-called ARM blocks. These are a feature supported by XP Language and reuse the try keyword:

  try ($expression[, $expression[, ...]]) {
    // Statements
  }

Any of the expressions are expected to be instanceof lang.Closeable and their close() methods are guaranteed to be called in the declaration order regardless of whether the block raises an exception or not.

Example
Here's a small demonstration of how it works:

  import static util.cmd.Console.writeLine;
  
  public class AutoClose {
    
    public static void main(string[] $args) {
      $c= new Closeable() {
        public void close() { writeLine('Closing'); }
      };
      
      try ($c) {
        writeLine('Inside try, args.length= ', $args.length);
        if ($args.length > 0) throw new IllegalArgumentException($args[0]);
      }
    }
  }

If invoked with no arguments, no exception will be raised:

  $ xp AutoClose
  Inside try, args.length= 0
  Closing

If we pass an argument, an IAE will be raised with the argument as its message:

  $ xp AutoClose Hello
  Inside try, args.length= 1
  Closing
  Uncaught exception: Exception lang.reflect.TargetInvocationException (AutoClose::main() invocation failed)

This is especially useful when working with streams - we should close them regardless of what happens. Of course we could wait for their destructor to be called by PHP's garbage collection, but here's our shorthand for it, following the motto of the less code you write the less can be broken

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The new lang.Closeable interface and ARM blocks

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