What is the difference between MessageFormat.format and String.format in JDK 1.5?

Put simply, the main difference is in format string:

1. MessageFormat.format() format string accepts argument positions (eg. {0}, {1}). Example:

   "This is year {0}!"

   The developer doesn’t have to worry about argument types, because they are, most often, recognized and formated according to current Locale.

2. String.format() format string accepts argument type specifiers (eg. %d for numbers, %s for strings). Example:

   "This is year %d!"

   String.format() generally gives you much more control over how the argument is displayed thanks to many options you can specify with the type specifier. For instance, format string "%-6.2f" specifies to display a left-aligned floating point number with min. width 6 chars and precision of 2 decimal places.

Just have a look at javadoc of both methods to find out more details.