Apple has released Java for Mac OS X 10.5 Update 7 for Leopard users, updating Mac OS X's J2SE 5.0 to version 1.5.0_24 and Java SE 6 to 1.6.0_20 and delivering "improved reliability, security, and compatibility for J2SE 5.0 and Java SE 6." The only details given revolve around security fixes, so it's probably worth downloading this update even if you don't anticipate using Java applets much. Also note that J2SE 1.4.2 is no longer being updated and remains disabled in this update, which requires Mac OS X 10.5.8. (Free, 122 MB)
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