why is this allowed in Java?
See Java Language Specification (14.6. The Empty Statement):
An empty statement does nothing.
It's simply allowed and it's equivalent to (and will be translated to):
if (condition) { }
Which means, if the condition is true, do nothing.
If you're using eclipse, you can look here, you might find something useful (I'm not sure there exists such an option for semicolon terminator):
Window → Preferences → Java → Compiler → Error/Warnings
EDIT
As @nullptr pointed out in his answer, there exist an IDE warning for this, you need to set warning on Empty statement.
