- Go to the menu
Run, and then to the menu itemRun Configurations.

-
In the left panel, go to
Java Application, and then go toAssertions. -
In the right panel, choose the tab
Arguments. -
Under the field for
VM arguments, type-eato enable assertions.

- Click on the
Applyand thenRunbutton.
To globally set it as the default for everything:
-
Go to menu
Window(if you are on Windows), or go to menuEclipse(if you are on Mac). For Linux it might be something similar. -
Go to
Preferences. -
Choose
Java, and thenInstalled JREsfrom the left panel. -
Select your JRE, and then click the
Edit...button in the right panel. -
In the
Default VM argumentsfield, add-ea.