In Java, assertions are disabled by default for performance reasons. To enable assertions, you need to use the -ea (or -enableassertions) option when running the JVM.

Here is an example of how to enable assertions when running a Java program:

java -ea MyApp

In this example, we enable assertions when running the MyApp class.

You can also enable assertions for a specific package or class by using the -ea:<package> or -ea:<package>.<class> options. For example:

java -ea:com.mycompany.mypackage MyApp

In this example, we enable assertions for all classes in the com.mycompany.mypackage package.

You can also disable assertions for a specific package or class by using the -da:<package> or -da:<package>.<class> options. For example:

java -ea -da:com.mycompany.mypackage.MyClass MyApp

In this example, we enable assertions for all classes except com.mycompany.mypackage.MyClass.

It's important to note that assertions should not be used for normal error handling or input validation. Instead, they should be used to catch programming errors that should never occur during normal program execution.