Java - how to enable assertions?

In this article we whould like to show how to enable assertions in Java.

The article describle 2 ways how to do it:

• with command line (terminal),
• with IntelliJ IDE.

Read below sections to see details. 

1. Java with command line example

Running program from console it is necessary to add -ea or -enableassertions paramter for java virtual machine to use assertions.

Program.java file:

public class Program {

    public static void main(String[] args) {
        String a = "text"; // or = null by default

        // some code...

        a = null; // for example: some operation caused null value

        // some code...

        assert a != null : "Variable can not be null";

        System.out.println(a.length());
    }
}

1.1. Compilation form console:

$ javac Program.java

1.2. Running from console:

$ java -ea Program

or

$ java -enableassertions Program

Output:

Exception in thread "main" java.lang.AssertionError: Variable can not be null
        at Program.main(Program.java:12)

2. IntelliJ IDEA compilation example

IntelliJ requires to add in Run/Debug Configurations window, -ea or -enableassertions paramter VM Options field.

2.1. Step 1 - create Java project

2.2. Step 2 - open Run / Debug configurations

2.3. Step 3 - add -ea parameter for VM options

2.4. Step 4 - running program