Java assertion statements come in two forms:

assert boolean-expression;

assert boolean-expression: information-expression;

For example:

case 1,

// validating/Assert1.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Non-informative style of assert
// Must run using -ea flag:
// {java -ea Assert1}
// {ThrowsException}

public class Assert1 {
  public static void main(String[] args) {
    assert false; // false produce error
  }
}
/* My Output:
 Exception in thread "main" java.lang.AssertionError // since 1.4, parent class is Error
        at Assert1.main(Assert1.java:12)
*/

execute the above code

./gradlew validating:Assert1

or cd validating directory, run

$ javac Assert1.java

# -ea means -enableassertions
$ java -ea Assert1 

 case 2,

// validating/Assert2.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Assert with an information-expression
// {java Assert2 -ea}
// {ThrowsException}

public class Assert2 {
  public static void main(String[] args) {
    assert false : "Here's a message saying what happened";
  }
}
/* My Output:
Exception in thread "main" java.lang.AssertionError:
Here's a message saying what happened
        at Assert2.main(Assert2.java:11)
*/

references:

1. On Java 8 - Bruce Eckel

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2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/validating/Assert1.java

3. https://docs.oracle.com/javase/8/docs/api/java/lang/AssertionError.html

4. http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/AssertionError.java

5. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/validating/Assert2.java