In Java, the assert keyword is used to define an assertion, which is a statement that is expected to be true at a certain point in the program. Assertions are used to test for conditions that should never occur in the normal flow of the program, and can be used to help debug and test code.

The assert keyword is followed by a boolean expression and an optional message. If the boolean expression is false, the AssertionError exception is thrown with the optional message. If the boolean expression is true, the assertion is ignored and the program continues to execute.

Here is an example of how to use the assert keyword in Java:

public class Main {
  public static void main(String[] args) {
    int x = 5;
    assert x > 0 : "x is negative"; // valid, x is positive
    assert x < 0 : "x is positive"; // throws AssertionError with message "x is positive"
  }
}

In this example, the first assertion is valid because the value of x is positive, and it is ignored by the program. The second assertion is invalid because the value of x is not negative, and it throws an AssertionError with the message “x is positive”.

By default, assertions are disabled in Java and do not affect the execution of the program. To enable assertions, you can use the -ea command line flag when running the program.

For more information on the assert keyword and how to use it in Java, you can refer to the Java documentation:

• assert keyword: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/assert.html
• Assertions: https://docs.oracle.com/en/java/javase/11/language/assert.html