In Java, assert is a keyword that can be used to perform assertions in code. An assertion is a statement that checks whether a certain condition is true, and if it's not, it throws an error. The assert keyword provides a way to check assumptions about the code during development and testing.

The syntax of the assert statement is as follows:

assert condition;

The condition is an expression that should evaluate to a boolean value. If the condition is true, the program continues execution as normal. If the condition is false, an AssertionError is thrown, indicating that the assumption about the code has been violated.

By default, assertions are disabled in Java, which means that the assert statement has no effect. To enable assertions, you need to run the Java Virtual Machine (JVM) with the -ea (or -enableassertions) option. This option can be set at runtime, or it can be added to the command line when running the Java program:

java -ea MyProgram

In this example, the MyProgram class is executed with assertions enabled.

Here is an example of using assert in Java:

int x = 10;
assert x == 5 : "x is not equal to 5";

In this example, we create a variable x and assign it the value 10. We then use the assert statement to check whether x is equal to 5. Since x is not equal to 5, the assertion fails, and an AssertionError is thrown with the message "x is not equal to 5".

Assertions are typically used during development and testing to catch programming errors early. They can be used to check the validity of preconditions, postconditions, and invariants in the code. It's important to note that assertions should not be used to handle user input errors or to replace exception handling in production code.