Java - assert Keyword

Java assert keyword is used to define assertion in a Java program. An assertion is a statement in Java which ensures the correctness of any assumptions which have been done in the program. When an assertion is executed, it is assumed to be true. If the assertion is false, the JVM will throw an Assertion error. It finds it application primarily in the testing purposes. Assertion statements are used along with boolean expressions.

There are two ways in which an assert statement can be used.

Using Assertion - First Way

assert expression;

Using Assertion - Second Way

assert expression1 : expression2

By default, assertions are disabled in Java. In order to enable them we use the following command −

java -ea Example
(or)
java -enableassertions Example

Where Example is the name of the Java file.

Let us see an example for generation of an assertion error by the JVM −

Examples

public class Example {
   public static void main(String[] args) {
      int age = 14;
      assert age <= 18 : "Cannot Vote";
      System.out.println("The voter's age is " + age);
   }
}

Output

The above example will produce the following result −

The voter's age is 14

We can disable java assertion as well using following commands

java -da Example
(or)
java -disableassertions Example

where Example is the name of the Java file.