Assertions in Java

An assertion allows testing the correctness of any assumptions that have been made in the program. An assertion is achieved using the assert statement in Java. While executing assertion, it is believed to be true. If it fails, JVM throws an error named AssertionError. It is mainly used for testing purposes during development. 

The assert statement is used with a Boolean expression and can be written in two different ways.

First way:

assert expression;

Second way:

assert expression1 : expression2;

Example:

Java

// Java program to demonstrate syntax of assertion
import java.util.Scanner;

class Test {
    public static void main(String args[])
    {
        int value = 15;
        assert value >= 20 : " Underweight";
        System.out.println("value is " + value);
    }
}

Output

value is 15

After enabling assertions:

Output:

Exception in thread "main" java.lang.AssertionError: Underweight

Enabling Assertions

By default, assertions are disabled. We need to run the code as given. The syntax for enabling assertion statement in Java source code is: 

java –ea Test

Or

java –enableassertions Test

Here, Test is the file name.

Disabling Assertions

The syntax for disabling assertions in java is: 

java –da Test

Or  

java –disableassertions Test

Here, Test is the file name.

Why use Assertions

Wherever a programmer wants to see if his/her assumptions are wrong or not. 

• To make sure that an unreachable-looking code is actually unreachable.
• To make sure that assumptions written in comments are right.

if ((x & 1) == 1) {

}
else // x must be even
{
    assert (x % 2 == 0);
}

• To make sure the default switch case is not reached.
• To check the object’s state.
• At the beginning of the method
• After method invocation.

Assertion Vs Normal Exception Handling

Assertions are mainly used to check logically impossible situations. For example, they can be used to check the state a code expects before it starts running or the state after it finishes running. Unlike normal exception/error handling, assertions are generally disabled at run-time. 

Where to use Assertions 

• Arguments to private methods. Private arguments are provided by the developer’s code only and the developer may want to check his/her assumptions about arguments.
• Conditional cases.
• Conditions at the beginning of any method.

Where not to use Assertions 

• Assertions should not be used to replace error messages
• Assertions should not be used to check arguments in the public methods as they may be provided by the user. Error handling should be used to handle errors provided by users.
• Assertions should not be used on command line arguments.

Example:

Java

// Java program to demonstrate assertion in Java
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 voter's age is 14