What is the main function of the JUnit testing framework in Java application development?

The main function of the JUnit testing framework is to automate the unit testing of Java applications. JUnit allows developers to write and run tests for individual methods and classes to ensure that the code works correctly and that changes do not lead to regressions.

JUnit provides:

• Annotations to designate test methods (@Test, @Before, @After, etc.).
• Mechanisms for verifying expected results (assertions).
• Tools for organizing and grouping tests.

A simple example of a test with JUnit 5:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class CalculatorTest {
    @Test
    void additionTest() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
    }
}

Thus, JUnit helps improve code quality and simplifies project maintenance.