Arrange, act, assert

The methodology

Arrange: Set up the conditions for the test.

Act: Execute the functionality you're testing.

Assert: Check that the action has produced the expected result.

Simple example

We can use arrange, act, assert to ensure the togglePower() method works as expected:

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

public class SmartLightTest {

    @Test
    public void testTogglePowerTurnsLightOn() {
        // arrange
        SmartLight light = new SmartLight("white", 50);

        // act
        light.togglePower();

        // assert
        assertTrue(light.isOn());
    }
}

Testing an arrangement multiple times

Bear in mind that there is no guarantee to the order in which tests execute. If you want to test a particular arrangement several times, you need to do it in the same method, or else rearrange for each test.

good

public class SmartLightTest {

    @Test
    public void testLightIsOffByDefault() {
        // arrange
        SmartLight light = new SmartLight("white", 50);

        // assert
        assertFalse(light.isOn());
    }

    @Test
    public void testTogglePowerTurnsLightOn() {
        // arrange
        SmartLight light = new SmartLight("white", 50);

        // act
        light.togglePower();

        // assert
        assertTrue(light.isOn());
    }
}

ok

public class SmartLightTest {

    @Test
    public void testTogglePower() {
        // arrange
        SmartLight light = new SmartLight("white", 50);

        // assert
        assertFalse(light.isOn());

        // act
        light.togglePower();

        // assert
        assertTrue(light.isOn());
    }
}

bad

public class SmartLightTest {

    // arrange
    SmartLight light = new SmartLight("white", 50);

    @Test
    public void testLightIsOffByDefault() {
        // assert
        assertFalse(light.isOn());
    }

    @Test
    public void testTogglePowerTurnsLightOn() {
        // act
        light.togglePower();

        // assert
        assertTrue(light.isOn());
    }
}

The bad example will sometimes fail because the second test runs before the first one.