Arrange / Act / Assert Anatomy

Every clean unit test has three sections in order. Click a section to highlight it — or "Show all" to see the whole test.

// RoomTest.java
@Test
public void checkIn_RoomInitialStatus_RoomIsOccupiedAndDirty() {
    // Arrange
    Room room = new Room(2, 100.0);
    
    // Act
    room.checkIn();
    
    // Assert
    assertTrue(room.isOccupied());
    assertTrue(room.isDirty());
}

All three sections

Read top to bottom — like a tiny script

A clean unit test reads in three beats. Set up → do one thing → check the result. Anything else (extra method calls, multiple acts, complicated logic) belongs in a different test or in a private helper.

Test naming — 3 underscored parts

1. Method under test

checkIn

2. State under test

RoomInitialStatus

3. Expected behavior

RoomIsOccupiedAndDirty

Pattern: methodUnderTest_StateUnderTest_ExpectedBehavior. Each name reads as a tiny spec: which method, in what state, expecting what.
Other styles exist (e.g. shouldDoX_whenY, doX_givenY_thenZ). Pick one for your project — and stick with it. Mixing styles inside one test class is the actual problem.

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