2.11. Nested Tests

@Nested tests give the test writer more capabilities to express the relationship among several groups of tests. Here’s an elaborate example.

Nested test suite for testing a stack

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertFalse;
import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assertions.assertTrue;
import java.util.EmptyStackException;
import java.util.Stack;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Nested;
import org.junit.jupiter.api.Test;
@DisplayName("A stack")
class TestingAStackDemo {
    Stack<Object> stack;
    @Test
    @DisplayName("is instantiated with new Stack()")
    void isInstantiatedWithNew() {
        new Stack<>();
    }
    @Nested
    @DisplayName("when new")
    class WhenNew {
        @BeforeEach
        void createNewStack() {
            stack = new Stack<>();
        }
        @Test
        @DisplayName("is empty")
        void isEmpty() {
            assertTrue(stack.isEmpty());
        }
        @Test
        @DisplayName("throws EmptyStackException when popped")
        void throwsExceptionWhenPopped() {
            assertThrows(EmptyStackException.class, stack::pop);
        }
        @Test
        @DisplayName("throws EmptyStackException when peeked")
        void throwsExceptionWhenPeeked() {
            assertThrows(EmptyStackException.class, stack::peek);
        }
        @Nested
        @DisplayName("after pushing an element")
        class AfterPushing {
            String anElement = "an element";
            @BeforeEach
            void pushAnElement() {
                stack.push(anElement);
            }
            @Test
            @DisplayName("it is no longer empty")
            void isNotEmpty() {
                assertFalse(stack.isEmpty());
            }
            @Test
            @DisplayName("returns the element when popped and is empty")
            void returnElementWhenPopped() {
                assertEquals(anElement, stack.pop());
                assertTrue(stack.isEmpty());
            }
            @Test
            @DisplayName("returns the element when peeked but remains not empty")
            void returnElementWhenPeeked() {
                assertEquals(anElement, stack.peek());
                assertFalse(stack.isEmpty());
            }
        }
    }
}