java-tutorials/testing-modules/junit-5/src/test/java/com/baeldung/NestedUnitTest.java

package com.baeldung;

import java.util.EmptyStackException;
import java.util.Stack;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Nested;
import org.junit.jupiter.api.Test;

public class NestedUnitTest {

    Stack<Object> stack;

    @Test
    @DisplayName("is instantiated with new Stack()")
    void isInstantiatedWithNew() {
        new Stack<>();
    }

    @Nested
    @DisplayName("when new")
    class WhenNew {

        @BeforeEach
        void init() {
            stack = new Stack<>();
        }

        @Test
        @DisplayName("is empty")
        void isEmpty() {
            Assertions.assertTrue(stack.isEmpty());
        }

        @Test
        @DisplayName("throws EmptyStackException when popped")
        void throwsExceptionWhenPopped() {
            Assertions.assertThrows(EmptyStackException.class, () -> stack.pop());
        }

        @Test
        @DisplayName("throws EmptyStackException when peeked")
        void throwsExceptionWhenPeeked() {
            Assertions.assertThrows(EmptyStackException.class, () -> stack.peek());
        }

        @Nested
        @DisplayName("after pushing an element")
        class AfterPushing {

            String anElement = "an element";

            @BeforeEach
            void init() {
                stack.push(anElement);
            }

            @Test
            @DisplayName("it is no longer empty")
            void isEmpty() {
                Assertions.assertFalse(stack.isEmpty());
            }

            @Test
            @DisplayName("returns the element when popped and is empty")
            void returnElementWhenPopped() {
                Assertions.assertEquals(anElement, stack.pop());
                Assertions.assertTrue(stack.isEmpty());
            }

            @Test
            @DisplayName("returns the element when peeked but remains not empty")
            void returnElementWhenPeeked() {
                Assertions.assertEquals(anElement, stack.peek());
                Assertions.assertFalse(stack.isEmpty());
            }
        }
    }
}