feat: adds Nia's custom state object (#498)

* feat: implement State class with population management and exception handling

* feat: reintroduce State class with population management and exception handling

Author: nia-source

lesson_16/objects/objects_app/src/main/java/com/codedifferently/lesson16/states/PopulationException.java

@@ -0,0 +1,7 @@
+package com.codedifferently.lesson16.states;
+
+class PopulationTooLowException extends Exception {
+  public PopulationTooLowException(String message) {
+    super(message);
+  }
+}

lesson_16/objects/objects_app/src/main/java/com/codedifferently/lesson16/states/State.java

@@ -0,0 +1,49 @@
+package com.codedifferently.lesson16.states;
+
+import java.util.ArrayList;
+
+public class State {
+  private String name;
+  private String capital;
+  private int population;
+  private double area;
+  private Region region;
+  private ArrayList<String> majorCities;
+
+  enum Region {
+    NORTHEAST,
+    MIDWEST,
+    SOUTH,
+    WEST,
+    PACIFIC
+  }
+
+  public State(String name, String capital, int population, double area, Region region) {
+    this.name = name;
+    this.capital = capital;
+    this.population = population;
+    this.area = area;
+    this.region = region;
+    this.majorCities = new ArrayList<>();
+  }
+
+  public void updatePopulation(int newPopulation) throws PopulationTooLowException {
+    if (newPopulation < 10000) {
+      throw new PopulationTooLowException("Population too low for a state!");
+    }
+    this.population = newPopulation;
+    System.out.println("Population updated to " + newPopulation);
+  }
+
+  public void addMajorCity(String city) {
+    majorCities.add(city);
+    System.out.println(city = "added to major cities of " + name);
+  }
+
+  public void listMajorCities() {
+    System.out.println("Major cities in " + name + ":");
+    for (String city : majorCities) {
+      System.out.println("- " + city);
+    }
+  }
+}

lesson_16/objects/objects_app/src/test/java/com/codedifferently/lesson16/states/StateTest.java

@@ -0,0 +1,67 @@
+package com.codedifferently.lesson16.states;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class StateTest {
+
+  private State state;
+
+  @BeforeEach
+  public void setUp() {
+    state = new State("California", "Sacramento", 39500000, 423967, State.Region.WEST);
+  }
+
+  @Test
+  public void constructor_ShouldInitializeFieldsCorrectly() {
+    assertThat(state)
+        .extracting("name", "capital", "population", "area", "region")
+        .containsExactly("California", "Sacramento", 39500000, 423967.0, State.Region.WEST);
+  }
+
+  @Test
+  public void updatePopulation_ShouldUpdateCorrectly_WhenValid() throws PopulationTooLowException {
+    state.updatePopulation(5000000);
+    assertThat(state).extracting("population").isEqualTo(5000000);
+  }
+
+  @Test
+  public void updatePopulation_ShouldThrowException_WhenTooLow() {
+    assertThatThrownBy(() -> state.updatePopulation(9000))
+        .isInstanceOf(PopulationTooLowException.class)
+        .hasMessage("Population too low for a state!");
+  }
+
+  @Test
+  public void addMajorCity_ShouldAddCityToList() {
+    state.addMajorCity("Los Angeles");
+    state.addMajorCity("San Francisco");
+
+    // Check if the cities were added (accessing via reflection for private field)
+    assertThat(state)
+        .extracting("majorCities")
+        .asList()
+        .containsExactly("Los Angeles", "San Francisco");
+  }
+
+  @Test
+  public void listMajorCities_ShouldPrintCityList() {
+    state.addMajorCity("Los Angeles");
+    state.addMajorCity("San Diego");
+
+    // Capture the output
+    ByteArrayOutputStream outContent = new ByteArrayOutputStream();
+    System.setOut(new PrintStream(outContent));
+
+    state.listMajorCities();
+
+    System.setOut(System.out); // Reset back
+
+    String expectedOutput = "Major cities in California:\n- Los Angeles\n- San Diego\n";
+    assertThat(outContent.toString().trim()).isEqualToIgnoringNewLines(expectedOutput.trim());
+  }
+}