GenericExampleTest.java

package com.examplehub.basics.generic;

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

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import org.junit.jupiter.api.Test;

class GenericExampleTest {
  @Test
  void testNoGeneric() {
    List list = new ArrayList();
    list.add("hello");
    list.add(123);
    list.add(12.34);
    assertEquals("[hello, 123, 12.34]", list.toString());

    assertEquals("hello", (String) list.get(0));
    assertEquals(123, (int) list.get(1));
    assertEquals(12.34, (double) list.get(2));

    try {
      String num = (String) list.get(1);
      fail();
    } catch (ClassCastException e) {
      assertTrue(true);
    }
  }

  @Test
  void testGenericList() {
    List<String> list = new ArrayList<>();
    list.add("java");
    // list.add(123); //compile error
    list.add("python");
    assertEquals("[java, python]", list.toString());
  }

  @Test
  void testGenericInterface() {
    class Student implements Comparable<Student> {
      private String name;
      private int age;

      @Override
      public int compareTo(Student o) {
        return Integer.compare(this.age, o.age);
      }

      public Student(String name, int age) {
        this.name = name;
        this.age = age;
      }
    }
    assertTrue(new Student("jack", 20).compareTo(new Student("tom", 21)) < 0);
    assertTrue(new Student("jack", 22).compareTo(new Student("tom", 21)) > 0);
    assertTrue(new Student("jack", 20).compareTo(new Student("tom", 20)) == 0);

    String[] languages = {"java", "python", "cpp"};
    Arrays.sort(languages);
    assertEquals("[cpp, java, python]", Arrays.toString(languages));
  }
}