2.HashMap集合
2.1HashMap集合概述和特点【理解】
2.2HashMap集合应用案例【应用】
- 案例需求
- 创建一个HashMap集合，键是学生对象(Student)，值是居住地 (String)。存储多个元素，并遍历。
- 要求保证键的唯一性：如果学生对象的成员变量值相同，我们就认为是同一个对象
代码实现
学生类 public class Student { private String name; private int age; public Student() { } public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; Student student = (Student) o; if (age != student.age) return false; return name != null ? name.equals(student.name) : student.name == null; } @Override public int hashCode() { int result = name != null ? name.hashCode() : 0; result = 31 * result + age; return result; } }
测试类
public class HashMapDemo { public static void main(String[] args) { //创建HashMap集合对象 HashMap<Student, String> hm = new HashMap<Student, String>(); //创建学生对象 Student s1 = new Student("林青霞", 30); Student s2 = new Student("张曼玉", 35); Student s3 = new Student("王祖贤", 33); Student s4 = new Student("王祖贤", 33); //把学生添加到集合 hm.put(s1, "西安"); hm.put(s2, "武汉"); hm.put(s3, "郑州"); hm.put(s4, "北京"); //遍历集合 Set<Student> keySet = hm.keySet(); for (Student key : keySet) { String value = hm.get(key); System.out.println(key.getName() + "," + key.getAge() + "," + value); } } }
3.TreeMap集合
3.1TreeMap集合概述和特点【理解】
- TreeMap底层是红黑树结构
- 依赖自然排序或者比较器排序,对键进行排序
- 如果键存储的是自定义对象,需要实现Comparable接口或者在创建TreeMap对象时候给出比较器排序规则
3.2TreeMap集合应用案例【应用】
- 案例需求
- 创建一个TreeMap集合,键是学生对象(Student),值是籍贯(String),学生属性姓名和年龄,按照年龄进行排序并遍历
- 要求按照学生的年龄进行排序,如果年龄相同则按照姓名进行排序
代码实现
学生类 public class Student implements Comparable<Student>{ private String name; private int age; public Student() { } public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Student{" + "name='" + name + '\'' + ", age=" + age + '}'; } @Override public int compareTo(Student o) { //按照年龄进行排序 int result = o.getAge() - this.getAge(); //次要条件，按照姓名排序。 result = result == 0 ? o.getName().compareTo(this.getName()) : result; return result; } }
- 测试类
public class Test1 { public static void main(String[] args) { // 创建TreeMap集合对象 TreeMap<Student,String> tm = new TreeMap<>(); // 创建学生对象 Student s1 = new Student("xiaohei",23); Student s2 = new Student("dapang",22); Student s3 = new Student("xiaomei",22); // 将学生对象添加到TreeMap集合中 tm.put(s1,"江苏"); tm.put(s2,"北京"); tm.put(s3,"天津"); // 遍历TreeMap集合,打印每个学生的信息 tm.forEach( (Student key, String value)->{ System.out.println(key + "---" + value); } ); }
