• JAVA-Map接口概述、常用方法、排序、Hashtable面试题

    ①. Map接口的概述

    • ①. Map是双列集合与Collection的单列集合并列存在

    • ②. Map  这里的K 、V 都应该是引用数据类型

    • ③. HashSet底层依赖HashMap,单列底层依赖双列集合

    • ④. Map中的key可以为null,value也可以为null,注意key只能有一个null,value可以有多个null

    ②. Map常用方法

    • ①. 添加功能:如果键是第一次储存,就直接储存元素,返回null；如果键不是第一次存在,就用值把以前的值替换掉,返回以前的值

      V put(K key ,V value):添加元素,返回的是以前的值

    1. "prettyprint hljs go" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">Map map=new HashMap<>();
    2.         Integer i1=map.put("张三",23);
    3.         Integer i2=map.put("李四",23);
    4.         Integer i3=map.put("王五",24);
    5.         Integer i4=map.put("赵六",25);
    6.         Integer i5=map.put("张三",26);//相同的键不储存,把原来的值覆盖 把被覆盖的值返回
    7.  
    8.         System.out.println(map);//{李四=23, 张三=26, 王五=24, 赵六=25}
    9.  
    10.         //这里的i1--i4 都是null是因为开始的时候[张三,23][李四,23]....
    11.         // 在集合中没有,这样把原来的值覆盖了,返回的是原来的值
    12.         System.out.println(i1);//null
    13.         System.out.println(i2);//null
    14.         System.out.println(i3);//null
    15.         System.out.println(i4);//null
    16.         System.out.println(i5);//23
    • ②. 删除功能
    1. void clear():移除所有的键值对元素
    2. V remove(Object key):根据键删除键值对元素,并把值返回
    1. "prettyprint hljs go" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">Map map=new HashMap<>();
    2.        map.put("张三",23);
    3.        map.put("李四",23);
    4.        map.put("王五",24);
    5.        map.put("赵六",25);
    6.        //根据键删除元素,返回键对应的值
    7.        Integer value=map.remove("张三");
    8.        System.out.println(value);//23
  1. boolean containsKey(Objecct key):判断是否包含指定的键
  2. boolean containsValue(Objecct value):判断是否包含指定的值
  1. "prettyprint hljs go" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//判断是否包含传入的键
  2.        System.out.println(map.containsKey("张三"));//true
  3.        //判断是否包含传入的值
  4.        System.out.println(map.containsValue(23));//true
  1. "prettyprint hljs go" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">Map map=new HashMap<>();
  2.        map.put("张三",26);
  3.        map.put("李四",23);
  4.        map.put("王五",24);
  5.        map.put("赵六",25);
  6.  
  7.        Collectioncoll=map.values();
  8.        System.out.println(coll);//[23, 23, 24, 25]
  9.        System.out.println(map.size());//4

  1. V get(Object key):根据键获取值

  2. Set  keySet():获取集合中所有的键集合

  3. Collection  values:获取集合中所有值集合

  4. Set> entrySet():拿到所有的键值对对象

    K getKey()：得到entrySet中的键

    V getValue()：得到entrySet中的值

  1. class="prettyprint hljs go" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">Map<String, Integer> map = new HashMap<>();
  2.         map.put("张三", 23);
  3.         map.put("李四", 24);
  4.         map.put("王五", 25);
  5.         map.put("赵六", 23);
  1. class="prettyprint hljs swift" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//V get(Object key):根据键获取值
  2.         Integer i = map.get("张三");//26
  3.         Integer i2=map.get("小智");//没有的话返回null
  4.  
  5.         //1.获取所有的键
  6.         Set keySet = map.keySet();
  7.         //iterator遍历
  8.         Iterator iterator = keySet.iterator();
  9.         while (iterator.hasNext()) { 
  10.             String key = iterator.next();//获取每一个键
  11.             Integer value = map.get(key); //根据键获取值
  12.             System.out.println(key + "=" + value);
  13.         }
  14.         System.out.println("--------------");
  15.         //使用增强for循环
  16.         for (String key : map.keySet()) { 
  17.            Intger value=map.get(key)；
  18.             System.out.println(key + "=" +value );
  19.         }

  1. class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">interface  Inter{ 
  2.     interface  Inter2{ 
  3.         public  void  show();
  4.     }
  5. }
  6. //这里的Inter.Inter2 和Map.Entry一样的
  7. class  Demo  implements  Inter.Inter2{ 
  8.  
  9.     @Override
  10.     public  void  show()  { 
  11.  
  12.     }
  13. }
  1. class="prettyprint hljs dart" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//2.根据键值对对象,获取键和值
  2.         //Map.Entry说明Entry是Map的内部接口,将键和值封装成了Entry对象,并储存在Set集合中
  3.         Set<Map.Entry<String,Integer>>entrySet=map.entrySet();
  4.         //获取每一个对象
  5.         Iterator<Map.Entry<String,Integer>>it=entrySet.iterator();
  6.         while(it.hasNext()){ 
  7.             //获取每一个Entry对象
  8.             //static  class Entry implements Map.Entry{}
  9.             Map.Entry<String,Integer>en=it.next();//父类引用指向子类对象
  10.             //Entryen=it.next(); 子类对象
  11.             //根据键值对对象获取键
  12.             String key=en.getKey();
  13.             //根据键值对对象获取值
  14.             Integer value=en.getValue();
  15.             System.out.println(key+"="+value);
  16.         }
  17.  
  18.         //增强for循环
  19.         for(Map.Entry<String,Integer> en:map.entrySet()){ 
  20.             System.out.println(en.getKey()+"="+en.getValue());
  21.         }
  22.        for(Entry<String,Integer> en:map.entrySet()){ 
  23.             System.out.println(en.getKey()+"="+en.getValue());
  24.         }

③. HashMap概述

  1. class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">// HashMap集合键是Student值是String的案例
  2. public class  Student  { 
  3.     private String name;
  4.     private int age;
  5.  
  6.     public  Student()  { 
  7.     }
  8.  
  9.     public  Student(String name,  int age)  { 
  10.         this.name = name;
  11.         this.age = age;
  12.     }
  13.  
  14.     public  String  getName()  { 
  15.         return name;
  16.     }
  17.  
  18.     public  void  setName(String name)  { 
  19.         this.name = name;
  20.     }
  21.  
  22.     public  int  getAge()  { 
  23.         return age;
  24.     }
  25.  
  26.     public  void  setAge(int age)  { 
  27.         this.age = age;
  28.     }
  29.  
  30.     @Override
  31.     public  boolean  equals(Object o)  { 
  32.         if (this == o) return true;
  33.         if (o == null || getClass() != o.getClass()) return false;
  34.         Student student = (Student) o;
  35.         return age == student.age &&
  36.                 Objects.equals(name, student.name);
  37.     }
  38.  
  39.     @Override
  40.     public  int  hashCode()  { 
  41.         return Objects.hash(name, age);
  42.     }
  43.  
  44.     @Override
  45.     public  String  toString()  { 
  46.         return "Student{" +
  47.                 "name='" + name + '\'' +
  48.                 ", age=" + age +
  49.                 '}';
  50.     }
  51. }
  1. class="prettyprint hljs go" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">HashMap<Student,String>hm=new HashMap<>();
  2.         hm.put(new Student("张三",23),"北京");
  3.         String i=hm.put(new Student("张三",23),"上海");
  4.         hm.put(new Student("李四",24),"广州");
  5.         hm.put(new Student("王五",25),"深圳");
  6.         System.out.println(i);
  7.         //没有重写equals方法之前,调用的是Object类的,会比较地址值
  8.         //没有重写equals方法和 hashCode方法的时候map中的元素有4个,重写后只有3个
  9.         //注意这里的张三对应的值是上海,因为把北京覆盖了
  10.         System.out.println(hm);//北京
  11.         //{Student{name='张三', age=23}=上海, Student{name='李四', age=24}=广州, Student{name='王五',

④. LinkedHashMap

  1. class="prettyprint hljs go" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">LinkedHashMap<String,Integer>lhm=new LinkedHashMap<>();
  2.     lhm.put("张三",23);
  3.     lhm.put("李四",24);
  4.     lhm.put("王五",25);
  5.     lhm.put("赵六",26);
  6.     System.out.println(lhm);//{张三=23, 李四=24, 王五=25, 赵六=26}

⑤. TreeMap排序

  1. class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//自然排序
  2. public class  Student  implements  Comparable<Student>  { 
  3.     private String name;
  4.     private int age;
  5.  
  6.     public  Student()  { 
  7.     }
  8.  
  9.     public  Student(String name,  int age)  { 
  10.         this.name = name;
  11.         this.age = age;
  12.     }
  13.  
  14.     public  String  getName()  { 
  15.         return name;
  16.     }
  17.  
  18.     public  void  setName(String name)  { 
  19.         this.name = name;
  20.     }
  21.  
  22.     public  int  getAge()  { 
  23.         return age;
  24.     }
  25.  
  26.     public  void  setAge(int age)  { 
  27.         this.age = age;
  28.     }
  29.  
  30.     @Override
  31.     public  String  toString()  { 
  32.         return "Student{" +
  33.                 "name='" + name + '\'' +
  34.                 ", age=" + age +
  35.                 '}';
  36.     }
  37.  
  38.     @Override
  39.     public  int  compareTo(Student s)  { 
  40.         //以年龄为主要条件,姓名为次要条件
  41.         int num=this.age-s.age;
  42.         return num==0?this.name.compareTo(s.name):num;
  43.     }
  44. }
  1. class="prettyprint hljs tcl" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">TreeMap<Student,String> tm=new TreeMap<>();
  2.          tm.put(new Student("张三",23),"北京");
  3.          tm.put(new Student("李四",13),"上海");
  4.          tm.put(new Student("王五",33),"广州");
  5.          tm.put(new Student("赵六",43),"北京");
  6.          System.out.println(tm);
  7.          //{Student{name='李四', age=13}=上海, Student{name='张三', age=23}=北京,
  8.         // Student{name='王五', age=33}=广州, Student{name='赵六', age=43}=北京}
  1. class="prettyprint hljs dart" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//选择器排序
  2.   TreeMap tm=new TreeMap<>(new Comparator() { 
  3.             @Override
  4.             public int compare(Student s1, Student s2) { 
  5.                 //按照姓名比较
  6.                 int num=s1.getName().compareTo(s2.getName());
  7.                 return num==0?s1.getAge()-s2.getAge():num;
  8.             }
  9.         });
  10.         tm.put(new Student("张三",23),"北京");
  11.         tm.put(new Student("李四",13),"上海");
  12.         tm.put(new Student("王五",33),"广州");
  13.         tm.put(new Student("赵六",43),"北京");
  14.         System.out.println(tm);
  15.         //{Student{name='张三', age=23}=北京, Student{name='李四', age=13}=上海,
  16.         //Student{name='王五', age=33}=广州, Student{name='赵六', age=43}=北京}

⑥. Hashtable面试题

  1. class="prettyprint hljs yaml" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">HashMap和Hashtable的区别
  2. 共同点：
  3. 底层都是哈希算法,都是双列集合
  4. 区别
  5. 1.HashMap是线程不安全的,效率高,jdk1.2版本
  6.   Hashtable是现场安全的,效率低,jdk1.0版本
  7. 2.HashMap可以存储null键和null值
  8.   Hashtable不可以存储null键和null值
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  • 原文地址：https://blog.csdn.net/weixin_62421895/article/details/126174624