java_day16

Java基础

Java集合框架

Map接口

• 用于存储任意键值对key-value
• 键：无序、无下标、不允许重复
• 值：无序、无下标、运行重复

方法

• put(key,value) 将对象存入到集合中，关联键值。若key值重复则覆盖原值

• get(Object key) 根据键获取对应的值

• Set(key) 返回所有的key

• values() 返回包含所有值的collection集合

• Set<Map.Entry<k,v>> 键值匹配的set集合

  /**
   * Map接口的使用
   * 特点
   *      1、存储键值对
   *      2、键不能重复、值可以
   *      3、无序
   * @author xue
   */
  public class Demo1 {
      public static void main(String[] args) {
          //创建Map集合
          Map<String, String> hashMap = new HashMap<>();
  
          //添加元素
          hashMap.put("cn","china");
          hashMap.put("uk","english");
          hashMap.put("usa","amarica");
          hashMap.put("usa","shabi");//后面出现相同的key值是，value值会被替换最新
  
          System.out.println("元素个数"+hashMap.size());
          System.out.println(hashMap.toString());
  
          //删除元素
          hashMap.remove("usa");//可以只指定key值，也可以key-value都指定
  
          //遍历
          System.out.println("使用keyset集合");
          Set<String> keySet = hashMap.keySet();//返回的是所有key的set集合
          for (String str:keySet) {
              System.out.println(str+":"+hashMap.get(str));
          }
  
          System.out.println("使用entrySet方法");
          Set<Map.Entry<String,String>> entries = hashMap.entrySet();
          for (Map.Entry<String,String> entry:entries) {
              System.out.println(entry);
              System.out.println("4444");
              System.out.println(entry.getKey()+":"+entry.getValue());
          }
          
          //entrySet的效率要由于keySet
  
  
      }
  }
  

HashMap

• 线程不安全，运行效率快

@Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }





/**
 * Map接口的使用
 * 存储结构：哈希表（数组+链表+红黑树）
 * key值唯一的原理是使用key的hashcode和euals方法为依据
 * 也就是说重写这两个方法可以改变key值比较的规则
 * @author xue
 */
public class Demo2 {
    public static void main(String[] args) {
        //创建集合
        HashMap<Student,String> hashMap = new HashMap<>();

        //添加元素
        Student stu1 = new Student("aaa", 12);
        Student stu2 = new Student("bbb", 13);
        Student stu3 = new Student("ccc", 18);
        Student stu4 = new Student("ddd", 10);

        hashMap.put(stu1,"1");
        hashMap.put(stu2,"2");
        hashMap.put(stu3,"4");
        hashMap.put(stu4,"3");
        //重写hashCode与equals方法后下面的元素便无法添加
        hashMap.put(new Student("ddd", 10),"3");

        System.out.println("元素个数："+hashMap.size());
        System.out.println(hashMap);

        //删除
        hashMap.remove(stu1);
        //重写hashCode与equals方法后下面的元素这样可以删除
        hashMap.remove(new Student("ddd", 10));
        System.out.println("元素个数："+hashMap.size());
        System.out.println(hashMap);

        //遍历
        System.out.println("keySet");
        Set<Student> keySet = hashMap.keySet();
        for (Student stu:keySet) {
            System.out.println(stu+":"+hashMap.get(stu));
        }

        System.out.println("entrySet");
        Set<Map.Entry<Student, String>> entries = hashMap.entrySet();
        for (Map.Entry entry:entries) {
            System.out.println(entry.getKey()+":"+entry.getValue());
        }

    }
    
}

TreeMap

• 实现了SortedMap接口，可以对key自动排序

  /**
   * TreeMap的使用
   * 存储结构：红黑树
   * @author xue
   */
  public class Demo3 {
  
      public static void main(String[] args) {
          //创建TreeMap集合
          TreeMap<Student, String> treeMap = new TreeMap<Student, String>();
  
          //添加元素
          Student stu1 = new Student("aaa", 12);
          Student stu2 = new Student("bbb", 13);
          Student stu3 = new Student("ccc", 18);
          Student stu4 = new Student("ddd", 10);
          Student stu5 = new Student("ddd", 10);
  
          treeMap.put(stu1,"1");
          treeMap.put(stu2,"2");
          treeMap.put(stu4,"4");
          treeMap.put(stu3,"3");
          treeMap.put(stu5,"3");
  //        treeMap.put(stu1,"1");
  
          System.out.println("元素个数："+treeMap.size());
          System.out.println(treeMap);
  
          //删除
          treeMap.remove(stu1);
  
          //遍历
          System.out.println("keySet");
          Set<Student> keySet = treeMap.keySet();
          for (Student stu:keySet) {
              System.out.println(stu+":"+treeMap.get(stu));
          }
  
          System.out.println("entrySet");
          Set<Map.Entry<Student, String>> entries = treeMap.entrySet();
          for (Map.Entry entry:entries) {
              System.out.println(entry.getKey()+":"+entry.getValue());
          }
  
      }
  }
  

​ 看了源码后就能直到HashSet与TreeSet其实都是调用了HashMap与TreeMap，有兴趣可以看一看

Colletions工具类

• 集合的工具类，定义了一下集合的常用方法

  /**
   * Collections工具类的使用
   * @author xue
   */
  public class Demo4 {
      public static void main(String[] args) {
          ArrayList<Integer> aList = new ArrayList<>();
          aList.add(234);
          aList.add(2);
          aList.add(34);
          aList.add(346);
          aList.add(56457);
          System.out.println("排序前");
          System.out.println(aList);
          System.out.println("排序后");
          Collections.sort(aList);//也可以自己写比较规则
          System.out.println(aList);
  
          //二分查找
          System.out.println( Collections.binarySearch(aList, 34));//返回的是int类型的位置序号
  
          //翻转
          System.out.println("翻转后");
          Collections.reverse(aList);
          System.out.println(aList);
  
          //打乱
          System.out.println("打乱后");
          Collections.shuffle(aList);
          System.out.println(aList);
  
          //list转数组
          Integer[] arr = (Integer[]) aList.toArray(new Integer[aList.size()]);
          System.out.println(arr.length);
          System.out.println(Arrays.toString(arr));
  
          //数组转List
          List<Integer> list  = Arrays.asList(arr);//受限集合：只读
  //        list.add(12);//添加就报错
          System.out.println(list);
  
          //这两个方法有很多限制
  
  
      }
  }