标签：java hashMap System day16 Student println new out

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


    }
}

标签：java,hashMap,System,day16,Student,println,new,out
From： https://www.cnblogs.com/onlyxue/p/16813593.html

java_day16