hashmap使用

hashmap作为dao对象存储数据库数据

list是把每一个数据库的字段都映射了，而hashmap则是唯一id:数据库字段作为key

hashmap遍历方式

public class Main {

    //使用迭代器（Iterator）EntrySet
        public static void main(String[] args) {
            // 创建并赋值 HashMap
            Map<Integer, String> map = new HashMap();
            map.put(1, "Java");
            map.put(2, "JDK");
            map.put(3, "Spring Framework");
            map.put(4, "MyBatis framework");
            map.put(5, "Java");
            // 遍历
            Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
            while (iterator.hasNext()) {
                Map.Entry<Integer, String> entry = iterator.next();
                System.out.println(entry.getKey());
                System.out.println(entry.getValue());
            }
        }

    //ForEach EntrySet
    @Test
    public void test1(){
        // 创建并赋值 HashMap
        Map<Integer, String> map = new HashMap();
        map.put(1, "Java");
        map.put(2, "JDK");
        map.put(3, "Spring Framework");
        map.put(4, "MyBatis framework");
        map.put(5, "Java");
        // 遍历
        for (Map.Entry<Integer, String> entry : map.entrySet()) {
            System.out.print(entry.getKey());
            System.out.print(entry.getValue());
        }
    }

    //ForEach KeySet
    @Test
    public void test2(){
        // 创建并赋值 HashMap
        Map<Integer, String> map = new HashMap();
        map.put(1, "Java");
        map.put(2, "JDK");
        map.put(3, "Spring Framework");
        map.put(4, "MyBatis framework");
        map.put(5, "Java");
        // 遍历
        for (Integer key : map.keySet()) {
            System.out.print(key);
            System.out.print(map.get(key));
        }
    }

    //Lambda
    @Test
    public void test3(){
        // 创建并赋值 HashMap
        Map<Integer, String> map = new HashMap();
        map.put(1, "Java");
        map.put(2, "JDK");
        map.put(3, "Spring Framework");
        map.put(4, "MyBatis framework");
        map.put(5, "Java中文社群");
        // 遍历
        map.forEach((key, value) -> {
            System.out.print(key);
            System.out.print(value);
        });
    }

    //Streams API 单线程
    @Test
    public void test4(){
        // 创建并赋值 HashMap
        Map<Integer, String> map = new HashMap();
        map.put(1, "Java");
        map.put(2, "JDK");
        map.put(3, "Spring Framework");
        map.put(4, "MyBatis framework");
        map.put(5, "Java中文社群");
        // 遍历
        map.entrySet().parallelStream().forEach((entry) -> {
            System.out.print(entry.getKey());
            System.out.print(entry.getValue());
        });
    }
}