2021-05-08

课堂代码
public static void main(String[]args){
HashSet<String> set = new HashSet<>(); //<>钻石语法
set.add("111");
set.add("222");
set.add("333");
set.add("小明");
set.add("小华");
set.add("aaa");
set.add("bbb");
set.add("xxx");
System.out.println(set);
set.remove("xxx"); //根据内容移除
System.out.println(set);
int size = set.size();
boolean empty = set.isEmpty();
System.out.println(size);
System.out.println(empty);
set.clear(); //清除
System.out.println(set);
}
}

public static void main(String[]args){
    TreeSet<String> set = new TreeSet<>();
    set.add("111");
    set.add("222");
    set.add("历史");
    set.add("地理");
    set.add("asd");
    set.add("ASD");
    System.out.println(set);
    //set.clear();
    System.out.println(set.pollFirst());
    System.out.println(set);
    /*
        add    添加
        addAll 添加一个新的元素
        Clear  清除所有元素
        CLone  克隆
        contains  包含
        first  第一个元素
        isEmpty 是否为空
        iterator 选代器
        last     最后一个
        remove   抽取第一个

     */
}

public static void main(String[]args){
HashMap<String,String> map = new HashMap<>();
map.put("henan","河南"); // entry 实例
map.put("hebei","河北");
map.put("hubei","湖北"); //键重复会覆盖掉原有的值
map.put("null","空1");
map.put(null,"空2");
System.out.println(map);

    for (Map.Entry<String ,String > m :map.entrySet() ){
        System.out.println(m);
    }
    for ( String k:map.keySet() ) {
        System.out.println( k + "=" + map.get(k));
    }
    for (String v:map.values()){
        System.out.println(v);
    }
}

public static void main(String[]args){
    HashMap<String,String> map = new TreeMap<>();
    map.put("henan","河南");     // entry 实例
    map.put("hebei","河北");
    map.put("hubei","湖北");  //键重复会覆盖掉原有的值
    /*map.put("null","空1");
    map.put(null,"空2");
    System.out.println(map)*/
    System.out.println(map);

    for (Map.Entry<String ,String > m :map.entrySet() ){
        System.out.println(m);
    }
    for ( String k:map.keySet() ) {
        System.out.println( k + "=" + map.get(k));
    }
    for (String v:map.values()){
        System.out.println(v);
    }
}

public static void main(String[]args){
    String str [] = {"中文","计算机","ABC","123","qq@qq.com"};
    List<String>list = Arrays.asList(str); //1 Arrays.asList()将数组转换成列表
    System.out.println(list);

    // 2 将数组转换成列表（链式列表） newArrayList(Arrays.asList(str))
    ArrayList<String>alist = new ArrayList<>(Arrays.asList(str));
    alist.remove("ABC");
    alist.add("1321123");
    System.out.println(alist);
    System.out.println(alist);

    // 3 将列表转换成数组  列表 .toArray();
    Object[] str2 = alist.toArray();
    for (Object O:str2){
        System.out.println(O + " ");
    }
    System.out.println();
    Object[] str3 = alist.toArray();
    for (Object o:str3){
        System.out.println(o +"");
    }
}