//声明 new String().var
        String s = new String();

        //判空 方式1: 变量声明后,ifn; 方式2: s.null
        if (s == null) {

        }

        //判非空 方式1: s.notnull; 方式2: s.nn
        if (s != null) {

        }

        //遍历 list.for
        ArrayList<Object> list = new ArrayList<>();
        for (Object o : list) {

        }

        //带索引遍历 从小到大: list.fori; 从大到小: list.forr
        for (int i = 0; i < list.size(); i++) {

        }

        for (int i = list.size() - 1; i >= 0; i--) {

        }

        //取反 success.not
        Boolean success = true;
        //success.not --> !success

        //条件判断 if success.if
        if (success) {

        }

        //强转 cast 方式1: name.cast --> (String)name; 方式2: name.castvar --> String name2 = (String) name;
        Object name = "xz";
        String name1 = (String) name;
        String name2 = (String) name;

        //返回 return name.return --> return name;
        //return name;