java中String，数组，ArrayList三者之间的转换

ArrayList list_int = new ArrayList<Integer>(); //Integer的List
list_int.add(1);
list_int.add(3);
list_int.add(5);
list_int.add(9);

ArrayList<String> list_str = new ArrayList<String>(); //String的List
list_str.add("xiaoling");
list_str.add("xiaoling");

String toString = list_int.toString(); //ArrayList转换为String，带中括号的
String substring = toString.substring(1, toString.length() - 1);//去除中括号。小心java的substring是左闭右开的
String result = substring.replaceAll("\\s", "");//去除所有的空字符
System.out.println(result);//1,3,5,9

String str = String.join(",", list_str);//将list_str中的元素使用逗号连接
System.out.println(str);//xiao,ling

Integer[] ints = new Integer[list_int.size()];//先造一个长度相同的Integer数组
Object[] objects = list_int.<Integer>toArray(ints);// 使用toArray(T[] a) 方法将List的所有元素放入a数组中
for (Integer i : ints){
  System.out.print(i+"\t");//1 3 5 9 
}
System.out.println();
//for (Object i : objects){ //打印结果显示返回的数组和自定义的数组内容是一样的
//  System.out.print(i+"\t");//1 3 5 9 
//}

String str_int = "1,3,5,9";
String str_str = "jiang,chen,xiao,ling";

String[] split = str_int.split(","); //转为String[]

//再将String[]转为Integer[]
Integer[] ints1 = new Integer[split.length];
for (int i = 0; i < ints1.length; i++) {
  ints1[i] = Integer.parseInt(split[i]);
}

System.out.println(Arrays.asList(ints1)); //[1, 3, 5, 9]

ArrayList arrayList = new ArrayList(Arrays.asList(str_int.split(",")));
System.out.println(arrayList); //[1, 3, 5, 9]

Integer[] arr_int = {1,3,7,11};
String[] arr_str = {"xiao","ling"};

//先转为Arrays.ArrayList,再使用它的toString方法
String s = Arrays.asList(str_int).toString();
System.out.println(s);

//如果是字符串类型则直接
String join = String.join(",", str_str);
System.out.println(join);

ArrayList list1 = new ArrayList(Arrays.asList(arr_int));
System.out.println(list1);

ArrayList list2 = new ArrayList(Arrays.asList(arr_str));
System.out.println(list2);