1, ArrayList indexOf () method
The indexOf(Object obj) method returns the index value of the element in the dynamic array.
The returned value is:
Returns the index value of the obj element that first appears in the array, and the index subscript starts from 0
If the specified element does not exist in the dynamic array, the indexOf() method returns -1
ArrayList<String> aa = new ArrayList<>();
aa.add("上海");
aa.add("北京");
aa.add("a");
aa.add("p");
aa.add("p");
System.out.println(aa); // [上海, 北京, a, p, p]
int b = aa.indexOf("app");
int e = aa.indexOf("海");
int f = aa.indexOf("北京");
int h = aa.indexOf("p");
System.out.println(b);//结果为 -1 因为不存在 app 的值
System.out.println(e);//结果为 -1 因为不存在 海 的值
System.out.println(f);//结果为 1 北京出现的位置索引下标为1
System.out.println(h);//结果为 3 p第一次出现的位置索引下标为32, String indexOf(String str) method
The indexOf(String str) method returns the first occurrence of a specified string value in the string, and the index subscript starts from 0
Returns -1 if no matching string is found.
String a = "A,B2,CDF,D,F";
int c = a.indexOf("B");
int g = a.indexOf("D");
int j = a.indexOf("F");
int k = a.indexOf("上");
int u = a.indexOf("D",7);
System.out.println(c);//结果为：2 B出现的位置索引下标为2
System.out.println(g);//结果为：6 D出现的位置索引下标为6
System.out.println(j);//结果为：7 F出现的位置索引下标为7
System.out.println(k);//结果为：-1 上 未出现返回-1
System.out.println(u);//结果为：9 在第7个位置开始查找字符 "D" 第一次出现的位置，所以是93、string.indexOf(String str,int fromIndex)方法
str: the string value to be retrieved
fromIndex: specifies the position in the string to start searching
String a = "A,B2,CDF,D,F";
int u = a.indexOf("D",7);
System.out.println(u);//结果为：9 在第7个位置开始查找字符 "D" 第一次出现的位置，所以是94. The difference between the two
ArrayList: The indexOf(Object obj) method returns the first occurrence of the index value of the element in the dynamic array
String: The indexOf(String str) method returns the position of the first occurrence of a specified string value in the string
5. The most misleading place is
For example, the above example:
There is a , (comma) in the String string , which is also counted as an index subscript
And ArrayList: return [Shanghai, Beijing, a, p, p] Among them , (comma) is not counted as a subscript, it is to check the subscript according to the value of the array,
The query int b = aa.indexOf("app") is not [a,p,p] in the array
If there is [Shanghai, Beijing, a, p, p, app] in the array, return 5
6. Extension: ArrayList lastIndexOf() method and String lastIndexOf() method
ArrayList: The lastIndexOf(Object obj) method returns the last position of the index value of the element in the dynamic array
String: The lastIndexOf(String str) method returns the position of the last occurrence of a specified string value in the string