String的indexof方法用于查找任何字符或者子字符串的索引。

indexof方法可以将char或者string作为参数拍摄，也可以提供fromIndex(索引之后我们要使用索引方法)。

方法：

public int indexof(int ch)返回字符串中第一次出现字符的索引

Public Int IndexOf(int ch，int flowindex)返回字符串内的第一次出现字符的索引，从特定索引"fromIndex"公共int indexof(string)返回字符串公共int index中的子字符串的第一次出现索引(int ch，int fromIndex)从特定索引"fromIndex"开始，返回字符串内第一次出现子字符串的索引

如果字符串中不存在char或substring，则所有indexOf方法都返回-1

字符串索引示例：

package org.igi.theitroad;
 
public class StringIndexOfExample {
 /*
  * @ Author : igi Mandliya
  */
 public static void main(String args[]) {
  
  String str="Hello world from theitroad";
  //Using index of method with character
  System.out.println("---------------------------------------------------");
  System.out.println("Using indexOf method with character");
  System.out.println("---------------------------------------------------");
  
  System.out.println("Index of character b in "Hello world from theitroad" is : "+str.indexOf('b'));
  System.out.println();
  
  String str1="from";
  System.out.println("---------------------------------------------------");
  System.out.println("Using indexOf method with fromIndex with character");
  System.out.println("---------------------------------------------------");
  System.out.println("Index of character o from index 6 in "Hello world from theitroad" is : "+str.indexOf('o',6));
  System.out.println();
  
  //Using index of method with String
  System.out.println("---------------------------------------------------");
  System.out.println("Using indexOf method with String");
  System.out.println("---------------------------------------------------");
  
  System.out.println("Index of String "from" in "Hello world from theitroad" is : "+str.indexOf(str1));
  System.out.println();
  
  String howDo="How do you do";
  System.out.println("---------------------------------------------------");
  System.out.println("Using indexOf method with fromIndex with String");
  System.out.println("---------------------------------------------------");
  System.out.println("Index of String "do" from index 9 in "How do you do" is : "+howDo.indexOf("do",9));
  System.out.println();
  
  System.out.println("---------------------------------------------------");
  System.out.println("If String or char is not available");
  System.out.println("---------------------------------------------------");
  System.out.println("Index of "from" in "How do you do" is : "+howDo.indexOf("from"));
  
  
 }
}

运行上面的程序时，我们将得到以下输出：

-------------------
Using indexOf method with character
-------------------
Index of character b in "Hello world from theitroad" is : 22
 
-------------------
Using indexOf method with fromIndex with character
-------------------
Index of character o from index 6 in "Hello world from theitroad" is : 7
 
-------------------
Using indexOf method with String
-------------------
Index of String "from" in "Hello world from theitroad" is : 12
 
-------------------
Using indexOf method with fromIndex with String
-------------------
Index of String "do" from index 9 in "How do you do" is : 11
 
-------------------
If String or char is not available
-------------------
Index of "from" in "How do you do" is : -1