问题描述：在字符串中寻找特定字符

1、第1次出现位置

实现函数原型： int indexOf(char c)

Java代码：

import java.io.*;
public class Way_1
{
    public static void main(String args[])
	 {
			String str = "Geeks for Geeks is a computer science portal";
			System.out.println(str.length());
			
			int firstIndex = str.indexOf('s');
			System.out.println("First occurrence of char 's' is found at: " + firstIndex);
			
			int firstIn = str.indexOf('z');
			System.out.println("First occurrence of char 'z' is found at: " + firstIn);        
    }
}

样例字符串： Geeks for Geeks is a computer science portal

样例输出：

2、最后一次出现位置

实现函数原型： public int lastIndexOf(char c)

Java代码：

import java.io.*;
public class Way_2
{
    public static void main(String args[])
	 {
			String str = "Geeks for Geeks is a computer science portal";
			System.out.println(str.length());
			
			int lastIndex = str.lastIndexOf('s');
			System.out.println("Last occurrence of char 's' is found at: " + lastIndex);			
			int lastIn = str.lastIndexOf('z');
			System.out.println("Last occurrence of char 'z' is found at: " + lastIn);        
    }
}

样例字符串： Geeks for Geeks is a computer science portal

样例输出：

3、指定位置后的首次出现

实现函数原型： public int IndexOf(char c, int indexFrom)

说明： 除-1外，该函数的返回值必然大于或等于其第2个参数indexFrom。

Java代码：

import java.io.*;

public class String_Search
{
    public static void main(String [] args)
    {
        String str = "Geeks For Geeks is a computer  science portal";
        System.out.println(str.length());
        
        int first_in = str.indexOf('s', 10);
        System.out.println("First occurrence of char 's' after index 10: " + first_in);
         int first_not_in = str.indexOf('z', 10);
        System.out.println("First occurrence of char 'z' after index 10: " + first_not_in);
    }
}

样例字符串： Geeks For Geeks is a computer science portal

样例输出：

4、指定位置前的末次出现

实现函数原型： public int lastIndexOf(char c, int fromIndex)

说明： 除-1外，该函数的返回值必然小于或等于其第2个参数fromIndex。

Java代码：

import java.io.*;

public class String_Search_2
{
    public static void main(String [] args)
    {
        String str = "Geeks For Geeks is a computer science portal";
        System.out.println(str.length());

        int lastIndex = str.lastIndexOf('s', 20);
        System.out.println("Last occurrence of char 's' before index 20: " + lastIndex);
        int last_not_in = str.lastIndexOf('z', 20);
        System.out.println("Last occurrence of char 'z' before index 20: " + last_not_in);
    }
}

样例字符串： Geeks For Geeks is a computer science portal

样例输出：

5、指定位置的字符

实现函数原型： char charAt(int indexNumber)

说明： 若指定的下标indexNumber超出字符串长度范围，则抛出StringIndexOutOfBounds异常。

Java代码：

import java.io.*;

public class Character_At
{
    public static void main(String [] args)
    {
        String str = "Geeks For Geeks is a computer science portal";
        System.out.println(str.length());

        int char_at = str.charAt(20);
        System.out.println("Character at location 20: " + char_at);
        
        int char_not_at = str.charAt(60);
    }
}

样例字符串： Geeks For Geeks is a computer science portal

样例输出：