Java中String indexOf() 方法解读

翻译自https://www.geeksforgeeks.org/java-string-indexof/

1 String indexOf(char c)：此方法返回c字符在string中的下标【不存在则返回-1】

解释代码：

// Java code to demonstrate the working 
// of String indexOf() 
public class Index1 { 
public static void main(String args[]) 
    { 
  
        // Initialising String 
        String gfg = new String("Welcome to geeksforgeeks"); 
  
        System.out.print("Found g first at position : "); 
  
        // Initial index of 'g' will print 
        // prints 11 
        System.out.println(gfg.indexOf('g')); 
    } 
} 

 

 

输出： 

Found g first at position : 11

 2 String indexOf(char c, int strt)：此方法从下标strt处开始查找c字符在string中的下标【不存在则返回-1】

解释代码：

// Java code to demonstrate the working 
// of String indexOf(char ch, int strt) 
public class Index2 { 
public static void main(String args[]) 
    { 
  
        // Initialising String 
        String gfg = new String("Welcome to geeksforgeeks"); 
  
        System.out.print("Found g after 13th index at position : "); 
  
        // 2nd index of 'g' will print 
        // prints 19 
        System.out.println(gfg.indexOf('g', 13)); 
    } 
} 

 

 

输出：

Found g after 13th index at position : 19

3 String indexOf(String str)：此方法返回字符串在string中的下标【不存在则返回-1】

解释代码：

// Java code to demonstrate the working 
// of String indexOf(String str) 
public class Index3 { 
public static void main(String args[]) 
    { 
  
        // Initialising string 
        String Str = new String("Welcome to geeksforgeeks"); 
  
        // Initialising search string 
        String subst = new String("geeks"); 
  
        // print the index of initial character 
        // of Substring 
        // prints 11 
        System.out.print("Found geeks starting at position : "); 
        System.out.print(Str.indexOf(subst)); 
    } 
} 

输出：

Found geeks starting at position : 11

4 String indexOf(String str, int strt)：此方法从下标strt处开始查找字符str在string中的下标【不存在则返回-1】

解释代码：

// Java code to demonstrate the working 
// of String indexOf(String str, int strt) 
public class Index4 { 
public static void main(String args[]) 
    { 
  
        // Initialising string 
        String Str = new String("Welcome to geeksforgeeks"); 
  
        // Initialising search string 
        String subst = new String("geeks"); 
  
        // print the index of initial character 
        // of Substring aftr 14th position 
        // prints 19 
        System.out.print("Found geeks(after 14th index) starting at position : "); 
        System.out.print(Str.indexOf(subst, 14)); 
    } 
} 

 

输出：

Found geeks(after 14th index) starting at position : 19

5 其他相关的应用

Finding out if a given character (maybe anything upper or lower case) is a vowel or consonant.

Implementation is given below:

解释代码

class Vowels 
{ 
		// function to check if the passed 
		// character is a vovel 
	public static boolean vowel(char c) 
	{ 
		return "aeiouAEIOU".indexOf(c)>=0; 
	} 

		// Driver program 
	public static void main(String[] args) 
	{ 
		boolean isVowel = vowel('a'); 
		
				// Printing the output 
				if(isVowel) 
			System.out.println("Vowel"); 
		else
			System.out.println("Consonant"); 
	} 
} 

输出：

Vowel