Java中String indexOf() 方法解读

2021-12-07 12:13 570阅读 0赞

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

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