How to use the indexOf() function of the String class in Java to find the position of a specified character or substring in a string

Java is a widely used programming language, and the String class is one of the most commonly used classes. The String class provides a series of methods for operating strings, allowing us to easily perform string search, replacement and other operations. Among them, the indexOf() function is a very important method in the String class, which can be used to find the position of a specified character or substring in a string. This article will introduce how to use the indexOf() function to achieve this function and provide corresponding code examples.

First, let’s take a look at the definition of the indexOf() function:

public int indexOf(int ch)
public int indexOf(int ch, int fromIndex)
public int indexOf(String str)
public int indexOf(String str, int fromIndex)

In the parameters, ch represents the character to be searched, str represents the substring to be searched, and fromIndex represents the index to start searching. Location. If the specified character or substring is found, returns the index position of the first character or substring found; if not found, returns -1.

Next, we use several examples to demonstrate how to use the indexOf() function to search.

Example 1: Find the position of the specified character

String str = "Hello, World!";
char ch = 'o';
int index = str.indexOf(ch);
System.out.println("字符" + ch + "在字符串中的位置是：" + index);

Output:
The position of character o in the string is: 4

Example 2: Starting from the specified position Find the position of the specified character

String str = "Hello, World!";
char ch = 'o';
int fromIndex = 6;
int index = str.indexOf(ch, fromIndex);
System.out.println("从索引位置" + fromIndex + "开始，字符" + ch + "在字符串中的位置是：" + index);

Output:
Starting from index position 6, the position of character o in the string is: 9

Example 3: Find the position of the specified substring

String str = "Hello, World!";
String subStr = "World";
int index = str.indexOf(subStr);
System.out.println("子串" + subStr + "在字符串中的位置是：" + index);

Output:
The position of the substring World in the string is: 7

Example 4: Find the position of the specified substring starting from the specified position

String str = "Hello, World!";
String subStr = "World";
int fromIndex = 6;
int index = str.indexOf(subStr, fromIndex);
System.out.println("从索引位置" + fromIndex + "开始，子串" + subStr + "在字符串中的位置是：" + index);

Output :
Starting from index position 6, the position of the substring World in the string is: 7

Through the above example, we can see that the indexOf() function can be conveniently used to find in a string Specifies the position of a character or substring. We can set the index position to start searching as needed, or it can also be used to determine whether a string contains specified characters or substrings. In practical applications, we can use the indexOf() function to handle various string operations, such as determining whether user input is legal, searching for keywords in strings, and so on.

To summarize, the indexOf() function of the String class in Java provides a simple and efficient way to find the position of a specified character or substring in a string. Through different parameter settings, we can flexibly apply this function to meet different needs. We hope that through the introduction and sample code of this article, readers will have a deeper understanding and mastery of this function.

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