Indexof C# là gì

Java String indexOf() method is used to find the index of a specified character or a substring in a given String. There are 4 variations of this method in String class:

The indexOf() method signature

int indexOf(int ch): It returns the index of the first occurrence of character ch in a given String.

int indexOf(int ch, int fromIndex): It returns the index of first occurrence of character ch in the given string after the specified index fromIndex. For example, if the indexOf() method is called like this str.indexOf(A, 20) then it would start looking for the character A in string str after the index 20.

int indexOf(String str): Returns the index of string str in a particular String.

int indexOf(String str, int fromIndex): Returns the index of string str in the given string after the specified index fromIndex.

All the above variations returns -1 if the specified char/substring is not found in the particular String.

Java String indexOf() Method example

public class IndexOfExample{ public static void main(String args[]) { String str1 = new String("This is a BeginnersBook tutorial"); String str2 = new String("Beginners"); String str3 = new String("Book"); String str4 = new String("Books"); System.out.println("Index of B in str1: "+str1.indexOf('B')); System.out.println("Index of B in str1 after 15th char:"+str1.indexOf('B', 15)); System.out.println("Index of B in str1 after 30th char:"+str1.indexOf('B', 30)); System.out.println("Index of string str2 in str1:"+str1.indexOf(str2)); System.out.println("Index of str2 after 15th char"+str1.indexOf(str2, 15)); System.out.println("Index of string str3:"+str1.indexOf(str3)); System.out.println("Index of string str4"+str1.indexOf(str4)); System.out.println("Index of hardcoded string:"+str1.indexOf("is")); System.out.println("Index of hardcoded string after 4th char:"+str1.indexOf("is", 4)); } }

Output:

Index of B in str1: 10 Index of B in str1 after 15th char:19 Index of B in str1 after 30th char:-1 Index of string str2 in str1:10 Index of str2 after 15th char-1 Index of string str3:19 Index of string str4-1 Index of hardcoded string:2 Index of hardcoded string after 4th char:5

Another example of indexOf() method

Lets take a simple example with a short string where we are finding the indexes of given chars and substring using the indexOf() method.

public class JavaExample { public static void main(String[] args) { String str = "Java String"; char ch = 'J'; char ch2 = 'S'; String subStr = "tri"; int posOfJ = str.indexOf(ch); int posOfS = str.indexOf(ch2); int posOfSubstr = str.indexOf(subStr); System.out.println(posOfJ); System.out.println(posOfS); System.out.println(posOfSubstr); } }

Output: