String class in Java

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The String class in Java is a fundamental class in the Java Standard Library. It is one of the most widely used classes in the Java programming language. It represents a sequence of characters and provides a number of methods for manipulating strings.

One of the most important characteristics of the String class is that it is immutable, meaning that once a String object is created, its value cannot be changed. This makes String objects thread-safe, as they cannot be modified by multiple threads simultaneously.

Following are some examples of the methods available in the String class:

length()

This method counts the number of characters in the string and returns the value.

Let’s take an example of the following code:

String str = "Hello, world!";
int length = str.length();  // length is 13
System.out.println(length);

The output of the above code will be:

13

Because in the variable str there are 13 characters. Remember that the space is also a character.

charAt(int index)

This method returns the character at the specified index in the string.

Let’s take an example of the following code:

String str = "Hello, world!";
char c = str.charAt(1);  // c is 'e'
System.out.println(c);

The output of the following code will be:

e

Because the starting index is zero (0). Hence, e is in the 1th position of the variable str.

substring(int beginIndex, int endIndex)

This method returns a new string that is a substring of the original string. Starting at the specified beginIndex and ending at the specified endIndex - 1.

Let’s consider the following example:

String str = "Hello, world!";
String sub = str.substring(7, 12);  // sub is "world"
System.out.println(sub);

The output of the above code will be:

world

indexOf(String str)

This method returns the index of the first occurrence of the specified string in the original string. For example:

String str = "Hello, world!";
int index = str.indexOf("world");  // index is 7
System.out.println(index);

The output of the above code will be:

7

Since this method returns the index of the first occurrence of the specified string, the following code also returns the same output:

String str = "Hello, world from the Coder Sathi World.";
int index = str.indexOf("world");  // index is 7
System.out.println(index);

The output of the above code is also the same. i.e. 7.

toLowerCase() and toUpperCase()

These methods return a new string with all the characters in the original string converted to lowercase or uppercase, respectively. See the example below:

String str = "Hello, world!";
String lower = str.toLowerCase();  // lower is "hello, world!"
String upper = str.toUpperCase();  // upper is "HELLO, WORLD!"

trim()

This method returns a new string with leading and trailing whitespace removed. See the example below:

String str = "   Hello, world!   ";
String trimmed = str.trim();  // trimmed is "Hello, world!"

split(String regex)

This method splits the string into an array of substrings, using the specified regular expression as the delimiter. See the example below:

String str = "Hello,world,how,are,you,today";
String[] words = str.split(",");  // words is ["Hello", "world", "how", "are", "you", "today"]

There are many methods in the String class. You can refer to the official documentation to understand them in detail.

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