String class in Java

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:

concat()

This method concatenates (joins) the two strings and returns a new string. Let’s see an example below:

String firstName = "Virat";
String lastName = "Kohli";

To print the full name Virat Kohli, we need to concat these two strings. See the example below:

firstName.concat(lastName);
System.out.println(firstName);

If we print firstName then it will print Virat. This is because String is an immutable object. This basically means, once the string is created it can’t be modified. To create a mutable string we have StringBuilder and StringBuffer in Java.

To print fullName we need to assign it to a new variable like:

String fullName = firstName.concat(lastName);
System.out.println(fullName);

This prints the output:

Virat Kohli

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);
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); 
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"); 
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!"

If we print the length of the str before and after print we will get:

String str = "   Hello, world!   ";
System.out.println("Before trim: "+str.length());
String trimmed = str.trim();  
System.out.println("After trim: "+trimmed.length());

Output:

Before trim: 19
After trim: 13

Here, the original String value str has two spaces before and after Hello, world! text.

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.