isLetter method belongs to java.lang.Character class. It is a static method which takes a character as argument and tells if it is a letter or not.
A letter here means a character between A - Z or a - z. Digits and special characters are not considered as letters.
This method returns true if the supplied character is a letter and false if it is not a letter.

isLetter versions
There are two overloaded versions of isLetter method in java.lang.Character class.
Both of these are explained below.
1. isLetter(char c) : This method takes a character as argument and returns true if the supplied character is a letter(lies between A-Z and a-z). Returns false if the supplied character is not among these.

2. isLetter(int c): This method takes an integer as argument and returns true if the supplied integer value corresponds to a character in the range A - Z and a - z, that is between 65 – 90 and 97 – 122.
Remember that the integers represent the ASCII values of characters.
Using isLetter
As stated above, isLetter is a static method in java.lang.Character class.
Thus, it can be invoked directly by using the class name as shown below.

Character.isLetter(argument)

Here, the argument can be a char or an int as explained in the previous section.
isLetter Example
Following are the examples of isLetter method which accepts a character with different arguments.

public class IsLetterDemo {

   public static void main(String[] args) {
      //check for an alphabet
      char character = 'f';
      boolean isletter = Character.isLetter(character);
      System.out.println(character + " : " + isletter);
	
      // check for a digit
      character = '9';
      isletter = Character.isLetter(character);
      System.out.println(character + " : " + isletter);
	
      // check for a special character
      character = ',';
      isletter = Character.isLetter(character);
      System.out.println(character + " : " + isletter);
   }
}

Output of above example is

f : true
9 : false
, : false

Following are the examples of isLetter method which accepts an integer with different arguments.
Second one is a value that is a valid letter or alphabet(C), other values do not correspond to a letter and the output below confirms that.

public class IsLetterDemo {

   public static void main(String[] args) {
	int character = 12;
	boolean isletter = Character.isLetter(character);
	System.out.println(character + " : " + isletter);
		
	character = 67;
	isletter = Character.isLetter(character);
	System.out.println(character + " : " + isletter);
	
	character = 123;
	isletter = Character.isLetter(character);
	System.out.println(character + " : " + isletter);
   }
}

Output of above code is

12 : false
67 : true
123 : false

Use of isLetter
isLetter is a convenience method to check if a character is an alphabetic letter.
It can be used to check the presence of a number or a special character inside a string by iterating over all the characters and checking if it contains a character other than alphabets.

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