We will discuss how to add characters to a string at a specific position in a string in java. It can be explained as follows, as shown in the figure, what we are trying to do is as follows:

Illustration:

Input: 
Input custom string = Hello
Output: 
--> String to be added 'Geeks'
    --> If end position, Output: HelloGeeks
    --> If in beginning, Output: GeeksHello
    --> If at sat 3rd index, Output: HelGeekslo 

Methods: This can be done using a number of methods, the most common of which are listed below:

1. Using + operator
   • At the end
   • At the beginning
2. Using insert() method of StringBuffer class
3. Using substring() method

Let us discuss all three methods above listed in detail to get a fair understanding of the same

Method 1: Using + operator

1.1 At the end

Example:  The "+" operator can be used to add characters to the beginning of a character string.

// Java Program to Add Characters to a String
// At the End

// Importing input output classes
import java.io.*;

// Main class
public class GFG {

	// Main driver method
	public static void main(String args[])
	{

		// Input character and string
		char a = 's';
		String str = "GeeksforGeek";

		// Inserting at the end
		String str2 = str + a;

		// Print and display the above string
		System.out.println(str2);
	}
}

Output

GeeksforGeeks

1.2 At the beginning

Example: One can add character at the start of String using the ‘+’ operator. 

// Java Program to Add Characters to a String
// At the Beginning

// Importing input output classes
import java.io.*;

// Main class
public class GFG {

	// Main driver method
	public static void main(String args[])
	{
		// Input character and string
		char a = 'G';
		String str = "eeksforGeeks";

		// Inserting at the beginning
		String str2 = a + str;

		// Print and display the above string
		System.out.println(str2);
	}
}

Output

GeeksforGeeks

Method 2: Using insert() method of StringBuffer class 

StringBuffer is the equivalent of String, and it provides most of the functionality of strings. String represents a fixed-length, immutable sequence of characters, and StringBuffer represents a sequence of characters that can grow and be writable. StringBuffer may insert characters and substrings in the middle or append to the end. It grows automatically to make room for such additions, and it usually pre-allocates more characters than actually needed to make room for growth. You can use the StringBuffer class method, the insert() method, to add characters to the String at a given position. This method inserts the string representation of the given data type at the given position in the StringBuffer.

Syntax: 

 str.insert(int position,  char x);
 str.insert(int position,  boolean x);
 str.insert(int position,  char[] x);
 str.insert(int position, float x);
 str.insert(int position, double x);
 str.insert(int position, long x);
 str.insert(int position, int x);

position is the index in string where
we need to insert.

Return type: A reference to this object.

Example  

// Java Program to Add Characters to a String
// Using StringBuffer class insert() method

// Main class
// AddCharacterToStringAnyPosition
public class GFG {

	// Method 1
	// To add character to string
	public static String addCharToString(String str, char c,
										int pos)
	{

		// Creating an object of StringBuffer class
		StringBuffer stringBuffer = new StringBuffer(str);

		// insert() method where position of character to be
		// inserted is specified as in arguments
		stringBuffer.insert(pos, c);

		// Return the updated string
		// Concatenated string
		return stringBuffer.toString();
	}

	// Method 2
	// Main driver method
	public static void main(String[] args)
	{

		// Input string and character
		String blogName = "GeeksorGeeks";
		char two = 'f';

		// Calling the method 1 to
		// add character to a string

		// Custom string, character and position passed
		String cblogName
			= addCharToString(blogName, two, 5);

		// Print and display th above string
		System.out.println(cblogName);
	}
}

Output

GeeksforGeeks

Method 3: Using substring() method

You can also use String's substring method to add characters to a String at a given position. There are two variants of this method. It returns a new string that is a substring of the string, where the substring starts with the character at the specified index and extends to the end of the string.

Syntax:  

public String substring(int begIndex)

Parameters: The beginning index, inclusive.

Return Value: The specified substring.

Example  

// Java Program to Add Characters to a String
// Using substring() method

// Main class
// AddCharacterToStringAnyPosition
public class GFG {

	// Method 1
	// To add character to a string
	public static String
	addCharToStringUsingSubString(String str, char c,
								int pos)
	{
		return str.substring(0, pos) + c
			+ str.substring(pos);
	}

	// Method 2
	// Main driver method
	public static void main(String[] args)
	{
		// Custom input character and string
		String blogName = "GeeksorGeeks";
		char two = 'f';

		// Calling the Method 1 to
		// To add character to a string

		// Custom arguments
		String cblogName = addCharToStringUsingSubString(
			blogName, two, 5);

		// Print and display the above string on console
		System.out.println(cblogName);
	}
}

Output

GeeksforGeeks