Java String concat() Method with Examples (original) (raw)

Last Updated : 19 Nov, 2024

The string concat() methodconcatenates (appends) a string to the end of another string. It returns the combined string. It is used for **string concatenation in Java. It returns **NullPointerException if any one of the strings is Null.

In this article, we will learn **how to concatenate two strings in Java.

Program to Concatenate Two Strings using concat() Method in Java

Below is the simplest method to **concatenate the string in Java. We will be using the **concat() method of String class to append one string to the end of another and return the new string.

Java `

// Java Program to Illustrate concat() method of String. class GFG {

public static void main(String args[]) {
  
    // String Initialization
    String s = "Geeks";
   
    // Use concat() method for string concatenation
    s = s.concat("forGeeks");
    System.out.println(s);
}

}

`

Table of Content

• Syntax of concat() Method
• Java String concat() Examples
  • Sequential Concatenation of Multiple Strings
  • Handling NullPointerException in String concat()
  • Combining Two Strings with concat() Method
  • Reversing a String Using concat() Method

Syntax of concat() Method

public String **concat (String s);

**Parameters:

• A string to be concatenated at the end of the other string.

**Return Value:

• Concatenated(combined) string.

**Exception:

• **NullPointerException: When either of the string is **Null.

Java String concat() Examples

There are many ways to use the concat() method in Java:

Sequential Concatenation of Multiple Strings

The below example shows sequential concatenation using String concat() method in Java.

**Example:

java `

// Java program to Illustrate Working of concat() method // in strings where we are sequentially // adding multiple strings as we need public class GFG {

public static void main(String args[])
{
    String s1 = "Computer-";

    String s2 = "Science-";

    // Combining above strings by
    // passing one string as an argument
    String s3 = s1.concat(s2);

    // Print and display temporary combined string
    System.out.println(s3);

    String s4 = "Portal";
    String s5 = s3.concat(s4);
    System.out.println(s5);
}

}

`

Output

Computer-Science- Computer-Science-Portal

**Note: As perceived from the code we can do as many times as we want to concatenate strings bypassing older strings with new strings to be contaminated as a parameter and storing the resultant string in String datatype.

Handling NullPointerException in String concat()

The below example shows the **NullPointerException in String concat() method.

**Example:

Java `

// Java program to Illustrate NullPointerException public class GFG {

public static void main(String args[])
{
    String s1 = "Computer-";
    String s2 = null;

    // Combining above strings by
    // passing one string as an argument
    String s3 = s1.concat(s2);

    // It will raise NullPointerException
    System.out.println(s3);
}

}

`

**Output:

Exception in thread "main" java.lang.NullPointerException

Combining Two Strings with concat() Method

The below example combines two strings using concat() method of string class.

Example:

Java `

// Java Program to combine two strings with concat() class GFG {

public static void main(String args[]) {

    // String Initialization
    String s1 = "Geeksfor";
    String s2 = "Geeks";

    // Concatenate the strings s1 and s2 using the concat() method
    // and store the result back in s1.
    s1 = s1.concat(s2);

    System.out.println(s1);
}

}

`

Reversing a String Using concat() Method

We can reverse a string using the concat() method of string class. Below is the example to reverse a string in Java.

**Example:

Java `

// Java Program to reverse a string using concat() method public class ReverseString {

public static void main(String[] args) {
    
    // Declare original string variable
    String a = "Geeks";

    // Declare another string variable
    // and initialize it with an empty string
    String b = "";

    // Iterate through each character in string "a"
    // from the last index to the first.
    for (int i = a.length() - 1; i >= 0; i--) {
        
        // Extract the current character at index "i" of the "a" string
        char ch = a.charAt(i);

        // Convert the character to a String object 
        // using the "Character.toString" method
        String ch1 = Character.toString(ch);

        // Concatenate the converted character String 
        // to the end of the "b" string
        b = b.concat(ch1);
    }

    System.out.println("" + a);
    System.out.println("" + b);
}

}

`