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Difference between concat() and + operator in Java

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  • Difficulty Level : Basic
  • Last Updated : 07 Jan, 2022

Strings are defined as an array of characters. The difference between a character array and a string is the string is terminated with a special character ‘\0’. Since arrays are immutable(cannot grow), Strings are immutable as well. Whenever a change to a String is made, an entirely new String is created. Concatenation is the process of joining end-to-end.

concat() Method

The Java String concat() method of String class has been itself present inside java.util package concatenates one string to the end of another string. This method returns a string with the value of the string passed into the method, appended to the end of the string.

Illustration:

Input:  String 1   : abc
        String 2   : def
        String n-1 : ...
        String n   : xyz
Output: abcdef...xyz  

Example:

Java




// Java Program to Demonstrate Working of concat() method
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String args[])
    {
        // Custom input string 1
        String s = "Geeks ";
 
        // Custom input string 2 is passed as in arguments
        // Here we are adding it to end of same string
        s = s.concat("for Geeks");
 
        // Printing the concatenated string
        System.out.println(s);
    }
}

Output

Geeks for Geeks

Now let us do dwell on the next concept where we will be discussing 

 ‘+’ Operator

+ operator is used for concatenating strings on either side and is used just likely we are up to adding two numbers, henceforth providing us the flexibility to add on either side. 

Example:

Java




// Java Program to Demonstrate Working of + Operator
// over Strings
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String args[])
    {
        // Custom input strings
        String s1 = " Geeks ";
        String s2 = " for Geeks ";
 
        // Now using + operator over string where
        // flexibility is provided to add on either side
        String s3 = s1 + s2;
        String s4 = s2 + s1;
 
        // Printing strings formed after
        // using + operator
        System.out.println(s3);
        System.out.println(s4);
    }
}

Output

 Geeks  for Geeks 
 for Geeks  Geeks 

Although concat() method of Strings class and + operator are both used for the concatenation of strings, there are some differences between them which are depicted below in the tabular format as follows:

Factor 1: Number of arguments the concat() method and + operator takes

  • concat() method takes only one argument of string and concatenates it with other string.
  • + operator takes any number of arguments and concatenates all the strings.

Example

Java




// Java Program to Illustrate concat() method
// vs + operator in Strings
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Custom input string
        String s = "Geeks", t = "for", g = "geeks";
 
        // Printing combined string using + operator
        System.out.println(s + t + g);
 
        // Printing combined string using concat() method
        System.out.println(s.concat(t));
    }
}

Output

Geeksforgeeks
Geeksfor

Factor 2: Type of arguments

  • string concat() method takes only string arguments, if there is any other type is given in arguments then it will raise an error.
  • + operator takes any type and converts to string type and then concatenates the strings.

Factor 3: concat() method raises java.lang.NullPointer Exception

  • concat() method throws NullPointer Exception when a string is concatenated with null
  • + operator did not raise any Exception when the string is concatenated with null.

Example

Java




// Java Program to Illustrate Raise of NullPointer Exception
// in case of concat() Method
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Input string 1
        String s = "Geeks";
        // Input string 2
        String r = null;
 
        // Combining above strings using + operator and
        //  printing resultant string
        System.out.println(s + r);
 
        // Combining above strings using concat() method and
        // printing resultant string
        // It raises an NullPointer Exception
        System.out.println(s.concat(r));
    }
}

Output:

Factor 4: Creates a new String object

  • concat() method takes concatenates two strings and returns a new string object only string length is greater than 0, otherwise, it returns the same object.
  • + operator creates a new String object every time irrespective of the length of the string.

Example

Java




// Java Program to Illustrate Creation of New String object
// in concat() method and + Operator
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Input strings
        String s = "Geeks", g = "";
 
        // Using concat() method over strings
        String f = s.concat(g);
 
        // Checking if both strings are equal
 
        // Case 1
        if (f == s)
 
            // Identical strings
            System.out.println("Both are same");
 
        else
 
            // Non-identical strings
            System.out.println("not same");
 
        // Case 2
        String e = s + g;
 
        // Again checking if both strings are equal
        if (e == s)
 
            // Identical strings
            System.out.println("Both are same");
        else
 
            // Non-identical strings
            System.out.println("not same");
    }
}

Output

Both are same
not same

Factor 5: Performance

concat() method is better than the + operator because it creates a new object only when the string length is greater than zero(0) but the + operator always creates a new string irrespective of the length of the string.

Conclusion: From above two programs we draw see that both are somehow adding up two strings directly or indirectly. In concat method we are bound to while adding strings as we have to pass the string in the parameter while in other case of ‘+’ operator, it is simply adding up strings likely mathematics, so there is no bound we can add either side.

Below is the table to depict major differences between both the methods:

Basics concat() Method + Operator 
Number of arguments the concat() method and + operator takesTakes only one argument of string and concatenates it with another string. Takes any number of arguments and concatenates all the strings
Type of ArgumentsTakes only string arguments, if there is any other type is given in arguments then it will raise an error. Takes any type and converts to string type and then concatenates the strings. 
NullPointer ExceptionThrows NullPointer Exception when the string is concatenated with null It does not raise any Exception when the string is concatenated with null
Creating a new String objectTakes concatenates two strings and returns a new string object only if the length of the returned string is greater than the length of one of the concatenated strings, otherwise, it returns the same object. Creates a new string object every time irrespective of the length of the string.
Performanceconcat() method is better than the ‘+’ operator because it creates a new object only when the string length is greater than zero(0), so it uses less amount of memory.+ operator always creates a new string irrespective of the length of string therefore it takes more memory.

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