How to Optimize String Concatenation in Java?

Last Updated : 02 Dec, 2020

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. Let us take an example to understand what concatenation means in English.

Person 1 speaking to Person 2 - ItsCodingEra
Person 2 speaking to Person 1 - 2020India


Now, let us carry a process whose output is as follows- ItsCodingEra2020India

This process is called concatenation.

Example:

Input String = "Geeks" + "for" + "Geeks";
OutputString = "GeeksforGeeks" ;

There are numerous ways by which we can tell computers to do so which are called methods. Let us describe how a computer can perform an action via our methods in different ways.

This action can take place via 4 methods :

Using '+' operator
Using concat() inbuilt method
Using StringBuilder
Using StringBuffer

Lets us describe and implement them one by one.

Method 1: String Concatenation using '+' Operator

Java

// Java program to concatenate string
import java.lang.*;

class GFG {

    public static void main(String[] args)
    {
        String str = "";

        // timer-start time
        long startTime = System.currentTimeMillis();

        for (int i = 0; i < 100000; i++) {
            // string concatenation
            str += Integer.toString(0);
        }

        // timer-end time
        long endTime = System.currentTimeMillis();

        // display the result
        System.out.println(
            "Time taken to concatenate 100000 Strings using '+' operator : "
            + (endTime - startTime) + " ms");
    }
}

Output

Time taken to concatenate 100000 Strings using '+' operator : 2126 ms

Method 2: using concat() Inbuilt function

Concat(String str) method concatenates the specified String to the end of this string. This method appends the specified string at the end of the given string and returns the combined string.

Example :

String str="GeeksforGeeks";
s1 = s1.concat(".").concat("com");

Java

// Java program to concatenate string
import java.lang.*;

class GFG {
    public static void main(String[] args)
    {
        String str = "";
        long startTime = System.currentTimeMillis();

        for (int i = 0; i < 100000; i++) {
            str.concat(Integer.toString(0));
        }

        long endTime = System.currentTimeMillis();
        System.out.println(
            "Time taken to concatenate 100000 Strings using concat() method : "
            + (endTime - startTime) + " ms");
    }
}

Output

Time taken to concatenate 100000 Strings using concat() method : 46 ms

Method 3: By using StringBuilder (Best Way)

StringBuilder represents a mutable sequence of characters. Since the String Class in Java creates an immutable sequence of characters, the StringBuilder class provides an alternative to String Class, as it creates a mutable sequence of characters.

Example :

StringBuilder str  = new StringBuilder();  
str.append("GFG");

Time Complexity for concatenation using StringBuilder method:

Java

// Java program to concatenate string
import java.lang.*;

class GFG {
    public static void main(String[] args)
    {
        StringBuilder str = new StringBuilder();
        long startTime = System.currentTimeMillis();

        for (int i = 0; i < 100000; i++) {
            str.append(0);
        }
        long endTime = System.currentTimeMillis();
        System.out.println(
            "Time taken to concatenate 100000 Strings using StringBuilder append : "
            + (endTime - startTime) + " ms");
    }
}

Output

Time taken to concatenate 100000 Strings using StringBuilder append : 34 ms

Method 4: By using StringBuffer

StringBuffer is a peer class of String that provides much of the functionality of strings. The string represents fixed-length, immutable character sequences while StringBuffer represents growable and writable character sequences.

Example :

StringBuffer s = new StringBuffer("GeeksforGeeks");