To convert the ArrayList to a comma-separated string, these are the methods available in Java.

1. Use StringBuilder's append() method
2. Use toString() method
3. Use Apache Commons StringUtils class
4. Use streaming API
5. Use the join() method of the String class

Method 1(Using append() Method of StringBuilder): 

StringBuilder in java represents a mutable sequence of characters. within the below example, we used StringBuilder’s append() method. The append method is employed to concatenate or add a replacement set of characters within the last position of the prevailing string.

Syntax :

public StringBuilder append(char a)

Parameter: 

// Java program to Convert ArrayList to Comma Separated
// String

import java.util.ArrayList;

public class Main {

	public static void main(String[] args)
	{

		// Create an ArrayList
		ArrayList geeklist
			= new ArrayList();

		// Add elements to ArrayList
		geeklist.add("Hey");
		geeklist.add("Geek");
		geeklist.add("Welcome");
		geeklist.add("to");
		geeklist.add("geeksforgeeks");
		geeklist.add("!");

		StringBuilder str = new StringBuilder("");

		// Traversing the ArrayList
		for (String eachstring : geeklist) {

			// Each element in ArrayList is appended
			// followed by comma
			str.append(eachstring).append(",");
		}

		// StringBuffer to String conversion
		String commaseparatedlist = str.toString();

		// By following condition you can remove the last
		// comma
		if (commaseparatedlist.length() > 0)
			commaseparatedlist
				= commaseparatedlist.substring(
					0, commaseparatedlist.length() - 1);

		System.out.println(commaseparatedlist);
	}
}

Output

Hey,Geek,Welcome,to,geeksforgeeks,!

Here, arraylist is an object of the ArrayList class.

Return Value: It returns a string representation of the arraylist

// Java program to Convert ArrayList to Comma Separated
// String in Java

import java.util.ArrayList;

public class Main {

	public static void main(String[] args)
	{

		// Create an ArrayList
		ArrayList geekcourses
			= new ArrayList();

		// Add elements to ArrayList
		geekcourses.add("Data Structures");
		geekcourses.add("Algorithms");
		geekcourses.add("Operating System");
		geekcourses.add("Computer Networks");
		geekcourses.add("Machine Learning");
		geekcourses.add("Databases");

		/* toString method returns the output as [Data
		Structure,Algorithms,...] In order to replace
		'[', ']' and spaces with empty strings to get
		comma separated values.*/

		String commaseparatedlist = geekcourses.toString();

		commaseparatedlist
			= commaseparatedlist.replace("[", "")
				.replace("]", "")
				.replace(" ", "");

		System.out.println(commaseparatedlist);
	}
}

Output

DataStructures,Algorithms,OperatingSystem,ComputerNetworks,MachineLearning,Databases

Method 3(Using Apache Commons StringUtils class):

// Java program to Convert ArrayList to Comma Separated
// String in Java

import java.util.ArrayList;
import org.apache.commons.collections4.CollectionUtils;

public class Main {

	public static void main(String[] args)
	{

		// Create an ArrayList
		ArrayList geekcourses
			= new ArrayList();

		// Add elements to ArrayList
		geekcourses.add("Data Structures");
		geekcourses.add("Algorithms");
		geekcourses.add("Operating System");
		geekcourses.add("Computer Networks");
		geekcourses.add("Machine Learning");
		geekcourses.add("Databases");

		/*join method of StringUtils class is used
		which returns a single string along with defined
		separator in every iteration.*/

		String commalist
			= StringUtils.join(geekcourses, ",");
		System.out.println(commalist);
	}
}

Output

OutputDataStructures,Algorithms,OperatingSystem,ComputerNetworks,MachineLearning,Databases

Method 4(Using Stream API):

Stream API was introduced in Java 8 and is employed to process collections of objects. The joining() method of Collectors Class, in Java, is employed to hitch various elements of a personality or string array into one string object.

// Java program to Convert ArrayList to Comma Separated
// String in Java

import java.util.*;
import java.util.stream.Collectors;

public class Main {

	public static void main(String[] args)
	{

		// Create ArrayList of String
		ArrayList geeklist
			= new ArrayList();

		// add elements to ArrayList
		geeklist.add("welcome");
		geeklist.add("to");
		geeklist.add("geeks");
		geeklist.add("for");
		geeklist.add("geeks");

		// The collect method is used to return the result
		// of the intermediate operations performed on the
		// stream.
		String str = geeklist.stream().collect(
			Collectors.joining(","));
		System.out.println(str);
	}
}

Output

welcome,to,geeks,for,geeks

Method 5(Using String join() method: 

We can use StringJoiner to convert the ArrayList into a comma-separated string. StringJoiner is a class in the java.util package, used to construct a sequence of characters (strings) separated by separators, and can choose to start with the provided prefix and start with The suffix provided ends. The join() method of the String class can be used to construct the same content.

// Java program to Convert ArrayList to Comma Separated
// String in Java

import java.util.*;
import java.util.stream.Collectors;

public class Main {

	public static void main(String[] args)
	{

		// Create ArrayList of String
		ArrayList geeklist
			= new ArrayList();

		// add elements to ArrayList
		geeklist.add("welcome");
		geeklist.add("to");
		geeklist.add("geeks");
		geeklist.add("for");
		geeklist.add("geeks");

		// String.join() is used with a delimiter comma
		// along with the list.
		String str = String.join(",", geeklist);
		System.out.println(str);
	}
}

Output

welcome,to,geeks,for,geeks