Java 8 – Convert a Collection to a String

Introduction

Converting a collection to a string is a common task in Java, especially when you need to display the contents of a collection, log data, or create a formatted output. Java 8 introduced the Stream API, which provides a powerful and flexible way to convert collections to strings, offering various options for customization, such as specifying delimiters, prefixes, and suffixes. In this guide, we’ll explore how to convert different types of collections to strings using Java 8, covering common scenarios like lists, sets, and custom objects.

Table of Contents

• Problem Statement
• Solution Steps
• Java Program
  • Converting a List to a String
  • Converting a Set to a String
  • Converting a Collection of Custom Objects to a String
• Advanced Considerations
• Conclusion

Problem Statement

The task is to create a Java program that:

• Accepts a collection of elements.
• Converts the collection to a formatted string.
• Outputs the resulting string.

Example 1:

• Input: List of strings ["apple", "banana", "orange"]
• Output: "apple, banana, orange"

Example 2:

• Input: Set of integers [1, 2, 3, 4]
• Output: "1, 2, 3, 4"

Solution Steps

1. Input Collection: Start with a collection of elements that can either be hardcoded or provided by the user.
2. Convert Collection to String Using Streams: Use the Stream API and Collectors.joining() to convert the collection to a string.
3. Display the Result: Print the resulting string.

Java Program

Converting a List to a String

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Java 8 - Convert a List to a String Using Streams
 * Author: https://www.rameshfadatare.com/
 */
public class ConvertListToString {

    public static void main(String[] args) {
        // Step 1: Take input list
        List<String> fruits = Arrays.asList("apple", "banana", "orange");

        // Step 2: Convert the list to a string
        String result = fruits.stream()
                              .collect(Collectors.joining(", "));

        // Step 3: Display the result
        System.out.println("Converted string: " + result);
    }
}

Output

Converted string: apple, banana, orange

Explanation

• The stream() method is used to create a stream from the list of strings.
• The Collectors.joining(", ") method joins the elements into a single string, separated by a comma and a space.
• The resulting string "apple, banana, orange" is printed to the console.

Converting a Set to a String

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;

/**
 * Java 8 - Convert a Set to a String Using Streams
 * Author: https://www.rameshfadatare.com/
 */
public class ConvertSetToString {

    public static void main(String[] args) {
        // Step 1: Take input set
        Set<Integer> numbers = new HashSet<>(Arrays.asList(1, 2, 3, 4));

        // Step 2: Convert the set to a string
        String result = numbers.stream()
                               .map(String::valueOf)
                               .collect(Collectors.joining(", "));

        // Step 3: Display the result
        System.out.println("Converted string: " + result);
    }
}

Output

Converted string: 1, 2, 3, 4

Explanation

• The stream() method is used to create a stream from the set of integers.
• The map(String::valueOf) method converts each integer to a string.
• The Collectors.joining(", ") method joins the elements into a single string, separated by a comma and a space.
• The resulting string "1, 2, 3, 4" is printed to the console.

Converting a Collection of Custom Objects to a String

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

/**
 * Java 8 - Convert a Collection of Custom Objects to a String
 * Author: https://www.rameshfadatare.com/
 */
public class ConvertCustomObjectsToString {

    public static void main(String[] args) {
        // Step 1: Take input list of custom objects
        List<Student> students = Arrays.asList(
                new Student("Raj", 25),
                new Student("Anita", 30),
                new Student("Vikram", 22)
        );

        // Step 2: Convert the collection to a string
        String result = students.stream()
                                .map(student -> student.getName() + ": " + student.getAge())
                                .collect(Collectors.joining(", "));

        // Step 3: Display the result
        System.out.println("Converted string: " + result);
    }
}

// Custom class Student
class Student {
    private String name;
    private int age;

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

Output

Converted string: Raj: 25, Anita: 30, Vikram: 22

Explanation

• The stream() method is used to create a stream from the list of Student objects.
• The map() method formats each Student object into a string combining the name and age.
• The Collectors.joining(", ") method joins the formatted strings into a single string, separated by a comma and a space.
• The resulting string "Raj: 25, Anita: 30, Vikram: 22" is printed to the console.

Advanced Considerations

• Custom Delimiters, Prefixes, and Suffixes: The Collectors.joining() method allows you to specify custom delimiters, prefixes, and suffixes. For example:

  String result = fruits.stream()
                        .collect(Collectors.joining(", ", "[", "]"));
  

  This would result in the output "[apple, banana, orange]".

• Null Handling: If your collection may contain null values, consider filtering them out before joining:

  String result = list.stream()
                      .filter(Objects::nonNull)
                      .collect(Collectors.joining(", "));
  

• Sorting Elements: You can also sort the elements before converting them to a string:

  String result = list.stream()
                      .sorted()
                      .collect(Collectors.joining(", "));
  

Conclusion

This guide provides methods for converting a collection to a string using Java 8 Streams, covering both simple collections like lists and sets, as well as more complex collections like custom objects. Java 8 Streams offer a powerful and flexible way to format and join elements into strings, making your code more readable and maintainable. Depending on your specific use case, you can easily apply the techniques demonstrated in this guide to convert any collection into a string.