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1. Introduction

In this quick tutorial, we'll explain how to convert a List of elements to a String. This can be useful in certain scenarios, like printing the contents to the console in a human-readable form for inspection/debugging.

2. Standard toString() on a List

One of the simplest ways is to call the toString() method on the List:

@Test
public void whenListToString_thenPrintDefault() {
    List<Integer> intLIst = Arrays.asList(1, 2, 3);
 
    System.out.println(intLIst);
}

Output:

[1, 2, 3]

This technique internally utilizes the toString() method of the type of elements within the List. In our case, we're using the Integer type, which has a proper implementation of the toString() method.

If we're using our custom type, such as Person, then we need to make sure that the Person class overrides the toString() method and doesn't rely on the default implementation. If we don't properly implement the toString() method, we might get unexpected results:

[[email protected],
  [email protected],
  [email protected]]

3. Custom Implementation Using Collectors

Often, we might need to display the output in a different format.

Compared to the previous example, let's replace the comma (,) with a hyphen (-), and the square brackets ([, ]) with a set of curly braces ({, }):

@Test
public void whenCollectorsJoining_thenPrintCustom() {
    List<Integer> intList = Arrays.asList(1, 2, 3);
    String result = intList.stream()
      .map(n -> String.valueOf(n))
      .collect(Collectors.joining("-", "{", "}"));
 
    System.out.println(result);
}

Output:

{1-2-3}

The Collectors.joining() method requires a CharSequence, so we need to map the Integer to String. We can utilize this same idea with other classes, even when we don't have access to the code of the class.

4. Using an External Library

Now we'll use Apache Commons' StringUtils class to achieve similar results.

4.1. Maven Dependency

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.11</version>
</dependency>

The latest version of the dependency can be found here.

4.2. Implementation

The implementation is literally a single method call:

@Test
public void whenStringUtilsJoin_thenPrintCustom() {
    List<Integer> intList = Arrays.asList(1, 2, 3);
 
    System.out.println(StringUtils.join(intList, "|"));
}

Output:

1|2|3

Again, this implementation is internally dependent on the toString() implementation of the type we're considering.

5. Conclusion

In this article, we learned how easy it is to convert a List to a String using different techniques.

As always, the full source code for this article can be found over on GitHub.

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