Java多行字符串

1. Overview

In this tutorial, we'll learn how to declare multiline strings in Java.

Now that Java 15 is out, we can use the new native feature called Text Blocks.

We'll also review other methods if we can't use this feature.

2. Text Blocks

We can use Text Blocks by declaring the string with """ (three double quote marks):

public String textBlocks() {
    return """
        Get busy living
        or
        get busy dying.
        --Stephen King""";
}

It is by far the most convenient way to declare a multiline string. Indeed, we don't have to deal with line separators or indentation spaces, as noted in our dedicated article.

This feature is available in Java 15, but also Java 13 and 14 if we enable the preview feature.

In the following sections, we'll review other methods that are suitable if we use a previous version of Java or if Text Blocks aren't applicable.

3. Getting the Line Separator

Each operating system can have its own way of defining and recognizing new lines.

In Java, it's very easy to get the operating system line separator:

String newLine = System.getProperty("line.separator");

We're going to use this newLine in the following sections to create multiline strings.

4. String Concatenation

String concatenation is an easy native method that can be used to create multiline strings:

public String stringConcatenation() {
    return "Get busy living"
            .concat(newLine)
            .concat("or")
            .concat(newLine)
            .concat("get busy dying.")
            .concat(newLine)
            .concat("--Stephen King");
}

Using the + operator is another way to achieve the same thing.

Java compilers translate concat() and the + operator in the same way:

public String stringConcatenation() {
    return "Get busy living"
            + newLine
            + "or"
            + newLine
            + "get busy dying."
            + newLine
            + "--Stephen King";
}

5. String Join

Java 8 introduced String#join, which takes a delimiter along with some strings as arguments.

It returns a final string having all input strings joined together with the delimiter:

public String stringJoin() {
    return String.join(newLine,
                       "Get busy living",
                       "or",
                       "get busy dying.",
                       "--Stephen King");
}

6. String Builder

StringBuilder is a helper class to build Strings. StringBuilder was introduced in Java 1.5 as a replacement for StringBuffer.

It's a good choice for building huge strings in a loop:

public String stringBuilder() {
    return new StringBuilder()
            .append("Get busy living")
            .append(newLine)
            .append("or")
            .append(newLine)
            .append("get busy dying.")
            .append(newLine)
            .append("--Stephen King")
            .toString();
}

7. String Writer

StringWriter is another method that we can utilize to create a multiline string. We don't need newLine here because we use PrintWriter.

The println function automatically adds new lines:

public String stringWriter() {
    StringWriter stringWriter = new StringWriter();
    PrintWriter printWriter = new PrintWriter(stringWriter);
    printWriter.println("Get busy living");
    printWriter.println("or");
    printWriter.println("get busy dying.");
    printWriter.println("--Stephen King");
    return stringWriter.toString();
}

8. Guava Joiner

Using an external library just for a simple task like this doesn't make much sense. However, if the project already uses the library for other purposes, we can utilize it.

For example, Google's Guava library is very popular.

Guava has a Joiner class that is able to build multiline strings:

public String guavaJoiner() {
    return Joiner.on(newLine).join(ImmutableList.of("Get busy living",
        "or",
        "get busy dying.",
        "--Stephen King"));
}

9. Loading From a File

Java reads files exactly as they are. This means that if we have a multiline string in a text file, we'll have the same string when we read the file. There are a lot of ways to read from a file in Java.

It's actually a good practice to separate long strings from code:

public String loadFromFile() throws IOException {
    return new String(Files.readAllBytes(Paths.get("src/main/resources/stephenking.txt")));
}

10. Using IDE Features

Many modern IDEs support multiline copy/paste. Eclipse and IntelliJ IDEA are examples of such IDEs. We can simply copy our multiline string and paste it inside two double quotes in these IDEs.

Obviously, this method doesn't work for string creation in runtime, but it's a quick and easy way to get a multiline string.

11. Conclusion

In this article, we learned several methods to build multiline strings in Java.

The good news is that Java 15 has native support for multiline strings via Text Blocks.

All the other methods reviewed can be used in Java 15 or any previous version.

The code for all the methods in this article is available over on GitHub.