Java multiline string

2019-01-01 01:42发布

站内问答 / Java
645 30 5

Coming from Perl, I sure am missing the "here-document" means of creating a multi-line string in source code:

$string = <<"EOF"  # create a three-line string
text
text
text
EOF

In Java, I have to have cumbersome quotes and plus signs on every line as I concatenate my multiline string from scratch.

What are some better alternatives? Define my string in a properties file?

Edit: Two answers say StringBuilder.append() is preferable to the plus notation. Could anyone elaborate as to why they think so? It doesn't look more preferable to me at all. I'm looking for a way around the fact that multiline strings are not a first-class language construct, which means I definitely don't want to replace a first-class language construct (string concatenation with plus) with method calls.

Edit: To clarify my question further, I'm not concerned about performance at all. I'm concerned about maintainability and design issues.

30条回答
笑指拈花
2楼-- · 2019-01-01 02:00

This is an old thread, but a new quite elegant solution (with only 4 little drawbacks) is to use a custom annotation.

Check : http://www.adrianwalker.org/2011/12/java-multiline-string.html

A project inspired from that work is hosted on GitHub:

https://github.com/benelog/multiline

Example of Java code:

import org.adrianwalker.multilinestring.Multiline;
...
public final class MultilineStringUsage {

  /**
  <html>
    <head/>
    <body>
      <p>
        Hello<br/>
        Multiline<br/>
        World<br/>
      </p>
    </body>
  </html>
  */
  @Multiline
  private static String html;

  public static void main(final String[] args) {
    System.out.println(html);
  }
}

The drawbacks are

  1. that you have to activate the corresponding (provided) annotation processor.
  2. that String variable can not be defined as local variable.
  3. that String cannot contains other variables as in Visual Basic .Net with XML literal (<%= variable %>) :-)
  4. that String literal is delimited by JavaDoc comment (/**)

And you probably have to configure Eclipse/Intellij-Idea to not reformat automatically your Javadoc comments.

One may find this weird (Javadoc comments are not designed to embed anything other than comments), but as this lack of multiline string in Java is really annoying in the end, I find this to be the least worst solution.

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低头抚发
3楼-- · 2019-01-01 02:01

In the IntelliJ IDE you just need to type:

""

Then position your cursor inside the quotation marks and paste your string. The IDE will expand it into multiple concatenated lines.

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无色无味的生活
4楼-- · 2019-01-01 02:01

Use Properties.loadFromXML(InputStream). There's no need for external libs.

Better than a messy code (since maintainability and design are your concern), it is preferable not to use long strings.

Start by reading xml properties:

 InputStream fileIS = YourClass.class.getResourceAsStream("MultiLine.xml");
 Properties prop = new Properies();
 prop.loadFromXML(fileIS);


then you can use your multiline string in a more maintainable way...

static final String UNIQUE_MEANINGFUL_KEY = "Super Duper UNIQUE Key";
prop.getProperty(UNIQUE_MEANINGFUL_KEY) // "\n    MEGA\n   LONG\n..."


MultiLine.xml` gets located in the same folder YourClass:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">

<properties>
    <entry key="Super Duper UNIQUE Key">
       MEGA
       LONG
       MULTILINE
    </entry>
</properties>

PS.: You can use <![CDATA[" ... "]]> for xml-like string.

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一个人的天荒地老
5楼-- · 2019-01-01 02:03

String.join

Java 8 added a new static method to java.lang.String which offers a slightly better alternative:

String.join( CharSequence delimiter , CharSequence... elements )

Using it:

String s = String.join(
    System.getProperty("line.separator"),
    "First line.",
    "Second line.",
    "The rest.",
    "And the last!"
);
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一个人的天荒地老
6楼-- · 2019-01-01 02:05

It sounds like you want to do a multiline literal, which does not exist in Java.

Your best alternative is going to be strings that are just +'d together. Some other options people have mentioned (StringBuilder, String.format, String.join) would only be preferable if you started with an array of strings.

Consider this:

String s = "It was the best of times, it was the worst of times,\n"
         + "it was the age of wisdom, it was the age of foolishness,\n"
         + "it was the epoch of belief, it was the epoch of incredulity,\n"
         + "it was the season of Light, it was the season of Darkness,\n"
         + "it was the spring of hope, it was the winter of despair,\n"
         + "we had everything before us, we had nothing before us";

Versus StringBuilder:

String s = new StringBuilder()
           .append("It was the best of times, it was the worst of times,\n")
           .append("it was the age of wisdom, it was the age of foolishness,\n")
           .append("it was the epoch of belief, it was the epoch of incredulity,\n")
           .append("it was the season of Light, it was the season of Darkness,\n")
           .append("it was the spring of hope, it was the winter of despair,\n")
           .append("we had everything before us, we had nothing before us")
           .toString();

Versus String.format():

String s = String.format("%s\n%s\n%s\n%s\n%s\n%s"
         , "It was the best of times, it was the worst of times,"
         , "it was the age of wisdom, it was the age of foolishness,"
         , "it was the epoch of belief, it was the epoch of incredulity,"
         , "it was the season of Light, it was the season of Darkness,"
         , "it was the spring of hope, it was the winter of despair,"
         , "we had everything before us, we had nothing before us"
);

Versus Java8 String.join():

String s = String.join("\n"
         , "It was the best of times, it was the worst of times,"
         , "it was the age of wisdom, it was the age of foolishness,"
         , "it was the epoch of belief, it was the epoch of incredulity,"
         , "it was the season of Light, it was the season of Darkness,"
         , "it was the spring of hope, it was the winter of despair,"
         , "we had everything before us, we had nothing before us"
);

If you want the newline for your particular system, you either need to use System.getProperty("line.separator"), or you can use %n in String.format.

Another option is to put the resource in a text file, and just read the contents of that file. This would be preferable for very large strings to avoid unnecessarily bloating your class files.

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几人难应
7楼-- · 2019-01-01 02:05

JEP 326: Raw String Literals will implement multi-line Strings, so you will be able to write something like:

String s = `
    text
    text
    text
  `;

It is planned as a feature preview in JDK 12.

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