What is the syntax for HEREDOC string in Java? [duplicate]
Java SE 13 introduced Text Blocks (or Multiline Strings) feature. What are its differences and the similarities with the existing string representation?
What is a text block?
A text block is a multi-line string literal and the feature offers a clean way to format the string in a predictable way, without using most of the escape sequences. It starts and ends with a """ (three double-quotes marks) e.g.
public class Main {
public static void main(String[] args) {
String text = """
<html>
<head>
<title>Hello World</title>
</head>
<body>
Java rocks!
</body>
</html>""";
System.out.println(text);
}
}
Output:
<html>
<head>
<title>Hello World</title>
</head>
<body>
Java rocks!
</body>
</html>
With the traditional string representation, the code would look like
public class Main {
public static void main(String[] args) {
String text = "<html>\n"
+ " <head>\n"
+ " <title>Hello World</title>\n"
+ " </head>\n"
+ " <body>\n"
+ " Java rocks!\n"
+ " </body>\n"
+ "</html>";
System.out.println(text);
}
}
Another key difference is that a text block begins with three double-quote characters followed by a line terminator which is not the case with the traditional string representation. It means
-
The text block can not be put on a single line.
-
The contents of the text block can not follow the three opening double-quotes on the same line.
String str = "Hello World!"; // The traditional string String textBlock = """ Hello World! """; // The text block String notAllowed = """Hello World!"""; // Error // The following code will fail compilation String notAllowed2 ="""Hello World! """;
A note about indentation:
The compiler shifts the complete text block to the left and then retains the specified spacing.
String str1 = """
Hello""";
^^^<-----These three whitespace characters will be retained
Demo:
public class Main {
public static void main(String[] args) {
// Text block without a line break at the end
String str1 = """
Hello""";
// Text block with a line break at the end
String str2 = """
Hello
""";
// Text block with three whitespace in the beginning and a line break at the end
String str3 = """
World!
""";
System.out.println(str1);
System.out.println(str2);
System.out.println(str3);
System.out.println("Java rocks!");
}
}
Output:
Hello
Hello
World!
Java rocks!
Is it available only as a Preview Feature?
It remained available in Java SE 13 and Java SE 14 as a Preview Feature and has been standardised with Java SE 15. With Java SE 13 and 14, like any Preview Feature, it has to be compiled and executed with --enable-preview option e.g.
To compile:
javac --enable-preview --release 13 Main.java
To execute:
java --enable-preview Main
Are they stored in string pool?
Yes, they are. The text blocks are compiled to the same type as that of the traditional String value i.e. the byte code does not distinguish between a traditional String value and text block.
public class Main {
public static void main(String[] args) {
String str1 = "Hello World!";
String str2 = """
Hello World!""";
System.out.println(str1 == str2);
}
}
Output:
true
Can a text block be concatenated with another string?
Yes, a text block can be concatenated to a traditional string value or another text block in the same way, the traditional String values are concatenated. As already described above, the byte code does not distinguish between a traditional String value and text block.
public class Main {
public static void main(String[] args) {
String str1 = "Hello ";
String str2 = """
World!""";
String str3 = """
Java rocks!
""";
System.out.println(str1 + str2);
System.out.println(str1 + (str2 + str3));
}
}
Output:
Hello World!
Hello World! Java rocks!
Note that I have put whitespace after Hello in str1 and another whitespace before Java rocks! in str3.
Does it support Escape Sequences?
Yes, the escape sequences will be evaluated in the traditional way e.g.
public class Main {
public static void main(String[] args) {
String text = """
<html>
<head>
<title>Hello World</title>
</head>
<body>
Java\n\t\trocks!
</body>
</html>""";
System.out.println(text);
}
}
Output:
<html>
<head>
<title>Hello World</title>
</head>
<body>
Java
rocks!
</body>
</html>
Does it support replaceable parameter?
Yes, you can replace a parameter in the text block using %s or $<<replaceable-parameter>> as shown below:
public class Main {
public static void main(String[] args) {
String text = """
What is the distance between %s and %s?""";
System.out.println(String.format(text, "earth", "moon"));
System.out.println(String.format(text, "earth", "mercury"));
// Alternative-1
text = """
What is the distance between $celestial1 and $celestial2?""";
System.out.println(text.replace("$celestial1", "earth").replace("$celestial2", "moon"));
// Alternative-2 using the deprecated String#formatted
text = """
What is the distance between %s and %s?""";
System.out.println(text.formatted("earth", "moon"));
}
}
Output:
What is the distance between earth and moon?
What is the distance between earth and mercury?
What is the distance between earth and moon?
What is the distance between earth and moon?