Theprogrammersfirst: Java 13

2020-02-20

Java 13 - Text Blocks

A text block is a multi-line string literal that avoids the need for most escape sequences, automatically formats the string in a predictable way, and gives the developer control over format when desired.

In Java, embedding a snippet of HTML, XML, SQL, or JSON in a string literal "..." usually requires significant editing with escapes and concatenation before the code containing the snippet will compile. The snippet is often difficult to read and arduous to maintain.

HTML example

Using "one-dimensional" string literals

String html = "<html>\n" +
" <body>\n" +
" Hello, world\n" +
" </body>\n" +
"</html>\n";
Using a "two-dimensional" block of text

String html = """
<html>
<body>
Hello, world
</body>
</html>
""";

SQL example

Using "one-dimensional" string literals

String query = "SELECT `EMP_ID`, `LAST_NAME` FROM `EMPLOYEE_TB`\n" +
"WHERE `CITY` = 'INDIANAPOLIS'\n" +
"ORDER BY `EMP_ID`, `LAST_NAME`;\n";
Using a "two-dimensional" block of text

String query = """
SELECT `EMP_ID`, `LAST_NAME` FROM `EMPLOYEE_TB`
WHERE `CITY` = 'INDIANAPOLIS'
ORDER BY `EMP_ID`, `LAST_NAME`;
""";

Polyglot language example

Using "one-dimensional" string literals

ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");
Object obj = engine.eval("function hello() {\n" +
" print('\"Hello, world\"');\n" +
"}\n" +
"\n" +
"hello();\n");
Using a "two-dimensional" block of text

ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");
Object obj = engine.eval("""
function hello() {
print('"Hello, world"');
}

hello();
""");

A text block is a new kind of literal in the Java language. It may be used to denote a string anywhere that a string literal could appear, but offers greater expressiveness and less accidental complexity.

A text block consists of zero or more content characters, enclosed by opening and closing delimiters.

Compile-time processing

The content of a text block is processed by the Java compiler in three distinct steps:

Line terminators in the content are translated to LF (\u000A). The purpose of this translation is to follow the principle of least surprise when moving Java source code across platforms.
Incidental white space surrounding the content, introduced to match the indentation of Java source code, is removed.
Escape sequences in the content are interpreted. Performing interpretation as the final step means developers can write escape sequences such as \n without them being modified or deleted by earlier steps.

1. Line terminators

Line terminators in the content are normalized from CR (\u000D) and CRLF (\u000D\u000A) to LF (\u000A) by the Java compiler.

2. Incidental white space

The text blocks easier to read than their concatenated string literal counterparts.
Here is the HTML example using dots to visualize the spaces that the developer added for indentation:

String html = """
..............<html>
.............. <body>
.............. Hello, world
.............. </body>
..............</html>
..............""";

3. Escape sequences

A text block supports the same escape sequences as a string literal, such as \n, \t, \', \", and \\. Developers will have access to escape processing via String::translateEscapes, a new instance method.

String html = """
<html>\r
<body>\r
Hello, world\r
</body>\r
</html>\r
""";
Using Unicode escapes to visualize LF (\u000A) and CR (\u000D), the result is:

|<html>\u000D\u000A
| <body>\u000D\u000A
| Hello, world\u000D\u000A
| </body>\u000D\u000A
|</html>\u000D\u000A

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