Write snippets of code in your documentation and execute them as regression tests.

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Run the Java examples calling byexample as:

$ byexample -l java your-file-here                # byexample: +skip

You need the have installed jshell, an interactive interpreter for Java. It is installed by default as part of the openjdk.

Stability: experimental - non backward compatibility changes are possible or even removal between versions (even patch versions).

New in byexample 10.3.0.

Versions tested

We tested byexample with the following versions of the language and the underlying runner or interpreter:

Language Runner/Interpreter
11 11.0.15
13 13.0.11
15 15.0.7

Find interactive examples

For Java, byexample uses the j> string as the primary prompt and .. as the secondary prompt.

j> int a = 1
j> int b = 2
j> a + b
=> 3

j> int g(int a, int b, int c) {
..   c += a;
..   c += b;
..   return c;
.. }

j> g(1, 2, 3)
=> 6

Not necessary but it is sometimes convenient to use +norm-ws when printing nested data structures so the output can be arranged into multiple lines:

j> String[][] names = {             // byexample: +norm-ws
..             {"Alice", "Bob", "Charlie"},
..             {"Eve", "Mallory"}
..         };
=> String[2][] {
        String[3] { "Alice", "Bob", "Charlie" },
        String[2] { "Eve", "Mallory" }

The object returned

Because everything in jshell, the interpreter of Java, is evaluated as an expression, everything returns something.

This is annoying if you want to write several Java lines without checking the results.

For this reason, byexample suppress the representation of the object returned unless the example has a =>.

In the following case, the first example is executed but the value of its expression evaluated is not checked while in the second example it is checked.

j> 1 + 2

j> 1 + 2
=> 3

Notice how the second example has a => to mark the value of the expression to be compared. This mark is used by byexample to know when or when not suppress the value.

This affects only the value of the expression, it has no effect on the output of the example in general.

For example the prints are not suppressed:

j> System.out.println("hello")

You can change the behavior of byexample with:

  • +java-expr-print=true to print always the expression, disabling the suppression
  • +java-expr-print=false to never print the expression.
  • +java-expr-print=auto to let byexample decide when to suppress or not based on the mark =>. This is the default.

This and any other flag/option can be set in the example’s comment.

j> System.out.println("hello      world")       // byexample: +norm-ws
hello world

Currently the flags/options can only be set in the single-line comments (//); block comments are not supported (/* .. */).

Class-path and module-path

byexample allows you to change the class path and module path from the command line with +java-class-path and +java-module-path.

Modules and exports can be added with +java-add-modules and +java-add-exports.

$ byexample -l java -o '+java-class-path=test/ds/' somefiles        # byexample: +skip

Run byexample -l java --show-options for more information about those options and their syntax. Refer to the official Java and jshell documentation to know what they do exactly.

Known limitations

Type text

The type feature (+type) is not supported.

Terminal support

To work with the current Java interpreter, jshell, the ANSI terminal emulator is enabled by default (+term=ansi) and cannot be disabled.

Also, the terminal geometry cannot by changed after launching the interpreter so the option +geometry cannot be used in an example (but it can be used from the command line)

The amount of rows of the terminal has a minimum value of 128 and this limit is really important: if your outputs have more than 128 lines you will need to increase the geometry or the results may be undefined.

The same for the width of the terminal: minimum of 128 columns.

Java specific options

$ byexample -l java --show-options       # byexample: +norm-ws -capture
java's specific options
  +java-expr-print {auto,true,false}
                        print the expression's value (true); suppress it
                        (false); or print it only if the example has a =>
                        (auto, the default)
  +java-class-path <path>
                        List of directories, JAR archives, and ZIP archives to
                        search for class files separated by a colon (:). On
                        Windows use a semicolon (;).
  +java-module-path <path>
                        List of directories, JAR archives, and ZIP archives to
                        search for modules separated by a colon (:). On
                        Windows use a semicolon (;).
  +java-add-modules <name>[,<name>...]
                        Root modules to resolve in addition to the initial
                        module. <name> can also be ALL-DEFAULT, ALL-SYSTEM,
  +java-add-exports <module>/<package>=<target>[,<target>...]
                        Updates <module> to export <package> to <target-
                        module>, regardless of module declaration. <target-
                        module> can be ALL-UNNAMED to export to all unnamed
                        modules. In jshell, if the <target-module> is not
                        specified then ALL-UNNAMED is used.