Java 9 was released and JShell (i.e. The REPL of Java) is now real and we can use this to test an API.

For example if we want to check the reverse function StringUtils from the commons-lang3-3.1.jar.
1.) Using windows terminal (i.e. cmd command).
2.) From this directory execute to following command jshell command.
jshell --class-path .\commons-lang3-3.1.jar
Expect to see something similar to the following:
Welcome to JShell -- Version 9.0.1
For an introduction type: /help intro

3.) From the jshell prompt import the class org.apache.commons.lang3.StringUtils just like we normally do in java and then press enter.
jshell> import org.apache.commons.lang3.StringUtils
4.) To optionally check all the imported classes on the current jshell session we can use /imports command.
jshell> /imports
The sample output would be:
|    import*
|    import java.math.*
|    import*
|    import java.nio.file.*
|    import java.util.*
|    import java.util.concurrent.*
|    import java.util.function.*
|    import java.util.prefs.*
|    import java.util.regex.*
|    import*
|    import org.apache.commons.lang3.StringUtils
5.) Since we know that the StringUtils class was already loaded. We can now execute its reverse function.
jshell> StringUtils.reverse("abcde")
The output must be:
$2 ==> "edcba"
Note: if you want to exit jshell execute the /exit command.