"java" is a command in the JRE.

Most folks use all lower-case letters for JAR names.  This is not so much a
matter of convention as of laziness, but it is widespread.  However, the word
"I" in English should always be spelled with an initial upper-case letter.

Try using the right method of the Runtime class.  The line you show will not
compile, because getRuntime() does not take an argument.  Have you read the
Javadocs?
<http://java.sun.com/javase/6/docs/api/java/lang/Runtime.html#getRuntime()>

Read the Javadocs on the various Runtime exec() methods.  And next time,
prepare an SSCCE (Simple Self-Contained Correct (compilable) Example) that
evinces your difficulty.  The act of preparing it and reviewing its error
message(s) will likely show you the trouble and its solution on your own.

<http://www.physci.org/codes/sscce.html>

If you had done that, you'd've seen right away that your version of
getRuntime() won't compile, and the error message would've told you why.

Try this line.
Runtime.getRuntime().exec("java -jar DoIt.jar")
No JDK needed

Your conclusion is wrong. The "java" command *is* part of the JRE.

For example on my Windows, the JRE 1.5.0_10 is installed in
"C:\Programme\Java\jre1.5.0_10\". The java command of this JRE is
"C:\Programme\Java\jre1.5.0_10\bin\java.exe".
There is no need for any JDK to be installed to run your java
application. On the other hand: a JRE doesn't contain any development
tools (like "javac.exe", "jar.exe", ...).

However, if you have a JDK installed, you'll have a second JRE inside
that. For example on my Windows, the JDK 1.5.0_10 is installed in
"C:\Program Files\Java\jre1.5.0_10\". It contains a complete JRE in
"C:\Program Files\Java\jre1.5.0_10\jre\". Especially, there is a java
command in it: "C:\Program Files\Java\jre1.5.0_10\jre\bin\java.exe".

Yes.