Hadoop installation on windows
Most of the answers suggest to copy the JDK installation path. However, if you are not comfortable doing it, you can use the Windows short path to set the path name so that all applications can access the path without any hassles.
Notion to set env variable if it contains white spaces:
Progra~1 = 'Program Files'Progra~2 = 'Program Files(x86)'
Just faced the same issue (Win 8.1 + Hadoop 2.7.0 [build from sources]).
The problem turned out to be (ol' good) space in path name where java is located (under C:\Program Files\ dir). What i did was this:
1) Copy JDK dir to C:\Java\jdk1.8.0_40
2) edit \etc\hadoop\hadoop-env.cmd and change: set JAVA_HOME=c:\Java\jdk1.8.0_40
3) run cmd and execute hadoop-env.cmd
4) now check 'hadoop version' whether it's still complaining (my wasn't)