How do I find where JDK is installed on my windows machine?

Command line:

Run where java on Command Prompt.

GUI:

On Windows 10 you can find out the path by going to Control Panel > Programs > Java. In the panel that shows up, you can find the path as demonstrated in the screenshot below. In the Java Control Panel, go to the 'Java' tab and then click the 'View' button under the description 'View and manage Java Runtime versions and settings for Java applications and applets.'

This should work on Windows 7 and possibly other recent versions of Windows.

In Windows at the command prompt

where javac

If you are using Linux/Unix/Mac OS X:

Try this:

$ which java

Should output the exact location.

After that, you can set JAVA_HOME environment variable yourself.

In my computer (Mac OS X - Snow Leopard):

$ which java
/usr/bin/java
$ ls -l /usr/bin/java
lrwxr-xr-x  1 root  wheel  74 Nov  7 07:59 /usr/bin/java -> /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java

If you are using Windows:

c:\> for %i in (java.exe) do @echo.   %~$PATH:i

Windows > Start > cmd >

C:> for %i in (javac.exe) do @echo.   %~$PATH:i

If you have a JDK installed, the Path is displayed,
for example: C:\Program Files\Java\jdk1.6.0_30\bin\javac.exe