When you try to start a Java application do you see the following message:
Unable to launch the application
There are three possible solutions - try each one in turn:
Solution 1 - Clear the Java Cache
The most common reason for 'Unable to launch the application' is that the files in the Java cache are corrupt or damaged in some way. You can clear the Java cache and when you run the application again it'll download a fresh copy of the application files. Follow these steps to clear the Java cache:
Search for "java"
Click Java (you'll see the Java Control Panel)
On the 'General' tab, under 'Temporary Internet Files' click Settings
Click Delete Files (button near the bottom of the window)
The above is explained in more detail here:
Solution 2 - Ensure 'Keep temporary files on my computer' is enabled
I've found recently (with Java 8 update 65 64-bit in particular) that on going to clear the Java cache the buttons were greyed out (disabled). In fact the first option 'Keep temporary files on my computer' is sometimes disabled! This cases the cache to be turned off, meaning that Java will not work.
Ensure that 'Keep temporary files on my computer' is enabled (ticked) - as shown above.
Normally this will be fine but to be sure it is a good idea to restart your computer at this point.
Solution 3 - Reinstall Java
If all else fails, reinstall Java.
IMPORTANT: When you reinstall Java, first make sure you uninstall all existing copies of Java that are on your computer - they'll show up in the Control Panel programs list. Restart your computer. Then install a fresh copy of Java.
A fresh copy of Java can be downloaded from:
Clear the Java cache