Just set the path variable to JDK bin in environment variables.
Variable Name : PATH
Variable Value : C:\Program Files\Java\jdk1.8.0_31\bin
But the best practice is to set JAVA_HOME and PATH as follow.
Variable Name : JAVA_HOME
Variable Value : C:\Program Files\Java\jdk1.8.0_31
Variable Name : PATH
Variable Value : %JAVA_HOME%\bin