I want to make a battery meter for my computers, which all work on different OSs. I figured the best way would be in Java. Is there any way to do this WITHOUT external libraries? All it needs to do is show the battery level in a java.swing.JProgressBar.
No, there is no way to do this just using the standard JDK. It is up to the OS to expose details about the current power/battery state, and there is no built-in way to access this information in Java. To do what you want will require at least some external libraries, and almost certainly JNI as well.