So I came across a method like this in some source
public native void method();
There was code inside, but it was commented out,
/*MANUAL code code code */
The code looked a bit like C++. So what does this mean? I know it does something because eventually it was called in the class...
That's a declaration for a native method, indicated by the native modifier, as explained in the Java Language Specification, section §8.4.3.4 native Methods .
You won't find its implementing code in Java, it's referring to code that's either in the C libraries that are part of the implementation of the JDK in use, or part of a native library bundled with the code you're studying. Refer to the JNI documentation to understand how this gets called from a Java program.
The native keyword in Java signifies the method is written in another language. See this exact question: What is the native keyword in Java for?
the keyword native states that this method's implementation comes from a native language like c/cpp etc via JNI
A common example: a couple of methods in java.lang.Object class are implemented in native language.
The native keyword is applied to a method to indicate that the method is implemented in native code using JNI. It marks a method, that it will be implemented in other languages, not in Java in other word, native is implemented in platform-dependent code, typically written in another programming language.
You can find more example from here.
