You use "void" when you don't want to return anything. This applies to public void, public static void, private void etc. Static means you do not need an instance of the class (new Object ();). More on reddit.com

Your main, keyboard, and all 3 methods need to be static. Then... there's just a lot wrong with your methods. In EarthWeight, you're getting input, but then storing that in a local double and doing nothing with it. If you want to return the user's input, you need to actually return it. Seems like you want this to be: static double EarthWeight() { System.out.println("Please enter your current Earth weight: "); return keyboard.nextDouble(); } Same sort of thing for the other input. Lastly, just some style things: All method and variable names should start with a lower case letter Notice how you write System.out.println("Your weight would be " + RealWeight + " on that planet. "); 6 times. Can you shuffle things so you only have that once? More on reddit.com

Say you have Apples (the class, Apple). When you create a specific apple with Apple a = new Apple() you have a new instance of an apple called a. An Apple is an object. You can have multiple objects and multiple instances of objects. An Apple is a kind of fruit which is another type of object. Since an Apple is a fruit but a fruit is not an apple you can say that Apple derives from fruit. Objects can have different methods on them that are the callable for any instance of the object. A fruit can have a function isRipe() which is transferred to the apple. The apple can have a function called isCored() which is specific to apples. Each instance of an apple might have different results to these questions. Getters and Setters are concepts that academia fucking loves and transfers its love to college grads and infuriates me because the explanation is so bullshit in any modern codebase and they're really only applicable to very hard decoupled APIs like ones you're giving to other companies. I'll tell you the cs101 answer. An object can have a member variable, for instance an apple can have a type such as Fuji or Granny Smith. If the member variable is public then you can just set this, for instance with our apple a above: a.type = Fuji; However suppose you want to verify that the kind of apple is valid (maybe because you have a list of apple types that will work for your scenario or because I picked apples and I didn't think this example through fully), you can't do that because anyone can just change the type at any time. Instead you can have a setter function: Apple::set(type t) ( /* verify here */ type = t;) And you make type private. Now the apple type is verified every time and you can't accidentally set an invalid value. Because type is private you need a getter function which returns it to be able to access it. More on reddit.com

Java does not have functions, only methods. Functions are stand alone and exist outside the context of classes. Since this is not possible in Java, it doesn't have functions. Methods exist only in the context of classes. Even methods that one might consider functions abs, sin, etc. only exist in the context of the Math class and therefore are not functions. In Python, you can have both. If you just define a function outside any class, it is a function, yet if you define a function inside a class, it becomes a method. More on reddit.com