How does += get the right answer?

applePrice = 0. apple.addprice increases applePrice to 50. apple2.addPrice increases it by 100 to 150. Yes, you are right, these variable and method names are very expressive and explain what the are supposed to mean or do. You just have one problem. The addPrice(int) method is telling you what it intends to do however its body is empty. Means it does nothing at all and that means you should implement that method. You are supposed to write its functionality. For that you have access to two variables. a) The parameter of the method addPrice(int applePrice) This applePrice parameter is a copy (if we talk about primitive values like int, double, long etc.) of the value you pass as argument to it inside the method call. Take line 10 for example. Here you invoke the addPrice(int) method with a value of 50 -> apple.addPrice(50) A copy of this value you can acces inside the addPrice method using the variable applePrice b) The second value you can access is the public static variable applePrice (that's called a class variable). public static variables you can access from everywhere using either ClassName.variableName or just variableName if you are inside that very class and if there is no shadowing (a parameter with the same name like here -> you have to access it using Apple.applePrice). That you can try and if you code some debug output

public static void addPrice(int applePrice) {
    System.out.println("Parameter applePrice: " + applePrice);
    System.out.println("Class variable Apple.applePrice: " + Apple.applePrice);
}

after the first call of addPrice(int) in line 10, the parameter is 50, the static variable 0 (we did not touch it). The second call gives an output of 100 for the parameter (static variable is still 0, we still did not do anything with it).