为什么最后还要+three?

public class HelloWorld{
    public static void main(String[] args) {
        int one = 10 ;
        int two = 20 ;
        int three = 0 ;
        three =one +two;
        System.out.println("three =one +two==>"+three);
        three += one ;
        System.out.println("three += one ==>"+three);
        three -= one;
        System.out.println("three -=one ==>"+three);
        three *= one;
        System.out.println("three *=one ==>"+three);
        three /=one;
        System.out.println("three =one ==>"+three);