为什么第一串字符（three1）输出的是300

Question

public class HelloWorld{    public static void main(String[] args) {    int one = 10 ;        int two = 20 ;        int three = 0 ;        int three1=one+two;        int three2=three1+=one;        int three3=three1-=one;        int three4=three1*=one;        int three5=three4/=one;        int three6=three%=one;        System.out.println(three1);        System.out.println(three2);        System.out.println(three3);        System.out.println(three4);        System.out.println(three5);        System.out.println(three6);        

Answer 1

int three2=three1+=one;  --> three1 = 30+10 = 40
int three3=three1-=one;  --> three1 = 40-10 = 30   
int three4=three1*=one;  --> three1 = 30*10 = 300