求大神解释下模等于

public class HelloWorld{
    public static void main(String[] args) {
        int one = 10 ;
        int two = 20 ;
        int three = 0 ;
        three=one+two;//这一步解释下 ，为什么不加变量？      

     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);
        three%=one;
        System.out.println("three %= one ==> "+three);
        }
}