为什么赋值不能加int，这个不都是整数吗？

public class HelloWorld{

    public static void main(String[] args) {

   int one = 10 ;

        int two = 20 ;

        int three = 0 ;

        int three=one+two;

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

        int three+=one;

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

        int three-=one;

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

        int three*=tow/2;

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

        int three/=two*2;

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

        int three%=one;

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

}

}