Java String + 与 =

例子1：

System.out.println("is"+ 100 + 5);
System.out.println(100 + 5 +"is");
System.out.println("is"+ (100 + 5));

输出：
is1005
105is
is105

例子2：

public class Test{

    public static void main(String argv[]){
        String str = "hello";
        str += 100;  // hello100
        str = str+100; // hello100100
        str = 100; // error: can not convert from int to String
        str = (String)100; // error: can not cast from int to String
        System.out.println(str);
    }

}