Java将值传递给另一个类问题

Question

我有这个班级

public class A {
    private int number; //A number instance
    public A() { //At the beginning number is 0
        number=0;
    }
    public int getNumber() {
        return number;
    }
    public void setNumber(int number) {
        this.number = number;
    }
    public void Str(String v) { //for given string number is gonna string's length
        setNumber(v.length());
    }
}

我有这个B级

public class B {
    public static void main(String[] args) {
        A a1=new A();//A class instance
        a1.Str("hello");//giving hello string
        System.out.println(a1.getNumber()); //this one is 0
    }
}

当我运行此示例代码时，a1.getNumber为0.这里可能有什么问题？