诡异的 get set 方法NullPointerException

 private String orderNumber;


    public String getOrderNumber() {
    return orderNumber;
}
public void setOrderNumber(String orderNumber) {
    this.orderNumber = orderNumber;
}

/*  System.out.println(orderNumber);
    这为啥不行odfo.setOrderNumber(orderNumber);*/
    odfo.setOrderNumber(orderNumber);

报错如下
四月 21, 2017 12:59:16 下午 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet [dispatcher] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException

不赋值肯定是null

public class a{
private String orderNumber;
public String getOrderNumber() {
return orderNumber;
}
public void setOrderNumber(String orderNumber) {
this.orderNumber = orderNumber;
}
}
在别的类中调用，A a=new A();
a.setOrderNumber("a");
syste.........out(a.getOrderNumber());

odfo.setOrderNumber(orderNumber); odfo对象是不是空

你定义的对象是string的