Question

我很困惑为什么我的程序按特定顺序打印语句？

我有一个Student class，里面是Inner Class of Address。该程序的想法是首先将一个归属地址分配给学生对象，然后通过利用内部地址类分配一个大学/学期时间地址。

代码如下：

学生班（具有内部地址类）

 public class Student {

    private String name;
    private Address homeAddress, uniAddress;

    public Student(String name, int houseNumber, String homeStreet) {
        this.name = name;
        homeAddress = new Address(houseNumber, homeStreet);
    }

    public String getName() {
        return name;
    }

    public Address getHomeAddress() {
        String s = "n/a";

        if (homeAddress != null) {
            return homeAddress;
        } else {
          //  System.out.println(s);
            return null;
        }


    }

    public void setUniAddress(int num, String add) {

        uniAddress = new Address(num, add);
    }

    public Address getUniAddress() {

        String s = "n/aa";

        //If uniAddress isn't set, 
        // then "n/aa" gets printed before anything else i/e toString() method - WHY?

        if (uniAddress == null) {
           System.out.println(s);
            return null;
        } else {

            return uniAddress;
        }
    }

    @Override
    public String toString() {

        return "NAME: " + getName() + "\n"
             + "HOME ADDRESS: " + getHomeAddress() + "\n"
             + "TERM TIME ADDRESS: " + getUniAddress();

    }

    // Inner Class
    public class Address {

        private int number;
        private String street;

        public Address(int no, String street) {
            number = no;
            this.street = street;
        }

        @Override
        public String toString() {
            //return name + "\n" + number + " " + street;
            return number + " " + street;
        }
    }
}   // more Student methods .. }

TestStudent类（使用main方法）

  public class TestStudent {

    public static void main(String[] args) {
        //Home Address
        Student s1 = new Student("Cathy", 21, "Smithfield Drive");
        //Uni Address
        s1.setUniAddress(72, "Nottingham Drive");



        Student.Address anotherAddress = s1.new Address(8, "Deerfield Way");
        // note the use of new

        System.out.println(s1.toString());


    }
}

输出结果为：

n/aa
NAME: Cathy
HOME ADDRESS: 21 Smithfield Drive
TERM TIME ADDRESS: null

(all on new lines)

如果我没有为学生分配一个Uni地址（即如果我在主方法中注释掉相应的行 - 调用setUniAddress()方法），那么我很好奇，为什么'n /来自getUniAddress（）方法的aa'是在toString()方法之前打印的吗？（如上所述）

如果我调用setUniAddress（）方法，则输出为：

NAME: Cathy
HOME ADDRESS: 21 Smithfield Drive
TERM TIME ADDRESS: 72 Nottingham Drive

(all on new lines)

这似乎按预期工作。

我也想知道如何，而不是打印'null'到 TERM TIME ADDRESS :(当没有调用setUniAddress()方法时），我可以返回'n / aa'在它的位置 - 这就是我试图做的事情？

感谢。

Answer 1

getUniAddress()

toString()，这就是首先打印n/aa的原因。
如果要将“n / aa”打印为默认值 - 将其设置为默认值，例如，将声明更改为：

private Address homeAddress, uniAddress = "n/aa";

@Override
public String toString() {

return "NAME: " + getName() + "\n"
     + "HOME ADDRESS: " + getHomeAddress() + "\n"
     + "TERM TIME ADDRESS: " + getUniAddress(); // <-- here you call getUniAddress() which 
                                                // is why "n/aa" is printed first

}

在getUniAddress()中你有以下行打印“n / aa”：

System.out.println(s);

Answer 2

当执行toString（）方法中的return语句时，将调用get *（）方法。然后创建一个字符串并由toString（）方法返回。

因此在计算从toString（）返回的字符串时打印'n / aa'，然后toString（）方法返回字符串，main方法打印从toString（）生成的输出。

内部类 - 打印声明顺序 -

2 个答案: