总结: Java 新手,尝试查看其他帖子但没有找到答案。我正在学习继承,并且有一个由 Runner 类扩展的 AddressBook 类。当我编写程序来测试继承时,我创建了一个 Runner 对象。如果我得到第一个 String 参数,它返回正常,但是当我尝试获取第二个 String 参数时,它返回 null。
问题: 为什么第二个参数返回null?
package Assignment_1;
//Begin Class Definition
public class AddressBook {
// Member variables
private String businessPhone;
private String cellPhone;
private String facebookId;
private String firstName;
private String homeAddress;
private String homePhone;
private String lastName;
private String middleName;
private String personalWebSite;
private String skypeId;
//Constructors
public AddressBook (String firstName, String middleName, String lastName, String homeAddress, String businessPhone, String homePhone, String cellPhone, String skypeId, String facebookId, String personalWebSite) {
this.firstName = firstName;
this.middleName = middleName;
this.lastName = lastName;
this.homeAddress = homeAddress;
this.businessPhone = businessPhone;
this.homePhone = homePhone;
this.cellPhone = cellPhone;
this.skypeId = skypeId;
this.facebookId = facebookId;
this.personalWebSite = personalWebSite;
}
public AddressBook (String firstName) {
this.firstName = firstName;
}
public AddressBook(String firstName, String middleName) {
this.firstName = firstName;
this.middleName = middleName;
}
public AddressBook (String firstName, String middleName, String lastName) {
this.firstName = firstName;
this.middleName = middleName;
this.lastName = lastName;
}
// Getters and setters
public String getFirstName() {
return firstName;
}
public String getMiddleName() {
return middleName;
}
public String getLastName() {
return lastName;
}
public String getHomeAddress() {
return homeAddress;
}
public String getBusinessPhone() {
return businessPhone;
}
public String getHomePhone() {
return homePhone;
}
public String getCellPhone() {
return cellPhone;
}
public String getSkypeId() {
return skypeId;
}
public String getFacebookId() {
return facebookId;
}
public String getPersonalWebsite() {
return personalWebSite;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public void setMiddleName(String middleName) {
this.middleName = middleName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public void setHomeAddress(String homeAddress) {
this.homeAddress = homeAddress;
}
public void setBusinessPhone(String businessPhone) {
this.businessPhone = businessPhone;
}
public void setHomePhone(String homePhone) {
this.homePhone = homePhone;
}
public void setCellPhone(String cellPhone) {
this.cellPhone = cellPhone;
}
public void setSkypeId(String skypeId) {
this.skypeId = skypeId;
}
public void setFacebookId(String facebookId) {
this.facebookId = facebookId;
}
public void setPersonalWebSite(String personalWebSite) {
this.personalWebSite = personalWebSite;
}
// Public methods
public static void compareNames(String name1, String name2) {
if(name1.equals(name2)) {
System.out.println(name1);
System.out.println(name2);
System.out.println("The names are the same.");
} else {
System.out.println(name1);
System.out.println(name2);
System.out.println("The names appear to be different.");
}
}
************************************************************
package Assignment_1;
public class BanffMarathonRunner extends AddressBook {
// Member variables
private int time;
private int years;
// Constructors
public BanffMarathonRunner(String firstName, String lastName, int min, int yr) {
super(firstName, lastName);
time = min;
years = yr;
}
// Getters and Setters
public int getTime() {
return time;
}
public void setTime(int time) {
this.time = time;
}
public int getYears() {
return years;
}
public void setYears(int years) {
this.years = years;
}
}
************************************************************
package Assignment_1;
import Assignment_1.BanffMarathonRunner;
public class TestBanffMarathonRunner {
public static void main(String[] args) {
BanffMarathonRunner r1 = new BanffMarathonRunner("Elena", "Brandon", 341, 1);
System.out.print(r1.getLastName());
}
}
}
最佳答案
因为 lastName
为空。
您正在调用 AddressBook(String firstName, String middleName)
并设置 middleName
,而不是 lastName
。
关于java - 对象返回 null,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/30917078/