我正试图传递＆＃34; personListToPrint＆＃34;到main.java并在使用开关3时出现，但它不起作用。我不知道如何解决这个问题。任何想法??
这里是main.java
package hartman;
import java.util.ArrayList;
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Printer.printWelcome();
Scanner keyboard = new Scanner(System.in);
ArrayList<Person> personList = new ArrayList<>();
boolean keepRunning = true;
while (keepRunning) {
Printer.printMenu();
Printer.printPrompt("Please enter your operation: ");
String userSelection = keyboard.nextLine();
switch (userSelection) {
case "1":
Database.addPerson(personList);
break;
case "2":
Database.printDatabase(personList);
break;
case "3":
Printer.printSearchPersonTitle();
String searchFor = keyboard.nextLine();
Database.findPerson(searchFor);
Printer.printPersonList(personListToPrint);
break;
case "4":
keepRunning = false;
break;
default:
break;
}
}
Printer.printGoodBye();
keyboard.close();
}
}
这里是database.java
package hartman;
import java.util.ArrayList;
import java.util.Scanner;
public class Database {
static Scanner keyboard = new Scanner(System.in);
private static ArrayList<Person> personList = new ArrayList<Person>();
public Database() {
}
public static void addPerson(ArrayList<Person> personList) {
Printer.printAddPersonTitle();
Printer.printPrompt(" Enter first name: ");
String addFirstName = keyboard.nextLine();
Printer.printPrompt(" Enter last Name: ");
String addLastName = keyboard.nextLine();
Printer.printPrompt(" Enter social Security Number: ");
String addSocial = keyboard.nextLine();
Printer.printPrompt(" Enter year of birth: ");
int addYearBorn = Integer.parseInt(keyboard.nextLine());
System.out.printf("\n%s, %s saved!\n", addFirstName, addLastName);
Person person = new Person();
person.setFirstName(addFirstName);
person.setLastName(addLastName);
person.setSocialSecurityNumber(addSocial);
person.setYearBorn(addYearBorn);
personList.add(person);
}
public static void printDatabase(ArrayList<Person> personList) {
System.out
.printf("\nLast Name First Name Social Security Number Age\n");
System.out
.printf("=================== =================== ====================== ===\n");
for (Person p : personList) {
System.out.printf("%-20s%-21s%-24s%s\n", p.getLastName(),
p.getLastName(), p.getSocialSecurityNumber(), p.getAge());
}
}
public static ArrayList<Person> findPerson(String searchFor) {
ArrayList<Person> matches = new ArrayList<>();
for (Person p : personList) {
boolean isAMatch = false;
if (p.getFirstName().equalsIgnoreCase(searchFor)) {
isAMatch = true;
}
if (p.getLastName().equalsIgnoreCase(searchFor)) {
isAMatch = true;
}
if (p.getSocialSecurityNumber().contains(searchFor)) {
isAMatch = true;
;
} else if (String.format("%d", p.getAge()).equals(searchFor))
if (isAMatch) {
}
matches.add(p);
Printer.printPersonList(matches);
}
return matches;
}
}
最后，这是printer.java
package hartman;
import java.util.ArrayList;
public class Printer {
public static void printWelcome() {
System.out.printf("WELCOME TO PERSON DATABASE!\n");
}
public static void printGoodBye() {
System.out.printf("\nGOOD BYE!!\n");
}
public static void printMenu() {
System.out.printf("\nMain Menu\n");
System.out.printf("---------\n\n");
System.out.printf(" 1. Add a new Person to the database.\n");
System.out.printf(" 2. Print the database.\n");
System.out.printf(" 3. Search for a person in the database.\n");
System.out.printf(" 4. Exit the application.\n");
System.out.printf("\n");
}
public static void printPrintMenu() {
System.out.printf("Print\n\n");
}
public static void printAddPersonTitle() {
System.out.printf("\nAdd Person to Database\n\n");
}
public static void printPrompt(String promptForWhat) {
System.out.printf("%s", promptForWhat);
}
public static void printPersonSaved(Person personSaved) {
System.out.printf("%s", personSaved);
}
public static void printSearchPersonTitle() {
System.out.printf("\nSearch for Person in Database\n\n");
System.out.printf("Enter search value: ");
}
public static void printPersonList(ArrayList<Person> personListToPrint) {
System.out
.printf("\nLast Name First Name Social Security Number Age\n");
System.out
.printf("=================== =================== ====================== ===\n");
for (Person p : personListToPrint) {
System.out.printf("%-20s%-21s%-24s%s\n", p.getLastName(),
p.getLastName(), p.getSocialSecurityNumber(), p.getAge());
}
}
}
答案 0 :(得分：1)
您没有定义变量
personListToPrint
将案例3替换为下一个
case "3":
Printer.printSearchPersonTitle();
String searchFor = keyboard.nextLine();
ArrayList<Person> personListToPrint = Database.findPerson(searchFor);
Printer.printPersonList(personListToPrint);
break;
现在是可编辑的
答案 1 :(得分：0)
Printer.printPersonList(personListToPrint);
它应该来自哪里？ personListToPrint永远不会在任何地方定义。 （我按Ctrl-F搜索了您的代码示例并且它没有任何地方）
答案 2 :(得分：0)
直接调用打印机而不创建类的对象。
Printer printerObj = new Printer();
答案 3 :(得分：0)
我想我已经弄清楚了你的困惑，但解释它有点棘手。
在您的Printer类中，您有以下方法声明：
public static void printPersonList(ArrayList<Person> personListToPrint)
在这种情况下，personListToPrint是您将使用INSIDE此方法的变量的名称来引用从外部源调用时传递给该方法的ACTUAL ArrayList。
在您的情况下，您在Case＆＃34; 3＆＃34;中使用此调用来调用它。块：
Printer.printPersonList(personListToPrint);
即使该方法将其称为＆＃34; personListToPrint，＆＃34;当你进行上述调用时，你真的想传入personList。在Main中，personList引用一些ArrayList，让我们说在某些内存位置定义为＆＃34; 0100。＆＃34;当调用Printer.printPersonList时，您希望传入要打印的数组列表，在本例中为personList（驻留在内存＆＃34; 0100＆＃34;）。一旦进入printPersonList，则存储在＆＃34; 0100＆＃34;将使用变量名称personListToPrint引用。它是同一内存位置的两个不同名称。
另一个评论过你从未声明或实例化过personListToPrint这一事实的人是正确的，所以当你以现在的方式调用printPersonList（personListToPrint）时，你传递的是null，因为那个特定的变量是从未声明或实例化（实际上你应该得到一个编译错误）。
如果这令人困惑，请回复评论，我会尝试以不同的方式解释。