ArrayList仅添加最后一个元素

Question

下面有一个for循环代码。我通过调用一个自定义显示函数发现aBook arrayList对象仅添加了最后一个类对象三次。为什么会这样呢？

Scanner s = new Scanner(System.in);
    ArrayList<LiFiAddressBook> aBook = new ArrayList<LiFiAddressBook>();
    // taking input for every LifIAddressBook and adding them to the ArrayList.
    for (int i = 0; i < 3; i++) {
        System.out.println("Entry " + i+1);
        System.out.print("Please Enter First Name: ");
        String a = s.nextLine();
        System.out.println();
        System.out.print("Please Enter Last Name: ");
        String b = s.nextLine();
        System.out.println();
        System.out.print("Please Enter Street Address: ");
        String c = s.nextLine();
        System.out.println();
        System.out.print("Please Enter City: ");
        String d = s.nextLine();
        System.out.println();
        System.out.print("Please Enter Zip Code: ");
        int e = s.nextInt();
      // in the next line we need to fire a blank scan function in order consume the nextLine. because after executing s.nextInt compiler skip a scan function for a weird reason
        s.nextLine();
        System.out.println();

        LiFiAddressBook x = new LiFiAddressBook(a, b, c, d, e);
        aBook.add(x);


    }

这是我的LiFiAddressBook类

public class LiFiAddressBook {

static  String  first_name, last_name, street_address, city_state;
static int zip_code;

public LiFiAddressBook(String first, String last, String street, String city, int zip) {
  //constructor for class object.
    first_name = first;
    last_name = last;
    street_address = street;
    city_state = city;
    zip_code = zip;
}

public  String get_first() {
    return first_name;
}

public String get_last() {
    return last_name;
}

public String get_address() {
    return street_address;
}

public String get_city() {
    return city_state;
}

public String get_zip() {
    return Integer.toString(zip_code);
}

public static void display() {
    System.out.println("First Name: "+first_name);
    System.out.println("Last Name: "+last_name);
    System.out.println("Street Address"+street_address);
    System.out.println("City State: "+city_state);
    System.out.println("Zip Code: "+zip_code);


}

}

Answer 1

由于使用static关键字，每次
public LiFiAddressBook(String , String , String , String , int )
调用构造函数时，旧值都会被新值覆盖，并且在打印列表中的元素时，LiFiAddressBook类的对象变量将指向相同的对象。因此打印相似的对象。

需要明确的是，实际上有3个LiFiAddressBook实例。但是这些LiFiAddressBook实例的变量/属性引用的是同一对象。