Question

我的程序中的构造函数遇到了一些问题。它是一个存储客户数据的简单银行数据库。我必须实施在两个账户之间存入，提取和转移现金的方法。我已经实现了这种构造函数来添加新的银行帐户：

public Customer() {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter id of customer:");
        this.id = scan.nextLine();
        File folder = new File("CustomerDataBase" + File.separator + this.id + ".txt");
        if(folder.exists()) {
            System.out.println("Customer ID already exists!");
            System.exit(1);
        }  
        try {      
            System.out.println("Enter name of customer:");
            this.name = scan.nextLine();
            System.out.println("Enter surname of customer:");
            this.surname = scan.nextLine();
            System.out.println("Enter PESEL number of customer:");
            this.pesel = scan.nextLine();           
            System.out.println("Enter address of customer:");
            System.out.println("    Street:");
            this.adressStreet = scan.nextLine();
            System.out.println("    City:");
            this.adressCity = scan.nextLine();
            System.out.println("    Zip Code: ");
            this.zipCode = scan.nextLine();
            System.out.println("Enter funds of Customer:");
            this.funds = Double.parseDouble(scan.nextLine());
            this.saveCustomer();
            scan.close();
        }catch(NumberFormatException e) {
            System.out.println("Error : " + e);
            System.exit(1);
        }   
    }

然后我有一个方法withdraw：

static void withdraw (int amount, int id) {

    File f = new File("CustomerDataBase" + File.separator + String.valueOf(id) + ".txt");
    Scanner fRead;

    Customer tempCustomer = new Customer();

    try{
        fRead = new Scanner(f);
        tempCustomer.id = fRead.nextLine();
        tempCustomer.name = fRead.nextLine();
        tempCustomer.surname = fRead.nextLine();
        tempCustomer.pesel = fRead.nextLine();
        tempCustomer.adressStreet = fRead.nextLine();
        tempCustomer.adressCity = fRead.nextLine();
        tempCustomer.zipCode = fRead.nextLine();
        tempCustomer.funds = Double.parseDouble(fRead.nextLine()) - id;
        fRead.close();
        tempCustomer.saveCustomer();
    }catch(FileNotFoundException e) {
        System.out.println("File not found!");
        System.exit(1);
    }   
}

withdraw方法从文件中读取数据，并且必须将其存储在类中。所以我将对象创建为客户类型。但我想只使用＆＃34; plain＆＃34; （默认）Java在您未声明自己的时提供的构造函数。怎么做？我读到了super():语句，但如果我理解正确，它只有在你从另一个类继承时才有用。

Answer 1

每个类都带有一个在类本身中不可见的默认构造函数。 但是，请注意，如果指定默认构造函数以外的构造函数，则不能使用默认构造函数，请参阅下面的@Rustam注释。例如，假设您的Customer类如下所示：

public class Customer {

    private String name;

    private String lastName;

    private age int;

    private String ssn;

    //default constructor that is NOT visible
    Customer()
    {

    }

    //other constructor given name and lastName
    Customer(name, lastName)
    {
        this.name = name;

        this.lastName = lastName;
    }

    //getters and setters

}

默认情况下会创建构造函数Customer（），您无需在类中包含它。

然后，您可以使用默认构造函数创建客户实例，然后您需要使用setter来设置属性，如下所示：

public class Test {

    public static void main(String [] args){

        Customer c = new Customer();

        //setting parameters
        c.setName("Jose");

        c.setLastName("Mejia");

    }

}

Java使用默认构造函数

1 个答案: