Java。类参数不通过构造函数传递

Question

这是我创建和管理对象时的主程序。

public class ShoppingCartManager {

    public static void main(String[] args) {
       
           //when declaring the object it's not changing the values.
       ShoppingCart cart1 = new ShoppingCart("John Doe", "February 1, 2016");
       System.out.println(cart1.getCustomerName());
       
    }

}//end of class

////////////////////////////////////////////////////////////////////////////////////// ////////////////////////////////////////////////

这是类，构造函数应该写入类的私有变量，但它们不会更新。

import java.util.ArrayList;

public class ShoppingCart {

    private String customerName = "none";
    private String currentDate = "January 1, 2016";
    //ArrayList<ItemToPurchase> cartItems = new ArrayList<ItemToPurchase>();
    
    //constructor methods
        //this is constructer that it calls
    ShoppingCart(String Name, String Date) {
        //Parameterized constructor which takes the customer name and date as parameters
        String customerName = Name;
        String currentDate = Date;
    }
    ShoppingCart() {
        //String customerName - Initialized in default constructor to "none"
        //String currentDate - Initialized in default constructor to "January 1, 2016"
        String customerName = "none";
        String currentDate = "January 1, 2016";
    }

ShoppingCart(String Name) {
String customerName = Name;
}

    /////////////////////////////////////////////////////
    
    //Getter methods
    
    //getCustomerName
    public String getCustomerName(){
        return customerName;
    }
    //getDate
    public String getDate() {
        return currentDate;
    }
    
    /////////////////////////////////////////////////////

}//end of class

我尝试重新构建构造函数并创建新对象。但对象 customerName 变量将始终保持默认的“none”。

Answer 1

public class ShoppingCart {

private String customerName = "none";
private String currentDate = "January 1, 2016";
//ArrayList<ItemToPurchase> cartItems = new ArrayList<ItemToPurchase>();

// Parameterized constructor
ShoppingCart(String name, String date) {
    this.customerName = name; // Assign the parameter to the class field
    this.currentDate = date; // Assign the parameter to the class field
}

// Default constructor
ShoppingCart() {
    // The class fields are already initialized with default values, so you could leave this empty or remove it if you don't need to do anything else.
}

ShoppingCart(String name) {
    this.customerName = name; // Assign the parameter to the class field
    // currentDate will retain its default value unless explicitly set
}

// Getter methods

public String getCustomerName() {
    return customerName;
}

public String getDate() {
    return currentDate;
}

您在 ShoppingCart 类中遇到的问题源于您在构造函数中处理赋值的方式。当您在构造函数内为 customerName 和 currentDate 赋值时，您实际上是在创建局部变量并将值赋值给局部变量，而不是更新类字段。这就是为什么您的 customerName 始终保持默认值“none”。要解决此问题，您应该在设置值时删除构造函数内的数据类型声明。这样，您可以确保将提供的值分配给类的字段而不是新的局部变量。这应该有效。