理解数组列表和对象

Question

public class Product {
    // Variables.
    private String name;      // Name  
    private Double price;     // Price

    Product() {}    // Default constructor with no parameters.

    Product(String name, Double price) {    // Constructor with parameters.
        this.name = name;
        this.price = price;
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Double getPrice() {
        return price;
    }
    public void setPrice(Double price) {
        this.price= price;
    }
    public String toString() {    // Overriding "toString()".
        return "\nName: " + this.name + "\nPrice: " + this.price;
    }


public boolean equals(Object obj) {    // Overriding equals()
   if(this == obj) {
      return true;
   }
   if(obj == null || obj.getClass() != this.getClass()) {
      return false;
   }
   Product product = (Product) obj;
   return this.name.equals(product.name)&& this.price.equals(product.price);
}
    }

import java.util.*;
    import java.io.*;
    public class Main {
        private static BufferedReader r = new BufferedReader (new InputStreamReader(System.in));
        private static String readln() throws IOException{
            return r.readLine();
        }
        private static long readInput() throws IOException{    // Use this to read input for the menu options.
            return Integer.valueOf(readln());
        }
        public static void menu(){    // Menu
            System.out.println("-------------------------" +
                    "\nAdd new product(1)" +
                    "\nSearch for product(2)" +
                    "\nDelete product(3)" +
                    "\nShow all products(4)" +
                    "\nReturn the number of products(5)" +
                    "\nExit(-1)" +
                    "\n-------------------------");
        }

        public static void main (String args[]) throws IOException{
            // This is the ArrayList for the "Product".
            ArrayList<Product> products = new ArrayList<Product>();
            int option = 0;
            do {
                menu();
                option = (int)readInput();
                switch (option){
                    case 1:{
                        System.out.println("Insert product name: ");
                        String name= readln();
                        System.out.println("Insert product price: ");
                        Double price = Double.parseDouble(readln());
                        products.add(new Product(name, price));
                        break;
                    }
                    case 2:{
                        System.out.println("Insert product name: ");
                        String name= readln();
                        System.out.println("Insert product price: ");
                        Double price= Double.parseDouble(readln());
                        if ((products.contains(new Product (name, price)))){
                            System.out.println("Works!");
                        }

                        break;
                    }
                    case 3:{

                        break;
                    }
                    case 4:{

                        break;
                    }
                    case 5:{
                        System.out.println("Number of products: " + products.size());
    //This prints with no problems, therefor the objects DO exist in the ArrayList.
                        break;
                    }
                }
            }while((option > 0) && (option < 6));
        }
    }

编辑：对于equals()的最后一行，还可以放入：

if( (this.price.equals(product.getPrice())) && (this.name.equals(product.getName())) ) {
            return true;
        }

Answer 1

要使其工作，您还应该将equals方法重写为people对象内的compere字段，重写equals方法覆盖equals方法

参数化构造函数中可能存在一个bug。它看起来应该是：

Product(final String name, final Double price) {    // Constructor with parameters.
    this.name = name;
    this.price = price;
}

最后一个字阻止我们改变输入参数的值。

@Override
public boolean equals(Object obj) {    // Overriding "equals()".
    // at first check if objects are the same -> your code
    if (this == obj) {
        return true;
    }

    // secondly we chack if objects are instances of the same class if not return false
    if (obj != null && this.getClass() != obj.getClass()) {
        return false;
    }

    // then compare objects fields. If fields have the same values we can say that objects are equal.
    Product product = (Product) obj;
    return this.name.equals(product.name) && this.price.equals(product.price);
}

而不是做

products.contains(name, price);

试试看

products.contains(new Product(name, price))

要从列表中删除元素，首先可以找到元素的索引和use remove方法。

products.remove(products.indexOf(new Product(name, price)))