为什么ArrayList contains（）方法不能与对象一起使用？

Question

我正在学习Java并遇到问题。 为什么ArrayList contains（）方法不能与对象一起使用？ 它似乎适用于字符串和其他基本数据类型，但不适用于对象。 我使用了错误的方法吗？

程序在第8-11行中创建数据并从命令行接收数据。 命令行中的数据为：

avacado 10 .50 broccoli 20 .75 cauliflower 30 1.00 banana 300 .55

命令行上的最后一个条目“ banana 300 .55”与第11行上的数据匹配，并且应该被绕过，因为已经存在，但是contains（）表示不存在。 怎么会？ 救命。

这是我的代码：

package java21days;

import java.util.*;

public class FruitBasket3 {

    ArrayList<Fruit> cart;
    Fruit apple = new Fruit("apple", 100, .75F);
    Fruit orange = new Fruit("orange", 200, .90F);
    Fruit banana = new Fruit("banana", 300, .55F);
    Fruit kumkwat = new Fruit();

    public FruitBasket3(String[] userCodes) {
        cart = new ArrayList<>();
        int quantity;
        float price;

        addCode(apple);
        addCode(orange);
        addCode(banana);
        addCode(kumkwat);

        for (int j = 0; j < userCodes.length; j+=3) {
            quantity = Integer.parseInt(userCodes[j+1]);
            price = Float.parseFloat(userCodes[j+2]);
            addCode(new Fruit(userCodes[j],quantity,price));
        }

        // display all codes
        for (Fruit code : cart) {
            System.out.println("Fruit: " + code.getName() + " Price: " +
                    code.getQuantity() * code.getPrice());
        }
    }

    private void addCode(Fruit inFruit) {
        boolean match = false;
        System.out.println("Inside addCode with fruit: " + inFruit.getName() + 
                " Quantity: " + inFruit.getQuantity() + " Price: " + inFruit.getPrice());
        if (!cart.contains(inFruit)) {
            System.out.println("    Passed the contains test.");
            for (Fruit item : cart) {
                System.out.println("    Existing: " + item.getName() + 
                        " Quantity: " + item.getQuantity() + " Price: " + item.getPrice());
                System.out.println("    New:      " + inFruit.getName() + 
                        " Quantity: " + inFruit.getQuantity() + " Price: " + inFruit.getPrice());

                //if (inFruit.getName() == item.getName() &&
                //        inFruit.getQuantity() == item.getQuantity() &&
                //        inFruit.getPrice() == item.getPrice()){
                if (inFruit.getName().equals(item.getName()) &&
                        inFruit.getQuantity() == item.getQuantity() &&
                        inFruit.getPrice() == item.getPrice()){
                    match = true;
                    System.out.println("        Did not pass the individual element comparision test.");
                } else {
                    System.out.println("        Passed the individual element comparision test.");
                }
            }
            if (!match) {
                System.out.println("            >>>> adding.");
                cart.add(inFruit);
            } else {
                System.out.println("            >>>> did not add.");
            }
        } else {
            System.out.println("    Fruit: " + inFruit.getName() + " already on file ... bypassing.");
        }
    }

    public static void main(String[] arguments) {
        if (arguments.length % 3 != 0) {
            System.out.println("Error!!! Incorrect number of arguments.");
        } else {
            FruitBasket3 keeper = new FruitBasket3(arguments);
        }
    }

}

public class Fruit {
    private String name;
    private int quantity;
    private float price;

    public Fruit() {
        name = "";
        quantity = 0;
        price = 0.0F;
    }

    public Fruit(String inName, int inQuantity, float inPrice) {
        name = inName;
        quantity = inQuantity;
        price = inPrice;
    }

    public String getName(){
        return name;
    }

    public int getQuantity(){
        return quantity;
    }

    public float getPrice(){
        return price;
    }
}

输出为：

run:
Inside addCode with fruit: apple Quantity: 100 Price: 0.75
    Passed the contains test.
            >>>> adding.
Inside addCode with fruit: orange Quantity: 200 Price: 0.9
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      orange Quantity: 200 Price: 0.9
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: banana Quantity: 300 Price: 0.55
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit:  Quantity: 0 Price: 0.0
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:       Quantity: 0 Price: 0.0
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:       Quantity: 0 Price: 0.0
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:       Quantity: 0 Price: 0.0
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: avacado Quantity: 10 Price: 0.5
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: broccoli Quantity: 20 Price: 0.75
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing: avacado Quantity: 10 Price: 0.5
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: cauliflower Quantity: 30 Price: 1.0
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: avacado Quantity: 10 Price: 0.5
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: broccoli Quantity: 20 Price: 0.75
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: banana Quantity: 300 Price: 0.55
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      banana Quantity: 300 Price: 0.55
        Did not pass the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: avacado Quantity: 10 Price: 0.5
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: broccoli Quantity: 20 Price: 0.75
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: cauliflower Quantity: 30 Price: 1.0
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
            >>>> did not add.***
Fruit: apple Price: 75.0
Fruit: orange Price: 180.0
Fruit: banana Price: 165.0
Fruit:  Price: 0.0
Fruit: avacado Price: 5.0
Fruit: broccoli Price: 15.0
Fruit: cauliflower Price: 30.0
BUILD SUCCESSFUL (total time: 0 seconds)

Answer 1

它不适用于您的类，因为它没有有效的public boolean equals(...)方法重写。 给它一个，一个使用关键字段测试功能是否相等，并且包含一个就可以。

根据ArrayList API的contains方法部分 ：

如果此列表包含指定的元素，则返回true。 更正式地讲，当且仅当此列表包含至少一个元素（e == null？e == null：o.equals（e））时，返回true。

仅将名称字段用于相等性检验，并使其成为final字段，因为您不想使用可能更改的字段。 还给它一个hashCode替代，它也使用equals方法中测试的相同字段。

就像是：

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((name == null) ? 0 : name.hashCode());
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Fruit other = (Fruit) obj;
    if (name == null) {
        if (other.name != null)
            return false;
    } else if (!name.equals(other.name))
        return false;
    return true;
}

可以工作

Answer 2

ArrayList类中的contains（）方法使用equals（）方法评估两个对象。 如果两个对象都是SAME对象，则ArrayList equals（）可以工作。 您必须在类中重写equals（）方法，并定义评估标准以定义列表中是否已包含Fruit。 将此代码添加到您的Fruit类

@Override
public boolean equals(Object o1) {
if (this == obj) {
    return true;
}
Fruit fruit = (Fruit)o1
if (name != null && fruit.getName() != null) {
   if (name.equas(fruit.getName())) {
        return true;
   }
   return false;
}
return false;