Question

我正在尝试创建一个函数，如果ArrayList中尚不存在该项，则应将其添加到ArrayList中。

这是我目前拥有的。

帐户类别：

public Account() {}

public Account(String accountName, int accountNumber, double accountBalance)
{
    this.AccountName = accountName;
    this.AccountNumber = accountNumber;
    this.AccountBalance = accountBalance;
}

银行类别：

private ArrayList<Account> accountList;

public Bank() {
}

public boolean addAccount(Account account) {
    if(!accountList.contains(account)) {
        accountList.add(account);
        return true;
    }
    return false;
}

测试器：

public class Main {
    public static void main(String[] args){
        Bank bank = new Bank("qLOwyONvKsM58ZdV& &yo", Bank.BranchLocations.values()[0]);
        Account account = new Account("aucchQitgyzLV", 6329668, 479389.0);
        System.out.println(bank.addAccount(account));
    }
}

看起来我可以添加没有任何值的空帐户，但是如果给定3个值（名称，数字，余额）的可选参数，则它不接受。我不明白

编辑;

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
        Account acc = (Account) obj;
        return AccountBalance == acc.AccountBalance &&
                AccountNumber == acc.AccountNumber &&
                AccountName.equals(acc.AccountName);
    }

Answer 1

从ArrayList.contains（Object o）javadoc：

如果此列表包含指定的元素，则返回true。更多形式上，当且仅当此列表包含至少一个时，才返回true 元素e使得（o == null？e == null：o.equals（e））。

这意味着，要使您的方法正常工作，您必须在Account类中实现hashcode / equals方法。

我为您的代码创建了一个模拟实现：

未定义哈希码/等于方法->使用对象类的默认哈希码/等于方法，以不同方式对待每个新对象。

import java.util.ArrayList;
import java.util.List;

public class Test{


    static class Account {

        private String AccountName;
        private int AccountNumber;
        private double AccountBalance;

        public Account(String accountName, int accountNumber, double accountBalance)
        {
            this.AccountName = accountName;
            this.AccountNumber = accountNumber;
            this.AccountBalance = accountBalance;
        }

        public Account()
        {
        }

        public String getAccountName() {
            return AccountName;
        }

        public void setAccountName(String accountName) {
            AccountName = accountName;
        }

        public int getAccountNumber() {
            return AccountNumber;
        }

        public void setAccountNumber(int accountNumber) {
            AccountNumber = accountNumber;
        }

        public double getAccountBalance() {
            return AccountBalance;
        }

        public void setAccountBalance(double accountBalance) {
            AccountBalance = accountBalance;
        }       


    }

    static class Bank {
    private List<Account> accountList = new ArrayList<>();
    private String inputStr = "";
    private int inputInt = 0;

    public Bank() {
    }

    public Bank(String string, int i) {
        inputStr = string;
        inputInt = i;
    }

    public boolean addAccount(Account account) {
        if(!accountList.contains(account)) {
            accountList.add(account);
            return true;
        }
        return false;
    }

}

        public static void main(String[] args)
        {

            Bank bank = new Bank("qLOwyONvKsM58ZdV& &yo", 1);

            Account account = new Account("aucchQitgyzLV", 6329668, 479389.0);
            Account account5 = new Account("aucchQitgyzLV", 6329668, 479389.0);
            Account account2 = new Account();
            Account account3 = new Account();
            Account account4 = new Account("sgdgrt", 0, 0.1);
            Account account6 = new Account("sgdgrt", 0, 0.1);


            System.out.println( "account added: "+bank.addAccount(account));
            System.out.println("account5 added: "+bank.addAccount(account5));

            System.out.println( "account2 added: "+bank.addAccount(account2));
            System.out.println("account3 added: "+bank.addAccount(account3));

            System.out.println("account4 added: "+bank.addAccount(account4));
            System.out.println("account6 added: "+bank.addAccount(account6));

        }

}

结果：
  帐户已添加：true
  account5已添加：true
  account2已添加：true
  account3已添加：true
  account4已添加：true
  account6已添加：true

显然不正确。

如果要在Account类中插入以下哈希码/等于方法：

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    long temp;
    temp = Double.doubleToLongBits(AccountBalance);
    result = prime * result + (int) (temp ^ (temp >>> 32));
    result = prime * result + ((AccountName == null) ? 0 : AccountName.hashCode());
    result = prime * result + AccountNumber;
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Account other = (Account) obj;
    if (Double.doubleToLongBits(AccountBalance) != Double.doubleToLongBits(other.AccountBalance))
        return false;
    if (AccountName == null) {
        if (other.AccountName != null)
            return false;
    } else if (!AccountName.equals(other.AccountName))
        return false;
    if (AccountNumber != other.AccountNumber)
        return false;
    return true;
}

结果将是：
  添加的帐户：true
  account5已添加：false
  account2已添加：true
  account3已添加：false
  account4已添加：true
  account6已添加：false

因此您的方法将正确运行。

Answer 2

在对等值进行编辑后，您的代码必须能够按预期工作。

您可以list来代替使用java.util.Set并使用包含来检查重复项。它不允许重复。
如果要保留插入顺序，请改用LinkedHashSet。它保留插入顺序并禁止重复。您要做的就是重写equals和hashcode。

import java.util.LinkedHashSet;
import java.util.Set;
public class Bank {
private Set<Account> accountList= new LinkedHashSet<>();

public Bank() {
}

public boolean addAccount(Account account) {
    if(!accountList.contains(account)) {
        accountList.add(account);
        return true;
    }
    return false;
}}

Answer 3

我不知道发生了什么，但是我重新创建了您的程序，它可以按预期工作。问题可能出在您的银行分行逻辑上。我没有重写HashCode（），但这是一个很好的习惯。

    public class BankingDemo {

       public static void main(String[] args) {
          Bank bank = new Bank();
          Account account = new Account("aucchQitgyzLV", 6329668, 479389.0);
          System.out.println(bank.addAccount(account));
          System.out.println(bank.getList());
          System.out.println(bank.addAccount(account));
          System.out.println(bank.getList());
          account = new Account("new account", 29228, 29202.2);
          System.out.println(bank.addAccount(account));
          System.out.println(bank.addAccount(account));
          System.out.println(bank.getList());

       }
    }

    class Account {
       String AccountName;
       int    AccountNumber;
       double AccountBalance;

       public Account() {
       }

       public Account(String accountName, int accountNumber,
             double accountBalance) {
          this.AccountName = accountName;
          this.AccountNumber = accountNumber;
          this.AccountBalance = accountBalance;
       }
       @Override
       public boolean equals(Object obj) {
          if (this == obj) {
             return true;
          }
          if (obj == null || getClass() != obj.getClass()) {
             return false;
          }
          Account acc = (Account) obj;
          return AccountBalance == acc.AccountBalance
                && AccountNumber == acc.AccountNumber
                && AccountName.equals(acc.AccountName);
       }
       @Override
       public String toString() {
          return "{" + AccountName + "," + AccountNumber + "," + AccountBalance
                + "}";
       }
    }

    class Bank {

       private ArrayList<Account> accountList = new ArrayList<>();

       public Bank() {
       }

       public boolean addAccount(Account account) {
          if (!accountList.contains(account)) {
             accountList.add(account);
             return true;
          }
          return false;
       }

       public List<Account> getList() {
          return accountList;
       }
    }

为什么此函数不能像应有的那样向ArrayList添加项目？

编辑;

3 个答案: