java - 我尝试使用 CompareTo 方法对数组列表进行排序

Question

我正在尝试对银行帐户进行排序，首先按名称字母顺序排序，然后按帐户中的金额从多到少排序。不幸的是，CompareTo 方法似乎无法正常工作，唯一有效的部分是按金额排序的第二部分。

BankAccount 类

/**
 * A bank account has a balance, the name of the account holder,
 * and an account number. The balance can be changed by deposits
 * and withdrawals.
 */
public class BankAccount implements Comparable<BankAccount> {

/**
 * Constructs a bank account with a zero balance.
 * @param name the name of the account holder
 */
public BankAccount(String name) {
   this.name = name;
   balance = 0;
   accountNo = ++lastAccountNo;
}

/**
 * Constructs a bank account with a given balance.
 * @param initialBalance the initial balance
 * @param name the name of the account holder
 */
public BankAccount(String name, double initialBalance) { 
    this.name = name;
    balance = initialBalance;
    accountNo = ++lastAccountNo;
}

/**
 * Deposits money into the bank account.
 * @param amount the amount to deposit
 */
public void deposit(double amount) {
    double newBalance = balance + amount;
    balance = newBalance;
}

/**
 * Withdraws money from the bank account.
 * @param amount the amount to withdraw
 */
public void withdraw(double amount) {
    double newBalance = balance - amount;
    balance = newBalance;
}

/**
 * Gets the current balance of the bank account.
 * @return the current balance
 */
public double getBalance() {
    return balance;
}

/**
 * Gets the name of the account holder.
 * @returns the name of the account holder
 */
public String getName() {
    return name;
}

/**
 * Gets the account number of the account.
 * @returns the account number of the account
 */
public int getAccountNo() {
    return accountNo;
}

/**
 * Returns a String representation of the BankAccount. The format
 * is "name: accountNo balance"
 * @returns a String representation of the BankAccount.
 */
public String toString() {
    return name + ": AccountNo:" + accountNo + " balance:" + balance;
}

private double balance;
private String name;
private int accountNo;
private static int lastAccountNo=0;

public int compareTo(BankAccount b) {

    if(this.name.compareTo(b.name) == 0 && this.balance > b.balance) return 0;
    else if(this.name.compareTo(b.name) < 0 && this.balance < b.balance) return 1;
    else if(this.name.compareTo(b.name) > 0 && this.balance == b.balance) return -1;

    //else if(this.name.compareTo(b.name) == 0) return 0;
    //else if(this.name.compareTo(b.name) < 0) return 1;
    //else if(this.name.compareTo(b.name) > 0) return -1;
    else if(this.balance == b.balance) return 0;
    else if(this.balance < b.balance) return 1;
    else if(this.balance > b.balance) return -1;
    else return 0;
}
}

银行账户测试器

import java.util.ArrayList;
import java.util.Collections;

class BankAccountTester {
    public static void main(String args[]) {
        ArrayList<BankAccount> accounts = new ArrayList<BankAccount>();
        accounts.add(new BankAccount("Louis Oliphant", 100.0));
        accounts.add(new BankAccount("Louis Oliphant", 100.10));
        accounts.add(new BankAccount("Louis Oliphant", 100.0));
        accounts.add(new BankAccount("Jane Doe", 100.0));
        accounts.add(new BankAccount("Jane Doe", 99.0));
        accounts.add(new BankAccount("Jane Doe", 100.0));
        System.out.println("*****Unsorted******");
        for (BankAccount b : accounts) {
            System.out.println(b);
        }
        System.out.println();
        Collections.sort(accounts);
        System.out.println("******Sorted******");
        for (BankAccount b : accounts) {
            System.out.println(b);
        }
    }
}

Answer 1

你应该重新考虑你的逻辑。如果两个银行账户的名字不一样，那么金额就没有必要比较了，不是吗？

public int compare(BankAccount o1, BankAccount o2) {
    int compareName = o1.getName().compareTo(o2.getName());
    if (compareName == 0) {

        // When the balance of the current amount is greater than the
        // balance of the compared account, then the current accounts
        // "precedes" the compared account, and thus -1 is returned.
        // If the balance is less, than this account "follows" the
        // compared account, and 1 is preceded.
        // Otherwise, 0 is returned.
        return (0 - Double.compare(o1.getBalance(), o2.getBalance()));
    }
    else {
        return compareName;
    }
}

或更短:

public int compare(BankAccount o1, BankAccount o2) {
    if (o1.getName().compareTo(o2.getName()) < 0) { return -1; }
    else if (o1.getName().compareTo(o2.getName()) > 0) { return 1; }
    return (0 - Double.compare(o1.getBalance(), o2.getBalance()));
}

正如其他人提到的，我建议使用分开 Comparator<T>作为 sort() 的参数方法，而不是实现 compareTo()接口(interface)方法Comparable 。实现compareTo()定义对象的“自然”顺序。您的排序看起来像是特定于该情况的排序，而不是自然排序。请参阅Java : Comparable vs Comparator .

顺便说一句，在处理货币值时，您应该使用 BigDecimal s 而不是 double s。请参阅Double vs. BigDecimal? .

<小时/>

注1:上面的代码块返回账户名相等时的字典序差异，参见 String.compareTo(String) ，也就是说，它们并不总是返回 -1 , 0或1 .

注释 2:当您还计划覆盖 equals(Object) 时和hashCode()方法，请记住，尽管不是严格要求x.compareTo(y) == 0，但强烈建议使用它。等于x.equals(y) ，参见 Comparable<T>.compareTo(T object) .