java - 检查 arraylist 是否有值

Question

我正在尝试解决我的问题，我必须查看在将值输入到 arraylist 时是否已添加此值，如果是，我想沿名称行显示一条消息已存在

import java.util.*;

class Bank{

void command(){
 CommandReader commandReader=new CommandReader(this);
 commandReader.run();
}
ArrayList <Account> accounts=new ArrayList <Account>();

public static void main(String[] args){
 new Bank().command();

}
void print(String name){

System.out.println("Account of "+name);
int b=0;
for(int i=0; i<accounts.size(); i++){ 
  b=accounts.get(i).balance(name);
  break;
  } 
System.out.println("Balance: "+b);//dit moet naar account verwjizen op een of andere      manier zodat hij weet waar hij balance() moet zoeken
 }
void printAll(){

/*
for(int i =0; i<accounts.size()-1;i++){
 print(accounts[i].name);//klopt niet, weet niet wat er wel moet staan
}*/
}
void enroll(String name){
Account account=new Account(name,0);
accounts.add(account);
}
void deposit(String name, int amount){
for(int i=0; i<accounts.size(); i++){
  accounts.get(i).deposit(amount, name);
  } 
}


 void withdraw(String name, int amount){//hiervoor moet je denk ik een methode in account maken zodat e met balance kan rekenen?
for(int i=0; i<accounts.size(); i++){
  int x;
  x=accounts.get(i).balance(name);
  if (x>-1000){
  amount = amount*-1;
   accounts.get(i).deposit(amount, name);

  } 
}
}
  void printRed(){

}
void interest(double rate){
for(int i=0; i<accounts.size(); i++){
  accounts.get(i).inter(rate);
}
 }   

}

class Account{
String name;
int balance;

void inter(double rate){
 balance+=rate
   /100*balance;
}

Account(String n, int b){//weet niet of dit nodig is, denk het wel
 this.name=n;
 this.balance=b;
}
String name(){
  return this.name;      
} 
int balance(String name){
  if(name.equals(this.name)){
    return this.balance;
  }
  return 0;
}
void deposit(int b, String name){
  if(name.equals(this.name)){
  this.balance+=b;
  }
}
 }

是我通过方法将值添加到数组中的方法。（零可以忽略）

Answer 1

你可以通过两种方式做到这一点：

1. 自己写一个对比。然后遍历所有数组元素以验证对象是否已经存在。

2. 让您的Account类覆盖equals和hashCode方法，然后用于List#contains验证对象是否已存在于列表中。

   一个简短的例子：

   Account account = new Account(name,0);
   //this assumes that Account class has already overridden equals and hashCode methods
   if (!accounts.contains(account)) {
       accounts.add(account);
   }
   

如果您要使用后者，最好使用 aSet而不是 aList因为Sets 已经处理该对象在集合中应该是唯一的。

---

根据您的编辑，当前的错误是Accountclass must overrideequals和hashCodemethods。执行此操作时，代码将按预期运行。

为两者提供基本实现：

class Account {
    String name;
    int balance;

    void inter(double rate) {
        balance+=rate
       /100*balance;
    }

    Account(String n, int b) {
        this.name=n;
        this.balance=b;
    }
    String name() {
        return this.name;
    }
    int balance(String name) {
        if(name.equals(this.name)) {
            return this.balance;
        }
        return 0;
    }
    void deposit(int b, String name){
        if(name.equals(this.name)){
            this.balance+=b;
        }
    }
    //from here, this is the code you need
    @Override
    public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        if (o instanceof Account) {
            Account another = (Account)o;
            return this.name.equals(another.name);
        }
        return false;
    }
    @Override
    public int hashCode() {
        //if working with Java 7, uncomment below LoC
        //return java.util.Objects.hash(name, balance);
        //if using Java SE 6 or less
        return java.util.Arrays.hashCode(new Object[]{ name, balance });
    }
}

Answer 2

覆盖equals()and hashcode(), 使用Set而不是它的add()返回boolean取决于它是否被忽略