How to replace loop in loop in functional way?

I have code like this:

 List<Account> myAccounts = new ArrayList<>();

 List<String> numbers = new ArrayList<>();

 List<Account> resultList = new ArrayList<>();

        for (String number : numbers) {
            for (Account account : myAccounts) {
                if(number.equals(account.getNumber())){
                    resultList.add(account);
                }
            }
        }

I tried get speciefied accounts with specified numbers, but I do this via loop all numbers and compare to accounts numbers. How can I do this in functional style? Im not asking how to make it into a function, but how to get the same results without having to run two loops every time.

List<Account> myAccounts = new ArrayList<>();    
List<String> numbers = new ArrayList<>();    
List<Account> resultList = myAccounts.stream()
                .filter(account -> numbers.contains(account.getNumber()))
                .collect(Collectors.toList());

You can use this. only requirement is that you should be using at least Java8

Try it like this.

List<Account> results = numbers.stream().flatMap(numb -> myAccounts
                  .stream()
				  .filter(acct->numb.equals(acct.getNumber()))
                  .collect(Collectors.toList());

</details>



# 答案3
**得分**: -1

以下是翻译好的内容：

最佳实现方法是将您的账户对象加载到SQLite数据库中。（如果您已经有一个数据库，也可以将其添加到当前数据库中）

https://developer.android.com/training/data-storage/sqlite

一旦您将所有账户对象存储在数据库中，您只需编写一个查询来列出具有匹配号码的账户。

下面是一个更完整的示例，展示了从创建数据库到创建对象，最终到查询对象的整个过程。

https://www.androidauthority.com/creating-sqlite-databases-in-your-app-719366/

<details>
<summary>英文:</summary>

The best way to accomplish this would be to load your Account objects into a sql lite database.  ( Or add it to your current database if you already have one )

    https://developer.android.com/training/data-storage/sqlite

Then once you have all of the Account objects store in the database you could just write a query to list the accounts with matching numbers. 

Here is a more complete example on how to build everything from creating the database to creating the objects and eventually to querying for them.

    https://www.androidauthority.com/creating-sqlite-databases-in-your-app-719366/

</details>



# 答案4
**得分**: -2

用函数式方法，您可以将 for 循环构建为一个方法，如下所示：

```java
public void compareNumbers(List<String> numbers, List<Account> resultList) {
    numbers.forEach(number -> myAccounts.stream()
            .filter(account -> number.equals(account.getNumber()))
            .forEach(resultList::add));
}

public void compareNumbers(List<String> numbers, List<Account> resultList){
  for (String number : numbers) {
        for (Account account : myAccounts) {
            if(number.equals(account.getNumber())){
                resultList.add(account);
            }
   }
}