Ways (Data Structure) to store and query java beans In Memory?

This is probably a problem that is already solved. But I can't seem to stumble upon the answer. Consider I have 2 beans that are relational. And Corresponding Daos. The trouble is finding all the transactions for a particular account.

Account bean:

public class Account {
    private String accountNumber;
    private String accountHolder;
    // other properties, constructor, getter and setter methods
}

Transaction bean:

public class Transaction {
    private String transactionId;
    private String accountNumber;
    // other properties, constructors, getter and setters.
}

Find all account method in Account Dao:

public class InMemoryAccountDao implements AccountDao {

    public static Map<Integer, Account> accountStore = new HashMap<>();

    // create, update, delete and find by Id methods.

    public Collection<Account> findAllAccounts() {
        return accountStore.values();
    }

}

Find all transactions for an account in Transaction Dao:

public class InMemoryTransactionDao implements TransactionDao {

    public static Map<Integer, Transaction> transactionStore = new HashMap<>();

    // create, update, delete and find by Id methods.

    public Collection<Transaction> findAllTransactions(Account account) {
        Iterator<Transaction> transactionIterator = transactionStore.values().iterator();
        Collection<Transaction> transactionsThisAccount = new ArrayList<>();
        Transaction tran = null;
        while(transactionIterator.hasNext()) {
            tran = transactionIterator.next();
            if(tran.getAccountNumber() == account.getAccountNumber())
                transactionsThisAccount.add(tran);
        }
        return transactionsThisAccount;
    }

}

As you see, I am having to iterate over every transaction and check the account number, with the account number in question. Is there an efficient way of storing and 'querying'? What data structure should I use? If there is a design pattern for relational beans please share me the name/link or any details.