Spring boot hand over specific parameter to another api

I want to save the parameters of a Json Response from an API to hand over to another API. The other API should receive account balance and would say if the customer has a positive or negative balance.

JSON:

id:	       1
name:	   "Lars"
lastname:  "Günther"
birth:     "1995-10-10"
sex:       "m"
road:      "Bahnhofsstrasse 17"
town:      "Borken"
account:	

 0:	
    bankaccNr:	      3
    customerstatus:	  "private client"
    accountbalance:   -3127
    openingdate:      "2001-01-23"
    iban:	          "DE37500105174133576347"
  
 1:	
    bankaccNr:	      16
    customerstatus:	  "corporate client"
    accountbalance:   71764
    openingdate:      "2014-07-09"
    iban:	          "DE09500105177725527397"

postalcode:	46325

That is the JSON I get when I call this API. 0 and 1 are not part of the model. They are automatically generated when this to accounts get mapped to the customer.

Customer:

@ManyToMany(targetEntity = Account.class, mappedBy = "customer", cascade = CascadeType.ALL)
private Set<Account> account= new HashSet<>();
	
public Customer() {}
	
public Customer(String name, String lastname, Date birth, char sex, String road, int postalcode, String town) {
    this.name=name;
	this.lastname=lastname;
	this.birth=birth;
	this.sex=sex;
	this.road=road;
	this.postalcode=postalcode;
	this.town=town;			
}
//getters/setters...

Should I save it as an object or as a hashmap. How do I handle this object? What If a customer has more than one account?

@RequestMapping(value = { "/{id}/liquid" }, method = RequestMethod.GET)
public Konto getLiquid(@PathVariable(value = "id") Long customer_id){   
		
	String url = "http://localhost:8081/mybank/customer/data/{id}";
	Customer customer= restTemplate.getForObject(url, Kunde.class,kundeid);
	'object or hashmap?' 
	HashMap <String,Map> answer= restTemplate.getForObject(url, new HashMap<>().getClass(),kundeid);
	        
	String urlForServiceB = "http://localhost:8082/liquicontroller/liquiditat/{value}";
	    
    int firstvalue = ???
    // get http response --> true or false
	return restTemplate.getForObject(urlForServiceB, String.class,firstvalue);
}

How can I send multiple requests to the other API?

Customer should be having a list of account. If according to your business requirement if it many to many mapping then Account should be having list of customers.

From Above it looks like each customer having many accounts so we can have list of account in customer

@OneTOManyMapping  
List<Account> account

Resttemplate will map the json from entity to your customer.

List<Account> account = customer.getlistOfAccounts

for(Account ac:account){
// ac will have each account object

For above to work response should contain nested list within customer. If there is no such case then below has to be invoked again and again for each account id

RestTemplate restTemplate = new RestTemplate();
Customer Customer = restTemplate.getForObject(uri, Customer.class);