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Calling multiple external APIs in Spring Boot

I am working on a project but it requires me to call multiple external APIs. I basically have to call an API to get a player id by giving a name. Then use that player id to get a list of match ids. Then make calls for each match id to get details on each match. its alot and doesnt seem optimal but its the only way to do it. I was going to use rest template to make a call to the following
https://americas.api.riotgames.com/lol/match/v5/matches/by-puuid/HDzjdaStxhHcceGGd8qJcc4Vw45FOlOQ1PNXKQ0h9_iqfwHP3oI0spl1bLUOw_7_J49vzaIKylv5Vg/ids?start=0&count=20
I have to pass in headers as well such as
riot token : token
"Origin": "https://developer.riotgames.com"
I was wondering how I can do this in Java Spring boot. I saw RestTemplate would be used but I couldnt figure out how to include the headers. Any guidance would be appreciated.

You can call RestTemplate.exchange() using either the method signature with RequestEntity or with HttpEntity.
// Using RequestEntity
RequestEntity<?> request= RequestEntity.get(url).header(headerName, headerValue).build();
ResponseEntity<String> response = restTemplate.exchange(request, String.class);
// Using HttpEntity
HttpHeaders headers = new HttpHeaders();
headers.set(headerName, headerValue);
HttpEntity httpEntity = new HttpEntity(/* this is nullable */ requestBody, headers);
ResponseEntity,String> response = restTemplate.exchange(url, HttpMethod.GET, httpEntity, String.class);
Would also recommend reading through the following:
https://howtodoinjava.com/spring-boot2/resttemplate/spring-restful-client-resttemplate-example/
https://www.baeldung.com/rest-template
How to set an "Accept:" header on Spring RestTemplate request?

Receive Spring ResponseEntity with and without body

How to implement a REST call which can sometimes return no body?
My SpringBoot application calls an external service over REST HTTP and its implemented via the org.springframework.web.client Client and method public <T> ResponseEntity<T> exchange.
The client received till now always a String body -> ResponseEntity<String>. Some time ago the service we are calling returns us HTTP 202 without body, so the following exception is thrown:
java.lang.IllegalArgumentException: argument "content" is null.
How can I tell Spring to ignore the body for a 202 status code?

You can use Void as the generic type parameter for ResponseEntity if the service does not return a response body:
ResponseEntity<Void> response = restTemplate.exchange(url, HttpMethod.POST, requestEntity, Void.class)
If the service returns a response sometimes, you could handle the response as a String. For empty responses, you would receive an empty string. For non-empty responses, you would need to deserialize the returned payload yourself.
ResponseEntity<String> response = restTemplate.exchange(url, HttpMethod.POST, requestEntity, String.class)
if (Strings.isEmpty(res.getBody())) {
// handle empty response
} else {
// handle non-empty response
}

What is the purpose of the exchange method in the RestTemplate?

I am currently sending a resource to a client, I am using code that has been done already and I am modifying it, there is a line shown below in this code that I don't understand. Well I understand that I am sending or posting a resource, I understand this method takes the url of the client, that it takes the type of HTTP request for example in this case POST, but I dont understant why this method takes nService.getStringHttpEntityWithPayload(payLoad) and Resource.class? Also the response entity it is returning will it be a class only or a class with a status and a headers?
ResponseEntity<Resource> responseEntity = restTemplate.exchange(
eURL,
HttpMethod.POST,
nService.getStringHttpEntityWithPayload(payLoad),
Resource.class);

why this method takes nService.getStringHttpEntityWithPayload(payLoad) and Resource.class?
The method getStringHttpEntityWithPayload is returning a HttpEntity which is composed of a body and header data to be sent to a URL. The method is creating the request message by adding the content type header, letting the receiving service know that the body contains JSON data.
The parameter Resource.class is used to determine what class to deserialize the response body from the service into. It defines the generic type of the return value: ResponseEntity<Resource>.
Also the response entity it is returning will it be a class only or a class with a status and a headers?
I'm not sure what you mean by "class only". The ResponseEntity is similar to HttpEntity (in fact class ResponseEntity<T> extends HttpEntity<T>). The ResponseEntity class contains the response body and headers, as well as the HTTP Status code of the response.

RestTemplate not working with APPLICATION_FORM_URLENCODED and Request Pojo

I am using restTemplate to consume a service.
HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
HttpEntity request = new HttpEntity(countryRequest, headers);
CountryResponse response = restTemplate.postForObject(countryURL, request, CountryResponse.class);
countryRequest is a object of a POJO with just a string field code.
restTemplate has jackson2HttpMessageConverter and FormHttpMessageConverter in messageConverters.
I am getting the following exception :
org.springframework.web.client.RestClientException:
Could not write request: no suitable HttpMessageConverter found for request type [CountryRequest] and content type [application/x-www-form-urlencoded]
But if I use MultiValueMap instead of CountryRequest, I got the 200 response:
MultiValueMap<String, String> map= new LinkedMultiValueMap<>();
map.add(code, "usa");
HttpEntity<MultiValueMap<String, String>> request = new HttpEntity(map, headers);
Is there any way to replace the MultiValueMap approach here?

There is two main kinds of request serialization: as usual FORM data or as JSON object.
Form Data is simplest and oldest way which sends a simple key-value pairs of strings in POST payload. But when you need to send some object with nested properties or some list or even map then it becomes a problem.
That's why everybody tries to use a JSON format which can be more easily deserialized into POJO object. And this is a de facto standard for modern web.
So in your case for some reason RestTemplate tries to serialize the CountryRequest but it don't know how to serialize it into FORM data.
Try to replace the request with a pojo that you are sending:
CountryRequest request = new CountryRequest();
CountryResponse response = restTemplate.postForObject(countryURL, request, CountryResponse.class);
Then RestTemplate tries to serialize the CountryRequest into JSON (which is default behavior).

Spring RestTemplate post response

I'm not familiar with Spring RestTemplate.
But for this project I have to use Spring RestTemplate to send a POST call to consume a rest api.
I'm using this code:
String restCall = restTemplate.postForObject(url+restParm, null, String.class);
This is working fine.
I would like to retriveve the HTTP status code (E.g: 200 OK.). How could I do that ?
Thanks.

You use the postForEntity method as follows...
ResponseEntity<String> response = restTemplate.postForEntity(url+restParm, null, String.class);
HttpStatus status = response.getStatusCode();
String restCall = response.getBody();

It will be pretty weird if RestTemplate couldn't get the response,as others have suggested. It is simply not true.
You just use the postForEntity method which returns a
http://static.springsource.org/spring/docs/3.0.x/javadoc-api/org/springframework/http/ResponseEntity.html
And as the documentation suggests, the response entity has the status.