用另一个流中的字符串替换一个流的字符串

Question

Fun with streams... I have a List<String> of tokenized URLs, and a List<UrlParametersConfig.UrlParameter> of all the parameter values for those tokens. For example, my yaml file looks like:

urlParameterList:
  urlParameter:
  -
    parameter: propertyId
    value: c3329fc
 -
    parameter: cartId
    value: 123456
 ...

And the corresponding config class looks like this:

@Configuration
@ConfigurationProperties(prefix="urlParameterList")
public class UrlParametersConfig {

    private List<UrlParameter> urlParameters = new ArrayList<>();

    public UrlParametersConfig() {}

    public List<UrlParameter> getUrlParameter() { return this.urlParameters; }

    public static class UrlParameter {
        private String parameter;
        private String value;

        public String getParameter() {
            return parameter;
        }
        public void setParameter(String parameter) {
            this.parameter = parameter;
        }
        public String getValue() {
            return value;
        }
        public void setValue(String value) {
            this.value = value;
        }
    }
}

So I have a list of URLs that contains something like www.comany.com/v2/rest/v2/property/{propertyId} www.comany.com/v2/rest/v2/sale/{propertyId}/{cartId}

Everywhere {propertyId} is referenced I need to substitute c3329fc , and everywhere {cartId} is referenced I need to substitute 123456a . I figured out I'd be working with two streams, but I don't know which to stream first, the list of all URLs or the list of config parameters. Can anyone show me what this token replacement code should look like? So far this is what I have:

List<String> parameterizeUrls = uris.stream()
        .forEach(it -> {
            if (it.contains(parameters.stream()
                  .map(UrlParametersConfig.UrlParameter::getParameter)
                  .collect(Collectors.toList()))
   // do something?
        });

Answer 1

If I understood correclty the idea, we want to obtain the following:

www.comany.com/v2/rest/v2/property/c3329fc
www.comany.com/v2/rest/v2/sale/c3329fc/123456

One stream is necessary to loop on the list of original url s with tokens. For each url , another stream will loop on the list of parameters, executing reduction of the url for each parameter.

To be more clear and to avoid too much nesting, the algo is split in the following parts:

1) Main loop, transforming the original list of urls with tokens into urls with replaced values:

List<String> urlsTokenReplaced = urls.stream().map(url ->
  replaceTokens(url, params)
).collect(Collectors.toList());

2) For the specified url , replaces the occurrences of each token from the list.

private static String replaceTokens(String url, List<UrlParametersConfig.UrlParameter> params) {
  return params.stream()
               .reduce(url, {ClassName}::replaceToken, (prev, cur) -> cur);
}

Update

Details on how the reduction operation works: Java tutorials .

For this particular case:

• url param is the starting value of the loop, for example www.comany.com/v2/rest/v2/sale/{propertyId}/{cartId} .

• {ClassName}::replaceToken references the static method below, it is the so-called accumulator function, which takes the previous (or starting) value and produces a new value by applying the current UrlParameter on it. In our case, on the first pass it will produce www.comany.com/v2/rest/v2/sale/c3329fc/{cartId} and www.comany.com/v2/rest/v2/sale/c3329fc/123456 on the second.

• (prev, cur) -> cur) is the so-called collector function. It defines what to do with the prev ious and the cur rent (produced by the accumulator) values. Here we just forget about the previous value and return the current, which will be reused by the accumulator on next iteration of the loop and so on. In our case, on the first pass it will return the second value between www.comany.com/v2/rest/v2/sale/{propertyId}/{cartId} and www.comany.com/v2/rest/v2/sale/c3329fc/{cartId} . On the second pass, it will return the second value from www.comany.com/v2/rest/v2/sale/c3329fc/{cartId} and www.comany.com/v2/rest/v2/sale/c3329fc/123456 .

When all the UrlParameter s from the stream are passed through, the last value produced by the accumulator (and thus the collector here) is returned from the method. If the stream were empty, the starting value (first arg) would be returned.

enf-of-update

3) Replaces a single occurrence of the specified token in the url :

private static String replaceToken(String url, UrlParametersConfig.UrlParameter param) {
  String replaceRegex = "\\{" + param.getParameter() + "\\}";
  return url.replaceAll(replaceRegex, param.getValue());
}