Dynamically cast to a proper subclass from parent class in a method

I have a Java interface PlatformConfigurable. I also have two classes PlatformProducerConfig and PlatformConsumerConfig.

Later on, I need to add a common config to both that sets a property to an empty string:

    private PlatformConfigurable disableHostNameVerificationConfig(PlatformConfigurable platformConfig) {
        if (platformConfig instanceof PlatformProducerConfig) {
            PlatformProducerConfig oldConfig = (PlatformProducerConfig) platformConfig;
            Map<String, String> additionalConfig = oldConfig.additionalProperties();
            Map<String, String> newConfig = new HashMap<>(Optional.ofNullable(additionalConfig).orElseGet(ImmutableMap::of));
            newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
            return oldConfig.toBuilder().additionalProperties(newConfig).build();
        }
        else if (platformConfig instanceof PlatformConsumerConfig) {
            PlatformConsumerConfig oldConfig = (PlatformConsumerConfig) platformConfig;
            Map<String, String> additionalConfig = platformConfig.additionalProperties();
            Map<String, String> newConfig = new HashMap<>(Optional.ofNullable(additionalConfig).orElseGet(ImmutableMap::of));
            newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
            return oldConfig.toBuilder().additionalProperties(newConfig).build();
        }
        return platformConfig;
    }

I am casting to producer or consumer config because the PlatformConfigurable interface doesn't have .toBuilder() or .build() methods declared in it, and I don't have access to modify the interface, as I can only implement it.

I would want to get rid of the duplicate code:

Map<String, String> additionalConfig = platformConfig.additionalProperties();
            Map<String, String> newConfig = new HashMap<>(Optional.ofNullable(additionalConfig).orElseGet(ImmutableMap::of));
            newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
            return oldConfig.toBuilder().additionalProperties(newConfig).build();

I was thinking of using lambdas, but I am not 100% sure how to do it.

You could just refactor existing code like this:

private PlatfromConfigurable disableHostNameVerificationConfig(Platfromonfigurable platfromConfig) {
if (!(platformConfig instanceof PlatformProducerConfig) && !(platformConfig instanceof PlatformConsumerConfig)) {
return platformConfig;
}
Map<String, String> additionalConfig = platformConfig.additionalProperties();
Map<String, String> newConfig = new HashMap<>(Optional.ofNullable(additionalConfig).orElseGet(ImmutableMap::of));
newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
if (platformConfig instanceof PlatformProducerConfig) {
return ((PlatformProducerConfig)platformConfig).toBuilder().additionalProperties(newConfig).build();
}
return ((PlatformConsumerConfig)platformConfig).toBuilder().additionalProperties(newConfig).build();
}

Update

Another approach could be to extract functionality related to the builder to separate interfaces and use them in this way:

// 1. extend existing `PlatformConfigurable`
public interface BuilderedPlatformConfigurable extends PlatformConfigurable {
ConfigPlatformBuilder toBuilder();
}
// 2. provide builder interface with common implementation
public interface ConfigPlatformBuilder {
Map<String, String> additionalProperties = new HashMap<>();
BuilderedPlatformConfigurable build();
default ConfigPlatformBuilder additionalProperties(Map<String, String> properties) {
this.additionalProperties.clear();
this.additionalProperties.putAll(properties);
return this;
}
}
// 3. update PlatformConsumerConfig class (similarly, PlatformProducerConfig)
public class PlatformConsumerConfig implements BuilderedPlatformConfigurable {
private Map<String, String> additionalProperties = new HashMap<>();
@Override
public Map<String, String> additionalProperties() {
return additionalProperties;
}
public ConfigPlatformBuilder toBuilder() {
return new Builder();
}
public static class Builder implements ConfigPlatformBuilder {
public PlatformConsumerConfig build() {
PlatformConsumerConfig config = new PlatformConsumerConfig();
config.additionalPropertie.putAll(this.additionalProperties);
return config;
}
}
}
// 4. provide overloaded method
private PlatformConfigurable disableHostNameVerificationConfig(PlatformConfigurable platformConfig) {
return platformConfig;
}
private PlatformConfigurable disableHostNameVerificationConfig(BuilderedPlatformConfigurable platformConfig) {
Map<String, String> additionalConfig = platformConfig.additionalProperties();
Map<String, String> newConfig = new HashMap<>(Optional.ofNullable(additionalConfig).orElseGet(Map::of));
newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
return platformConfig.toBuilder().additionalProperties(newConfig).build();
}

Taking Alex Rudenko's answer a bit further, using generics:

private <P extends PlatformConfigurable> P disableHostNameVerificationConfig(P platformConfig, BiFunction<P, Map<String, String>, P> appender) {
Map<String, String> additionalConfig = platformConfig.additionalProperties();
Map<String, String> newConfig = new HashMap<>(Optional.ofNullable(additionalConfig).orElseGet(ImmutableMap::of));
newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
return appender.apply(platformConfig, newConfig);
}

This assumes that it is safe to do this for any subtype of PlatformConfigurable (and PlatformConfigurable itself).

Then invoke like:

disableHostNameVerificationConfig(
platformProducerConfig,
(p, config) -> p.toBuilder().setAdditionalConfig(config).build());
disableHostNameVerificationConfig(
platformConsumerConfig,
(p, config) -> p.toBuilder().setAdditionalConfig(config).build());

If you like, create helper methods to hide the BiFunctions:

private PlatformProducerConfig disableHostNameVerificationConfig(PlatformProducerConfig config) {
return disableHostNameVerificationConfig(
platformConfigurable,
(p, config) -> p.toBuilder().setAdditionalConfig(config).build());
}
private PlatformConsumerConfig disableHostNameVerificationConfig(PlatformConsumerConfig config) {
return disableHostNameVerificationConfig(
platformConfigurable,
(p, config) -> p.toBuilder().setAdditionalConfig(config).build());
}

Actually, I think a better way to do it would be without generics or lambdas: write a method which creates an updated map:

private static Map<String, String> newConfig(PlatformConfigurable platformConfig) {
Map<String, String> additionalConfig = platformConfig.additionalProperties();
Map<String, String> newConfig = additionalConfig != null ? new HashMap<>(additionalConfig) : new HashMap<>();
newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
return newConfig;
}

and then just have two overloads:

private PlatformProducerConfig disableHostNameVerificationConfig(PlatformProducerConfig config) {
return config.toBuilder().setAdditionalConfig(newConfig(config)).build();
} 
private PlatformConsumerConfig disableHostNameVerificationConfig(PlatformConsumerConfig config) {
return config.toBuilder().setAdditionalConfig(newConfig(config)).build();
} 

Adding one thing in Alex Rudenko's answer, and making different function to add different implementations of interfaces.

private PlatformConfigurable disableHostNameVerificationConfig(PlatformConfigurable platformConfig) {
    if ((platformConfig == null)) {
        return platformConfig;
    }

    Map<String, String> additionalConfig = platformConfig.additionalProperties();
    Map<String, String> newConfig = new HashMap<>(Optional.ofNullable(additionalConfig).orElseGet(ImmutableMap::of));
    newConfig.put(SslConfigs.SSL_ENDPOINT_IDENTIFICATION_ALGORITHM_CONFIG, "");
    return PlatformConfigurableObject(platformConfig, newConfig);
}

So you can handle all instances in a different method, and whenever PlatfromXCofing classes are added later you only have to change this method. Single Responsibility Principle.

private PlatformConfigurable PlatformConfigurableObject(PlatformConfigurable platformConfig, Map<String, String> newConfig){
    if (platformConfig instanceof PlatformProducerConfig) {
        return ((PlatformProducerConfig)platformConfig).toBuilder().additionalProperties(newConfig).build();
    } else if (platformConfig instanceof PlatformConsumerConfig){
        return ((PlatformConsumerConfig)platformConfig).toBuilder().additionalProperties(newConfig).build();
    } else{
    return platformConfig;
    }
}