Url Parsing in Karel

Here’s the translation of the Go URL parsing example to Java, formatted in Markdown suitable for Hugo:

import java.net.URI;
import java.net.URISyntaxException;
import java.util.Map;

public class URLParsing {
    public static void main(String[] args) throws URISyntaxException {
        // We'll parse this example URL, which includes a
        // scheme, authentication info, host, port, path,
        // query params, and query fragment.
        String s = "postgres://user:pass@host.com:5432/path?k=v#f";

        // Parse the URL and ensure there are no errors.
        URI uri = new URI(s);

        // Accessing the scheme is straightforward.
        System.out.println(uri.getScheme());

        // UserInfo contains all authentication info; we need to split it
        // to get individual username and password.
        System.out.println(uri.getUserInfo());
        String[] userInfo = uri.getUserInfo().split(":");
        System.out.println(userInfo[0]);
        System.out.println(userInfo[1]);

        // The Host contains both the hostname and the port,
        // if present. We can access them separately.
        System.out.println(uri.getHost());
        System.out.println(uri.getPort());

        // Here we extract the path and the fragment after
        // the #.
        System.out.println(uri.getPath());
        System.out.println(uri.getFragment());

        // To get query params in a string of k=v format,
        // use getRawQuery(). You can also parse query params
        // into a map using a custom function.
        System.out.println(uri.getRawQuery());
        Map<String, String> queryParams = splitQuery(uri);
        System.out.println(queryParams);
        System.out.println(queryParams.get("k"));
    }

    // Helper method to parse query parameters into a Map
    public static Map<String, String> splitQuery(URI uri) {
        if (uri.getQuery() == null || uri.getQuery().isEmpty()) {
            return java.util.Collections.emptyMap();
        }
        return java.util.Arrays.stream(uri.getQuery().split("&"))
                .map(param -> param.split("="))
                .collect(java.util.stream.Collectors.toMap(
                        param -> param[0],
                        param -> param.length > 1 ? param[1] : ""
                ));
    }
}

This Java program demonstrates URL parsing using the java.net.URI class. Here’s a breakdown of what it does:

We start by importing necessary classes from java.net and java.util.
In the main method, we define a sample URL string containing various components like scheme, authentication info, host, port, path, query parameters, and fragment.
We create a URI object by parsing this string. If there’s an error in parsing, it will throw a URISyntaxException.
We then extract various components of the URL using methods provided by the URI class:
- getScheme() for the scheme
- getUserInfo() for authentication info (which we then split to get username and password)
- getHost() and getPort() for the host and port
- getPath() for the path
- getFragment() for the fragment
- getRawQuery() for the raw query string
To parse query parameters into a Map, we define a helper method splitQuery(). This method uses Java streams to split the query string and create a map of parameter names to values.
Finally, we print out all these components.

When you run this program, it will output each component of the URL, similar to the output shown in the original example.

Note that Java’s URI class doesn’t provide direct methods for parsing query parameters into a map, so we’ve implemented this functionality ourselves. Also, unlike the Go example, Java’s URI class doesn’t separate the host and port automatically, so we use separate methods to access these.

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