Modern Java

public class PageTreeFactory {

	public static Page loadPageTree(/*...*/) {
		// [...]
	}

}

public URL url();
public Exception error();
public String content();
public int issueNumber();
public int prNumber();
public Set<Page> links();

public Stats evaluateStatistics();
public String toPrettyString();

record ExternalPage(URI url, String content) { }

public record ErrorPage(
	URI url, Exception ex) { }

public record ExternalPage(
	URI url, String content) { }

public record GitHubIssuePage(
	URI url, String content,
	int issueNumber, Set<Page> links) { }

public record GitHubPrPage(
	URI url, String content,
	int prNumber, Set<Page> links) { }

public sealed interface Page
		permits ErrorPage, SuccessfulPage {
	URI url();
}

public sealed interface SuccessfulPage
		extends Page permits ExternalPage, GitHubPage {
	String content();
}

public sealed interface GitHubPage
		extends SuccessfulPage
		permits GitHubIssuePage, GitHubPrPage {
	Set<Page> links();
}

public Stats evaluateStatistics();
public String toPrettyString();

private void evaluatePage(Page page) {
	// `numberOf...` are fields
	switch (page) {
		case GitHubIssuePage issue -> numberOfIssues++;
		case GitHubPrPage pr -> numberOfPrs++;
		case ExternalPage ext -> numberOfExternals++;
		case ErrorPage err -> numberOfErrors++;
	}
}

private static String createPrettyString(Page page) {
	return switch (page) {
		case GitHubIssuePage issue
			-> "🐈 ISSUE #" + issue.issueNumber();
		case GitHubPrPage pr
			-> "🐙 PR #" + pr.prNumber();
		case ExternalPage ext
			-> "💤 EXTERNAL: " + ext.url().getHost();
		case ErrorPage err
			-> "💥 ERROR: " + err.url().getHost();
	};
}

public static String createPrettyString(Page page) {
	return switch (page) {
		case GitHubIssuePage(
				var url, var content,
				int issueNumber, var links)
			-> "🐈 ISSUE #" + issueNumber;
		case ErrorPage(var url, var ex)
			-> "💥 ERROR: " + url.getHost();
		// ...
	};
}

private static String createPrettyString(Page page) {
	return switch (page) {
		case GitHubIssuePage(_, _, int issueNumber, _)
			-> "🐈 ISSUE #" + issueNumber;
		case GitHubPrPage(_, _, int prNumber, _)
			-> "🐙 PR #" + prNumber;
		case ExternalPage(var url, _)
			-> "💤 EXTERNAL: " + url.getHost();
		case ErrorPage(var url, _)
			-> "💥 ERROR: " + url.getHost();
	};
}

record GitHubCommitPage(/*…*/) implements GitHubPage {

	// ...

}

public sealed interface GitHubPage
		extends SuccessfulPage
		permits GitHubIssuePage, GitHubPrPage,
				GitHubCommitPage {
	// [...]
}

private static String createPrettyString(Page page) {
	return switch (page) {
		case GitHubIssuePage issue -> // ...
		case GitHubPrPage pr -> // ...
		case ExternalPage external -> // ...
		case ErrorPage error -> // ...
		// missing case: GitHubCommitPage
	};
}

private static String createPrettyString(Page page) {
	return switch (page) {
		case GitHubIssuePage issue -> // ...
		case GitHubPrPage pr -> // ...
		case GitHubCommitPage page -> // ...
		case ExternalPage external -> // ...
		case ErrorPage error -> // ...
	};
}

int x = 0;
if (x instanceof byte b)
	IO.println(b + " in [-128, 127]");

int y = 16_777_217;
if (y instanceof float f)
	IO.println(f + " is a float (lossless)");

record Point(int x, int y) { }

var point = // ...
switch (point) {
	// primitive pattern
	case Point(var x, _) when x == 0 -> // ...
	// constant pattern (probably)
	case Point(0, _) -> // ...
}

// ↙ INTERFACE   ↙ state description
interface Point(int x, int y) {
	// state description implicitly requires:
	// int x();
	// int y();
}

// would allow
var point = // ...
switch (point) {
	case Point(var x, var y) -> // ...
}

// `Point` is a type with state description
Point nextPoint() { /* ... */ }

// if you only need `x`
Point(int x, _) = nextPoint();

record Point(int x, int y) { }

var p0 = new Point(0, 0);
var p1 = p0 with { x = 1; };