Refactor to allow testing side effects

By externalizing the source of i/o and randomness for shuffling, we can
inject non-interactive and deterministic behavior during unit tests.

10_Blackjack/java/src/UserIo.java

@@ -0,0 +1,102 @@
+import java.io.BufferedReader;
+import java.io.EOFException;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.io.Reader;
+import java.io.UncheckedIOException;
+import java.io.Writer;
+import java.util.stream.IntStream;
+
+/**
+ * This class is responsible for printing output to the screen and reading input
+ * from the user. It must be initialized with a reader to get input data from
+ * and a writer to send output to. Typically these will wrap System.in and
+ * System.out respectively, but can be a StringReader and StringWriter when
+ * running in test code.
+ */
+public class UserIo {
+
+    private BufferedReader in;
+    private PrintWriter out;
+
+	/**
+	 * Initializes the UserIo with the given reader/writer. The reader will be
+	 * wrapped in a BufferedReader and so should <i>not</i> be a BufferedReader
+	 * already (to avoid double buffering).
+	 * 
+	 * @param in Typically an InputStreamReader wrapping System.in or a StringReader
+	 * @param out Typically an OuputStreamWriter wrapping System.out or a StringWriter
+	 */
+    public UserIo(Reader in, Writer out) {
+        this.in = new BufferedReader(in);
+        this.out = new PrintWriter(out, true);
+    }
+
+	public void println(String text) {
+		out.println(text);
+	}
+
+	public void println(String text, int leftPad) {
+		IntStream.range(0, leftPad).forEach((i) -> out.print(' '));
+		out.println(text);
+	}
+
+	public void print(String text) {
+		out.print(text);
+		out.flush();
+	}
+
+	private String readLine() {
+		try {
+			String line = in.readLine();
+			if(line == null) {
+				throw new UncheckedIOException("!END OF INPUT", new EOFException());
+			}
+			return line;
+		} catch (IOException e) {
+			throw new UncheckedIOException(e);
+		}
+	}
+
+	/**
+	 * Prompts the user for a "Yes" or "No" answer.
+	 * @param prompt The prompt to display to the user on STDOUT.
+	 * @return false if the user enters a value beginning with "N" or "n"; true otherwise.
+	 */
+	public boolean promptBoolean(String prompt) {
+		print(prompt + "? ");
+
+		String input = readLine();
+
+		if(input.toLowerCase().startsWith("n")) {
+			return false;
+		} else {
+			return true;
+		}
+	}
+
+	/**
+	 * Prompts the user for an integer.  As in Vintage Basic, "the optional
+	 * prompt string is followed by a question mark and a space." and if the
+	 * input is non-numeric, "an error will be generated and the user will be
+	 * re-prompted.""
+	 *
+	 * @param prompt The prompt to display to the user.
+	 * @return the number given by the user.
+	 */
+	public int promptInt(String prompt) {
+		print(prompt + "? ");
+
+		while(true) {
+			String input = readLine();
+			try {
+				return Integer.parseInt(input);
+			} catch(NumberFormatException e) {
+				// Input was not numeric.
+				println("!NUMBER EXPECTED - RETRY INPUT LINE");
+				print("? ");
+				continue;
+			}
+		}
+	}
+}