Add serial monitor command history

The behavior is as follows:
- Pressing the UP key will select older commands.
- Pressing the DOWN key will select newer commands, restoring the last unexecuted command if available.
- Pressing the ESC key will reset the input field to the latest unexecuted command and reset the traversal location. Pressing ESC while the latest unexecuted command is selected will clear the input field.

This fixes arduino#4891, with the difference that the proposed solution has a command history per Arduino session and this implementation has a command history per serial monitor start.

app/src/processing/app/CommandHistory.java

@@ -0,0 +1,118 @@
+package processing.app;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Keeps track of command history in console-like applications.
+ * @author P.J.S. Kools
+ */
+public class CommandHistory {
+
+  private List<String> commandHistory = new ArrayList<String>();
+  private int selectedCommandIndex = 0;
+  private final int maxHistorySize;
+
+  /**
+   * Create a new {@link CommandHistory}.
+   * @param maxHistorySize - The max command history size.
+   */
+  public CommandHistory(int maxHistorySize) {
+    this.maxHistorySize = (maxHistorySize < 0 ? 0 : maxHistorySize);
+    this.commandHistory.add(""); // Current command placeholder.
+  }
+
+  /**
+   * Adds the given command to the history and resets the history traversal
+   * position to the latest command. If the max history size is exceeded,
+   * the oldest command will be removed from the history.
+   * @param command - The command to add.
+   */
+  public void addCommand(String command) {
+
+    // Remove the oldest command if the max history size is exceeded.
+    if(this.commandHistory.size() >= this.maxHistorySize + 1) {
+      this.commandHistory.remove(0);
+    }
+
+    // Add the new command, reset the 'current' command and reset the index.
+    this.commandHistory.set(this.commandHistory.size() - 1, command);
+    this.commandHistory.add(""); // Current command placeholder.
+    this.selectedCommandIndex = this.commandHistory.size() - 1;
+  }
+
+  /**
+   * Gets whether a next (more recent) command is available in the history.
+   * @return {@code true} if a next command is available,
+   * returns {@code false} otherwise.
+   */
+  public boolean hasNextCommand() {
+    return this.selectedCommandIndex + 1 < this.commandHistory.size();
+  }
+
+  /**
+   * Gets the next (more recent) command from the history.
+   * @return The next command or {@code null} if no next command is available.
+   */
+  public String getNextCommand() {
+    return this.hasNextCommand()
+        ? this.commandHistory.get(++this.selectedCommandIndex) : null;
+  }
+
+  /**
+   * Gets whether a previous (older) command is available in the history.
+   * @return {@code true} if a previous command is available,
+   * returns {@code false} otherwise.
+   */
+  public boolean hasPreviousCommand() {
+    return this.selectedCommandIndex > 0;
+  }
+
+  /**
+   * Gets the previous (older) command from the history.
+   * When this method is called while the most recent command in the history is
+   * selected, this will store the current command as temporary latest command
+   * so that {@link #getNextCommand()} will return it. This temporary latest
+   * command gets reset when this case occurs again or when
+   * {@link #addCommand(String)} is invoked.
+   * @param currentCommand - The current unexecuted command.
+   * @return The previous command or {@code null} if no previous command is
+   * available.
+   */
+  public String getPreviousCommand(String currentCommand) {
+
+    // Return null if there is no previous command available.
+    if (!this.hasPreviousCommand()) {
+      return null;
+    }
+
+    // Store current unexecuted command if not traversing already.
+    if (this.selectedCommandIndex == this.commandHistory.size() - 1) {
+      this.commandHistory.set(this.commandHistory.size() - 1,
+          (currentCommand == null ? "" : currentCommand));
+    }
+
+    // Return the previous command.
+    return this.commandHistory.get(--this.selectedCommandIndex);
+  }
+
+  /**
+   * Resets the history location to the most recent command.
+   * @returns The latest unexecuted command as stored by
+   * {@link #getPreviousCommand(String)} or an empty string if no such command
+   * was set.
+   */
+  public String resetHistoryLocation() {
+    this.selectedCommandIndex = this.commandHistory.size() - 1;
+    return this.commandHistory.set(this.commandHistory.size() - 1, "");
+  }
+
+  /**
+   * Clears the command history.
+   */
+  public void clear() {
+    this.commandHistory.clear();
+    this.commandHistory.add(""); // Current command placeholder.
+    this.selectedCommandIndex = 0;
+  }
+}

app/src/processing/app/SerialMonitor.java

@@ -23,6 +23,9 @@
 
 import java.awt.Color;
 import java.awt.event.ActionEvent;
+import java.awt.event.ActionListener;
+import java.awt.event.KeyAdapter;
+import java.awt.event.KeyEvent;
 
 import static processing.app.I18n.tr;
 
@@ -32,6 +35,10 @@ public class SerialMonitor extends AbstractTextMonitor {
   private Serial serial;
   private int serialRate;
 
+  private static final int COMMAND_HISTORY_SIZE = 100;
+  private final CommandHistory commandHistory =
+      new CommandHistory(COMMAND_HISTORY_SIZE);
+
   public SerialMonitor(BoardPort port) {
     super(port);
 
@@ -54,11 +61,42 @@ public SerialMonitor(BoardPort port) {
     });
 
     onSendCommand((ActionEvent event) -> {
-      send(textField.getText());
+      String command = textField.getText();
+      send(command);
+      commandHistory.addCommand(command);
       textField.setText("");
     });
-    
+
     onClearCommand((ActionEvent event) -> textArea.setText(""));
+
+    // Add key listener to UP, DOWN, ESC keys for command history traversal.
+    textField.addKeyListener(new KeyAdapter() {
+      @Override
+      public void keyPressed(KeyEvent e) {
+        switch (e.getKeyCode()) {
+
+          // Select previous command.
+          case KeyEvent.VK_UP:
+            if (commandHistory.hasPreviousCommand()) {
+              textField.setText(
+                  commandHistory.getPreviousCommand(textField.getText()));
+            }
+            break;
+
+          // Select next command.
+          case KeyEvent.VK_DOWN:
+            if (commandHistory.hasNextCommand()) {
+              textField.setText(commandHistory.getNextCommand());
+            }
+            break;
+
+          // Reset history location, restoring the last unexecuted command.
+          case KeyEvent.VK_ESCAPE:
+            textField.setText(commandHistory.resetHistoryLocation());
+            break;
+        }
+      }
+    });
   }
 
   private void send(String s) {