Script Interpreter: keep a record of a 'current command'

The 'current command' is the command that is not yet in the history. It
is very convenient to be able to go back in history, just to have a
look, before continuing to craft the current command (and it would be
annoying if it was lost when going back in history). Of course, if the
user decides to execute a different command from the history instead,
the edits to the 'current command' are lost.

This behavior is most in line with the Unix shell behavior power users
might be used to.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

src/main/java/org/scijava/script/History.java

@@ -49,6 +49,7 @@ class History {
 	private final PrefService prefs;
 	private final String name;
 	private final LastRecentlyUsed<String> entries = new LastRecentlyUsed<String>(MAX_ENTRIES);
+	private String currentCommand = "";
 	private int position = -1;
 
 	/**
@@ -88,10 +89,14 @@ public void write() {
 	public void add(final String command) {
 		entries.add(command);
 		position = -1;
+		currentCommand = "";
 	}
 
 	public boolean replace(final String currentCommand) {
-		if (position < 0) return false;
+		if (position < 0) {
+			this.currentCommand = currentCommand;
+			return false;
+		}
 		return entries.replace(position, currentCommand);
 	}
 
@@ -106,7 +111,7 @@ public boolean replace(final String currentCommand) {
 	 */
 	public String next() {
 		position = entries.next(position);
-		return position < 0 ? null : entries.get(position);
+		return position < 0 ? currentCommand : entries.get(position);
 	}
 
 	/**
@@ -120,7 +125,7 @@ public String next() {
 	 */
 	public String previous() {
 		position = entries.previous(position);
-		return position < 0 ? null : entries.get(position);
+		return position < 0 ? currentCommand : entries.get(position);
 	}
 
 	@Override