Terminal handles the form feed and backspace character in a proper way. See #171

src/main/java/de/neemann/digital/gui/components/terminal/CharDeleter.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2018 Helmut Neemann.
+ * Use of this source code is governed by the GPL v3 license
+ * that can be found in the LICENSE file.
+ */
+package de.neemann.digital.gui.components.terminal;
+
+/**
+ * Used to delete a char in a string and keep track of the cursor position
+ */
+public class CharDeleter {
+    private String text;
+    private int pos;
+
+    /**
+     * Creaztes a new instance
+     *
+     * @param text the text
+     * @param pos  the cursor position
+     */
+    public CharDeleter(String text, int pos) {
+        this.text = text;
+        this.pos = pos;
+    }
+
+    /**
+     * @return the text
+     */
+    public String getText() {
+        return text;
+    }
+
+    /**
+     * @return the cursor position
+     */
+    public int getPos() {
+        return pos;
+    }
+
+    /**
+     * deletes the last char
+     *
+     * @return this for chained calls
+     */
+    public CharDeleter delete() {
+        final int len = text.length();
+        if (len > 0) {
+            char last = text.charAt(len - 1);
+            text = text.substring(0, len - 1);
+            pos--;
+
+            if (last == '\n') {
+                pos = 0;
+                int p = len - 2;
+                while (p >= 0 && text.charAt(p) != '\n') {
+                    pos++;
+                    p--;
+                }
+            }
+        }
+        return this;
+    }
+}

src/main/java/de/neemann/digital/gui/components/terminal/TerminalDialog.java

@@ -66,16 +66,28 @@ public class TerminalDialog extends JDialog {
      * @param value the character
      */
     public void addChar(char value) {
-        if (value == 13 || value == 10) {
-            pos = 0;
-            textArea.append("\n");
-        } else {
-            textArea.append("" + value);
-            pos++;
-            if (pos == width) {
+        switch (value) {
+            case 13:
+            case 10:
                 pos = 0;
                 textArea.append("\n");
-            }
+                break;
+            case 8:
+                CharDeleter cd = new CharDeleter(textArea.getText(), pos).delete();
+                textArea.setText(cd.getText());
+                pos = cd.getPos();
+                break;
+            case 12:
+                pos = 0;
+                textArea.setText("");
+                break;
+            default:
+                textArea.append("" + value);
+                pos++;
+                if (pos == width) {
+                    pos = 0;
+                    textArea.append("\n");
+                }
         }
     }
 }

src/test/java/de/neemann/digital/gui/components/terminal/CharDeleterTest.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2018 Helmut Neemann.
+ * Use of this source code is governed by the GPL v3 license
+ * that can be found in the LICENSE file.
+ */
+package de.neemann.digital.gui.components.terminal;
+
+import junit.framework.TestCase;
+
+public class CharDeleterTest extends TestCase {
+
+    public void testSimple() {
+        CharDeleter cd = new CharDeleter("test", 4).delete();
+        assertEquals("tes", cd.getText());
+        assertEquals(3, cd.getPos());
+    }
+
+    public void testLineFeed1() {
+        CharDeleter cd = new CharDeleter("\n\n", 0).delete();
+        assertEquals("\n", cd.getText());
+        assertEquals(0, cd.getPos());
+    }
+
+    public void testLineFeed2() {
+        CharDeleter cd = new CharDeleter("\ntest\n", 0).delete();
+        assertEquals("\ntest", cd.getText());
+        assertEquals(4, cd.getPos());
+    }
+
+    public void testLineFeed3() {
+        CharDeleter cd = new CharDeleter("test\n", 0).delete();
+        assertEquals("test", cd.getText());
+        assertEquals(4, cd.getPos());
+    }
+
+    public void testLineFeed4() {
+        CharDeleter cd = new CharDeleter("test\ntest\n", 0).delete();
+        assertEquals("test\ntest", cd.getText());
+        assertEquals(4, cd.getPos());
+    }
+
+}