Add all 23 GoF design pattern implementations

03-behavioral/command/DeleteCommand.java

@@ -0,0 +1,23 @@
+package command;
+
+public class DeleteCommand implements Command {
+    private final TextEditor editor;
+    private final int start;
+    private final int length;
+    private String deletedText;  // saved for undo
+
+    public DeleteCommand(TextEditor editor, int start, int length) {
+        this.editor = editor;
+        this.start  = start;
+        this.length = length;
+    }
+
+    @Override
+    public void execute() {
+        deletedText = editor.getText().substring(start, start + length);
+        editor.deleteText(start, length);
+    }
+
+    @Override public void undo() { editor.insertText(deletedText, start); }
+    @Override public String getDescription() { return "Delete " + length + " chars at " + start; }
+}