added command pattern example

src/learn/simple_command/ConcreteCommand.java

@@ -0,0 +1,14 @@
+package learn.simple_command;
+
+public class ConcreteCommand implements Command {
+    private Receiver receiver;
+
+    public ConcreteCommand(Receiver receiver) {
+        this.receiver = receiver;
+    }
+
+    @Override
+    public void execute() {
+        receiver.action();
+    }
+}