added strategy example

src/learn/simple_strategy/ConcreteStrategyA.java

@@ -0,0 +1,8 @@
+package learn.simple_strategy;
+
+public class ConcreteStrategyA implements Strategy {
+    @Override
+    public void algorithm() {
+        System.out.println("nice strategy A");
+    }
+}

src/learn/simple_strategy/ConcreteStrategyB.java

@@ -0,0 +1,8 @@
+package learn.simple_strategy;
+
+public class ConcreteStrategyB implements Strategy {
+    @Override
+    public void algorithm() {
+        System.out.println("nice strategy B");
+    }
+}

src/learn/simple_strategy/Context.java

@@ -0,0 +1,17 @@
+package learn.simple_strategy;
+
+public class Context {
+    private Strategy currentStrategy;
+
+    public Context(Strategy currentStrategy) {
+        this.currentStrategy = currentStrategy;
+    }
+
+    public void setCurrentStrategy(Strategy currentStrategy) {
+        this.currentStrategy = currentStrategy;
+    }
+
+    public void doSomeJob() {
+        currentStrategy.algorithm();
+    }
+}

src/learn/simple_strategy/Main.java

@@ -0,0 +1,11 @@
+package learn.simple_strategy;
+
+public class Main {
+    public static void main(String[] args) {
+        Context context = new Context(new ConcreteStrategyB());
+
+        context.doSomeJob();
+        context.setCurrentStrategy(new ConcreteStrategyA());
+        context.doSomeJob();
+    }
+}

src/learn/simple_strategy/Strategy.java

@@ -0,0 +1,5 @@
+package learn.simple_strategy;
+
+public interface Strategy {
+    void algorithm();
+}