#590 add explanation for State

state/README.md

@@ -12,8 +12,126 @@ tags:
 Objects for States
 
 ## Intent
-Allow an object to alter its behavior when its internal state
-changes. The object will appear to change its class.
+Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
+
+## Explanation
+Real world example
+
+> When observing a mammoth in its natural habitat it seems to change its behavior based on the situation. It may first appear calm but over time when it detects a threat it gets angry and dangerous to its surroundings.  
+
+In plain words
+
+> State pattern allows an object to change its behavior. 
+
+Wikipedia says
+
+> The state pattern is a behavioral software design pattern that allows an object to alter its behavior when its internal state changes. This pattern is close to the concept of finite-state machines. The state pattern can be interpreted as a strategy pattern, which is able to switch a strategy through invocations of methods defined in the pattern's interface.
+
+**Programmatic Example**
+
+Here is the state interface and its concrete implementations.
+
+```java
+public interface State {
+
+  void onEnterState();
+
+  void observe();
+}
+
+public class PeacefulState implements State {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(PeacefulState.class);
+
+  private Mammoth mammoth;
+
+  public PeacefulState(Mammoth mammoth) {
+    this.mammoth = mammoth;
+  }
+
+  @Override
+  public void observe() {
+    LOGGER.info("{} is calm and peaceful.", mammoth);
+  }
+
+  @Override
+  public void onEnterState() {
+    LOGGER.info("{} calms down.", mammoth);
+  }
+}
+
+public class AngryState implements State {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(AngryState.class);
+
+  private Mammoth mammoth;
+
+  public AngryState(Mammoth mammoth) {
+    this.mammoth = mammoth;
+  }
+
+  @Override
+  public void observe() {
+    LOGGER.info("{} is furious!", mammoth);
+  }
+
+  @Override
+  public void onEnterState() {
+    LOGGER.info("{} gets angry!", mammoth);
+  }
+}
+```
+
+And here is the mammoth containing the state.
+
+```java
+public class Mammoth {
+
+  private State state;
+
+  public Mammoth() {
+    state = new PeacefulState(this);
+  }
+
+  public void timePasses() {
+    if (state.getClass().equals(PeacefulState.class)) {
+      changeStateTo(new AngryState(this));
+    } else {
+      changeStateTo(new PeacefulState(this));
+    }
+  }
+
+  private void changeStateTo(State newState) {
+    this.state = newState;
+    this.state.onEnterState();
+  }
+
+  @Override
+  public String toString() {
+    return "The mammoth";
+  }
+
+  public void observe() {
+    this.state.observe();
+  }
+}
+```
+
+And here is the full example how the mammoth behaves over time.
+
+```java
+    var mammoth = new Mammoth();
+    mammoth.observe();
+    mammoth.timePasses();
+    mammoth.observe();
+    mammoth.timePasses();
+    mammoth.observe();
+    
+    // The mammoth gets angry!
+    // The mammoth is furious!
+    // The mammoth calms down.
+    // The mammoth is calm and peaceful.
+```
 
 ## Class diagram
 ![alt text](./etc/state_1.png "State")