Remove outdated Java doc and run demo code in CI (#10698)

Resolve Conflicts:
        java/tutorial/pom.xml
        java/tutorial/pom_template.xml

java/test/src/main/java/io/ray/docdemo/RayDemo.java

@@ -0,0 +1,69 @@
+package io.ray.docdemo;
+
+import io.ray.api.ActorHandle;
+import io.ray.api.ObjectRef;
+import io.ray.api.Ray;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * This class contains demo code of the Ray introduction doc
+ * (https://docs.ray.io/en/master/index.html and  https://docs.ray.io/en/master/ray-overview/index.html).
+ *
+ * Please keep them in sync.
+ */
+public class RayDemo {
+
+  public static int square(int x) {
+    return x * x;
+  }
+
+  public static class Counter {
+
+    private int value = 0;
+
+    public void increment() {
+      this.value += 1;
+    }
+
+    public int read() {
+      return this.value;
+    }
+  }
+
+  public static void main(String[] args) {
+    // Intialize Ray runtime.
+    Ray.init();
+    {
+      List<ObjectRef<Integer>> objectRefList = new ArrayList<>();
+      // Invoke the `square` method 4 times remotely as Ray tasks.
+      // The tasks will run in parallel in the background.
+      for (int i = 0; i < 4; i++) {
+        objectRefList.add(Ray.task(RayDemo::square, i).remote());
+      }
+      // Get the actual results of the tasks with `get`.
+      System.out.println(Ray.get(objectRefList));  // [0, 1, 4, 9]
+    }
+
+    {
+      List<ActorHandle<Counter>> counters = new ArrayList<>();
+      // Create 4 actors from the `Counter` class.
+      // They will run in remote worker processes.
+      for (int i = 0; i < 4; i++) {
+        counters.add(Ray.actor(Counter::new).remote());
+      }
+
+      // Invoke the `increment` method on each actor.
+      // This will send an actor task to each remote actor.
+      for (ActorHandle<Counter> counter : counters) {
+        counter.task(Counter::increment).remote();
+      }
+      // Invoke the `read` method on each actor, and print the results.
+      List<ObjectRef<Integer>> objectRefList = counters.stream()
+          .map(counter -> counter.task(Counter::read).remote())
+          .collect(Collectors.toList());
+      System.out.println(Ray.get(objectRefList));  // [1, 1, 1, 1]
+    }
+  }
+}

java/test/src/main/java/io/ray/docdemo/UsingActorsDemo.java

@@ -8,7 +8,7 @@ import java.util.concurrent.TimeUnit;
 import org.testng.Assert;
 
 /**
- * This class contains demo code of the Ray core Using Actors doc (https://docs.ray.io/en/latest/actors.html).
+ * This class contains demo code of the Ray core Using Actors doc (https://docs.ray.io/en/master/actors.html).
  *
  * Please keep them in sync.
  */

java/test/src/main/java/io/ray/docdemo/WalkthroughDemo.java

@@ -11,7 +11,7 @@ import java.util.concurrent.TimeUnit;
 import org.testng.Assert;
 
 /**
- * This class contains demo code of the Ray core walkthrough doc (https://docs.ray.io/en/latest/walkthrough.html).
+ * This class contains demo code of the Ray core walkthrough doc (https://docs.ray.io/en/master/walkthrough.html).
  *
  * Please keep them in sync.
  */