Added Concat Operator usage.

Author: Ramakrishna

app/src/main/java/com/example/reactiveprogrammingusingrxjava2/_operators/Rx_d_ConcatOperator.java

@@ -0,0 +1,28 @@
+package com.example.reactiveprogrammingusingrxjava2._operators;
+
+import android.support.v7.app.AppCompatActivity;
+
+import com.example.reactiveprogrammingusingrxjava2.BaseActivity;
+
+import io.reactivex.Observable;
+import io.reactivex.functions.Consumer;
+
+public class Rx_d_ConcatOperator extends BaseActivity {
+
+    @Override
+    protected void onResume() {
+        super.onResume();
+
+        Observable aSeriesObservable = Observable.just("A1","A2","A3","A4","A5","A6","A7", "A8", "A9", "A10");
+        Observable bSeriesObservable = Observable.just("B1","B2","B3","B4","B5","B6","B7", "B8", "B9", "B10");
+
+        Observable
+                .concat(bSeriesObservable, aSeriesObservable)
+                .subscribe(new Consumer() {
+                    @Override
+                    public void accept(Object o) throws Exception {
+                        logMessage(o.toString()); //First gets all items from first observable passed to concat and then second observable items
+                    }
+                });
+    }
+}