Added toMutableList collector to mark the need for a mutable list

It's useful in cases where we need to modify the resulting list, which
is not possible with the immutable list returned by the Stream#toList
method. We use this method to differentiate between cases where a
mutable list is necessary and to prevent the IDE from suggesting the use
of Stream#toList.

Issue: #3197

name.abuchen.portfolio.ui/src/name/abuchen/portfolio/ui/views/dashboard/DashboardView.java

@@ -1,5 +1,7 @@
 package name.abuchen.portfolio.ui.views.dashboard;
 
+import static name.abuchen.portfolio.util.CollectorsUtil.toMutableList;
+
 import java.text.MessageFormat;
 import java.util.ArrayList;
 import java.util.HashMap;
@@ -11,7 +13,6 @@
 import java.util.concurrent.Executors;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.function.Supplier;
-import java.util.stream.Collectors;
 
 import javax.annotation.PostConstruct;
 import javax.annotation.PreDestroy;
@@ -879,7 +880,7 @@ private void duplicateColumn(Dashboard.Column column, Composite columnControl)
         Dashboard.Column newColumn = new Dashboard.Column();
         dashboard.getColumns().add(index, newColumn);
 
-        newColumn.setWidgets(column.getWidgets().stream().map(Widget::copy).collect(Collectors.toList()));
+        newColumn.setWidgets(column.getWidgets().stream().map(Widget::copy).collect(toMutableList()));
 
         getClient().touch();
 

name.abuchen.portfolio/src/name/abuchen/portfolio/util/CollectorsUtil.java

@@ -0,0 +1,25 @@
+package name.abuchen.portfolio.util;
+
+import java.util.List;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
+
+public class CollectorsUtil
+{
+    private CollectorsUtil()
+    {
+    }
+
+    /**
+     * This method returns a collector that gathers the elements into a mutable
+     * ArrayList. It's useful in cases where we need to modify the resulting
+     * list, which is not possible with the immutable list returned by the
+     * Stream#toList method. We use this method to differentiate between cases
+     * where a mutable list is necessary and to prevent the IDE from suggesting
+     * the use of Stream#toList.
+     */
+    public static <T> Collector<T, ?, List<T>> toMutableList()
+    {
+        return Collectors.toList();
+    }
+}