diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/TaskFieldAdapters.java b/opentasks/src/main/java/org/dmfs/tasks/model/TaskFieldAdapters.java
index f1195736d..0c4766d59 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/TaskFieldAdapters.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/TaskFieldAdapters.java
@@ -62,7 +62,7 @@ public final class TaskFieldAdapters
      * Adapter for the status of a task.
      */
     public final static IntegerFieldAdapter STATUS = (IntegerFieldAdapter) new IntegerFieldAdapter(Tasks.STATUS, Tasks.STATUS_NEEDS_ACTION)
-            .addContraint(new AdjustPercentComplete(PERCENT_COMPLETE));
+            .addConstraint(new AdjustPercentComplete(PERCENT_COMPLETE));
 
     /**
      * Adapter for the priority value of a task.
@@ -108,7 +108,7 @@ public final class TaskFieldAdapters
      * Adapter for the checklist of a task.
      */
     public final static ChecklistFieldAdapter CHECKLIST = (ChecklistFieldAdapter) new ChecklistFieldAdapter(Tasks.DESCRIPTION)
-            .addContraint(new ChecklistConstraint(STATUS, PERCENT_COMPLETE));
+            .addConstraint(new ChecklistConstraint(STATUS, PERCENT_COMPLETE));
 
     /**
      * Private adapter for the start date of a task. We need this to reference DTSTART from DUE.
@@ -119,12 +119,12 @@ public final class TaskFieldAdapters
     /**
      * Adapter for the due date of a task.
      */
-    public final static FieldAdapter<Time> DUE = new CustomizedDefaultFieldAdapter<Time>(_DUE, new DefaultAfter(_DTSTART)).addContraint(new After(_DTSTART));
+    public final static FieldAdapter<Time> DUE = new CustomizedDefaultFieldAdapter<Time>(_DUE, new DefaultAfter(_DTSTART)).addConstraint(new After(_DTSTART));
 
     /**
      * Adapter for the start date of a task.
      */
-    public final static FieldAdapter<Time> DTSTART = new CustomizedDefaultFieldAdapter<Time>(_DTSTART, new DefaultBefore(DUE)).addContraint(
+    public final static FieldAdapter<Time> DTSTART = new CustomizedDefaultFieldAdapter<Time>(_DTSTART, new DefaultBefore(DUE)).addConstraint(
             new BeforeOrShiftTime(DUE));
 
     /**
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/XmlModel.java b/opentasks/src/main/java/org/dmfs/tasks/model/XmlModel.java
index ae2e5156f..18558e4d2 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/XmlModel.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/XmlModel.java
@@ -212,11 +212,11 @@ public XmlModel finish(ElementDescriptor<XmlModel> descriptor, XmlModel object,
                         // add UpdateAllDay constraint of due or start fields are missing to keep the values in sync with the allday flag
                         if (!state.hasDue)
                         {
-                            ((FieldAdapter<Boolean>) state.alldayDescriptor.getFieldAdapter()).addContraint(new UpdateAllDay(TaskFieldAdapters.DUE));
+                            ((FieldAdapter<Boolean>) state.alldayDescriptor.getFieldAdapter()).addConstraint(new UpdateAllDay(TaskFieldAdapters.DUE));
                         }
                         if (!state.hasStart)
                         {
-                            ((FieldAdapter<Boolean>) state.alldayDescriptor.getFieldAdapter()).addContraint(new UpdateAllDay(TaskFieldAdapters.DTSTART));
+                            ((FieldAdapter<Boolean>) state.alldayDescriptor.getFieldAdapter()).addConstraint(new UpdateAllDay(TaskFieldAdapters.DTSTART));
                         }
                     }
                     return object;
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/BooleanFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/BooleanFieldAdapter.java
index fa7fd0ca7..06251371b 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/BooleanFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/BooleanFieldAdapter.java
@@ -18,9 +18,12 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
+import org.dmfs.tasks.utils.BooleanBinaryInt;
 
 
 /**
@@ -79,7 +82,7 @@ public BooleanFieldAdapter(String fieldName, Boolean defaultValue)
 
 
     @Override
-    public Boolean get(ContentSet values)
+    public Boolean get(@NonNull ContentSet values)
     {
         Integer value = values.getAsInteger(mFieldName);
 
@@ -88,7 +91,7 @@ public Boolean get(ContentSet values)
 
 
     @Override
-    public Boolean get(Cursor cursor)
+    public Boolean get(@NonNull Cursor cursor)
     {
         int columnIdx = cursor.getColumnIndex(mFieldName);
         if (columnIdx < 0)
@@ -100,35 +103,35 @@ public Boolean get(Cursor cursor)
 
 
     @Override
-    public Boolean getDefault(ContentSet values)
+    public Boolean getDefault(@NonNull ContentSet values)
     {
         return mDefaultValue;
     }
 
 
     @Override
-    public void set(ContentSet values, Boolean value)
+    public void set(@NonNull ContentSet values, @Nullable Boolean value)
     {
-        values.put(mFieldName, value ? 1 : 0);
+        values.put(mFieldName, new BooleanBinaryInt(value).value());
     }
 
 
     @Override
-    public void set(ContentValues values, Boolean value)
+    public void set(@NonNull ContentValues values, @Nullable Boolean value)
     {
-        values.put(mFieldName, value ? 1 : 0);
+        values.put(mFieldName, new BooleanBinaryInt(value).value());
     }
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mFieldName, initalNotification);
     }
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mFieldName);
     }
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ChecklistFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ChecklistFieldAdapter.java
index 1c0bd9191..940a6e4d4 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ChecklistFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ChecklistFieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.CheckListItem;
 import org.dmfs.tasks.model.ContentSet;
@@ -77,8 +79,9 @@ public ChecklistFieldAdapter(String fieldName, List<CheckListItem> defaultValue)
     }
 
 
+    @Nullable
     @Override
-    public List<CheckListItem> get(ContentSet values)
+    public List<CheckListItem> get(@NonNull ContentSet values)
     {
         // return the check list
         return extractCheckList(values.getAsString(mFieldName));
@@ -86,7 +89,7 @@ public List<CheckListItem> get(ContentSet values)
 
 
     @Override
-    public List<CheckListItem> get(Cursor cursor)
+    public List<CheckListItem> get(@NonNull Cursor cursor)
     {
         int columnIdx = cursor.getColumnIndex(mFieldName);
         if (columnIdx < 0)
@@ -98,14 +101,14 @@ public List<CheckListItem> get(Cursor cursor)
 
 
     @Override
-    public List<CheckListItem> getDefault(ContentSet values)
+    public List<CheckListItem> getDefault(@NonNull ContentSet values)
     {
         return mDefaultValue;
     }
 
 
     @Override
-    public void set(ContentSet values, List<CheckListItem> value)
+    public void set(@NonNull ContentSet values, @Nullable List<CheckListItem> value)
     {
         String oldDescription = DescriptionStringFieldAdapter.extractDescription(values.getAsString(mFieldName));
         if (value != null && !value.isEmpty())
@@ -130,7 +133,7 @@ public void set(ContentSet values, List<CheckListItem> value)
 
 
     @Override
-    public void set(ContentValues values, List<CheckListItem> value)
+    public void set(@NonNull ContentValues values, @Nullable List<CheckListItem> value)
     {
         String oldDescription = DescriptionStringFieldAdapter.extractDescription(values.getAsString(mFieldName));
         if (value != null && !value.isEmpty())
@@ -156,14 +159,14 @@ public void set(ContentValues values, List<CheckListItem> value)
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mFieldName, initalNotification);
     }
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mFieldName);
     }
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ColorFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ColorFieldAdapter.java
index a3ffe9b87..1f00e3244 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ColorFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/ColorFieldAdapter.java
@@ -18,6 +18,9 @@
 
 import android.database.Cursor;
 import android.graphics.Color;
+import android.support.annotation.ColorInt;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 
@@ -88,28 +91,34 @@ public ColorFieldAdapter(String fieldName, Integer defaultValue, float darkenThr
 
 
     @Override
-    public Integer get(ContentSet values)
+    public Integer get(@NonNull ContentSet values)
     {
         return darkenColor(super.get(values), mDarkenThreshold);
     }
 
 
     @Override
-    public Integer get(Cursor cursor)
+    public Integer get(@NonNull Cursor cursor)
     {
         return darkenColor(super.get(cursor), mDarkenThreshold);
     }
 
 
     @Override
-    public Integer getDefault(ContentSet values)
+    public Integer getDefault(@NonNull ContentSet values)
     {
         return darkenColor(super.getDefault(values), mDarkenThreshold);
     }
 
 
-    private static int darkenColor(int color, float maxLuminance)
+    @ColorInt
+    private static Integer darkenColor(@Nullable Integer color, float maxLuminance)
     {
+        if (color == null)
+        {
+            return null;
+        }
+
         float[] hsv = new float[3];
         Color.colorToHSV(color, hsv);
         hsv[2] = hsv[2] * hsv[2] * hsv[2] * hsv[2] * hsv[2] * (maxLuminance - 1) + hsv[2];
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/CustomizedDefaultFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/CustomizedDefaultFieldAdapter.java
index da3aa6779..d95d3cc15 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/CustomizedDefaultFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/CustomizedDefaultFieldAdapter.java
@@ -17,6 +17,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -59,15 +61,17 @@ public CustomizedDefaultFieldAdapter(FieldAdapter<Type> fieldAdapter, Default<Ty
     }
 
 
+    @Nullable
     @Override
-    public Type get(ContentSet values)
+    public Type get(@NonNull ContentSet values)
     {
         return mFieldAdapter.get(values);
     }
 
 
+    @Nullable
     @Override
-    public Type get(Cursor cursor)
+    public Type get(@NonNull Cursor cursor)
     {
         return mFieldAdapter.get(cursor);
     }
@@ -81,8 +85,9 @@ public Type get(Cursor cursor)
      *
      * @return A default Value
      */
+    @Nullable
     @Override
-    public Type getDefault(ContentSet values)
+    public Type getDefault(@NonNull ContentSet values)
     {
         Type defaultValue = mFieldAdapter.getDefault(values);
         return mDefault.getCustomDefault(values, defaultValue);
@@ -90,28 +95,28 @@ public Type getDefault(ContentSet values)
 
 
     @Override
-    public void set(ContentSet values, Type value)
+    public void set(@NonNull ContentSet values, @Nullable Type value)
     {
         mFieldAdapter.set(values, value);
     }
 
 
     @Override
-    public void set(ContentValues values, Type value)
+    public void set(@NonNull ContentValues values, @Nullable Type value)
     {
         mFieldAdapter.set(values, value);
     }
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initialNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initialNotification)
     {
         mFieldAdapter.registerListener(values, listener, initialNotification);
     }
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         mFieldAdapter.unregisterListener(values, listener);
     }
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/DescriptionStringFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/DescriptionStringFieldAdapter.java
index 888319ce7..6cdad9681 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/DescriptionStringFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/DescriptionStringFieldAdapter.java
@@ -17,6 +17,8 @@
 package org.dmfs.tasks.model.adapters;
 
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 
@@ -56,21 +58,21 @@ public DescriptionStringFieldAdapter(String fieldName, String defaultValue)
 
 
     @Override
-    public String get(ContentSet values)
+    public String get(@NonNull ContentSet values)
     {
         return extractDescription(super.get(values));
     }
 
 
     @Override
-    public String get(Cursor cursor)
+    public String get(@NonNull Cursor cursor)
     {
         return extractDescription(super.get(cursor));
     }
 
 
     @Override
-    public void set(ContentSet values, String value)
+    public void set(@NonNull ContentSet values, @Nullable String value)
     {
         String oldValue = super.get(values);
         if (oldValue != null && oldValue.length() > 0)
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FieldAdapter.java
index d38a8de76..f7961c49d 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -52,7 +54,8 @@ public abstract class FieldAdapter<Type>
      *
      * @return The value.
      */
-    public abstract Type get(ContentSet values);
+    @Nullable
+    public abstract Type get(@NonNull ContentSet values);
 
     /**
      * Get the value from the given {@link Cursor}
@@ -62,7 +65,8 @@ public abstract class FieldAdapter<Type>
      *
      * @return The value.
      */
-    public abstract Type get(Cursor cursor);
+    @Nullable
+    public abstract Type get(@NonNull Cursor cursor);
 
     /**
      * Get a default value for this Adapter.
@@ -72,7 +76,8 @@ public abstract class FieldAdapter<Type>
      *
      * @return A default Value
      */
-    public abstract Type getDefault(ContentSet values);
+    @Nullable
+    public abstract Type getDefault(@NonNull ContentSet values);
 
     /**
      * Set a value in the given {@link ContentSet}.
@@ -82,7 +87,7 @@ public abstract class FieldAdapter<Type>
      * @param value
      *         The new value to store.
      */
-    public abstract void set(ContentSet values, Type value);
+    public abstract void set(@NonNull ContentSet values, @Nullable Type value);
 
     /**
      * Set a value in the given {@link ContentValues}.
@@ -92,7 +97,7 @@ public abstract class FieldAdapter<Type>
      * @param value
      *         The new value to store.
      */
-    public abstract void set(ContentValues values, Type value);
+    public abstract void set(@NonNull ContentValues values, @Nullable Type value);
 
 
     /**
@@ -103,7 +108,7 @@ public abstract class FieldAdapter<Type>
      * @param value
      *         The new value to store.
      */
-    public void validateAndSet(ContentSet values, Type value)
+    public final void validateAndSet(@NonNull ContentSet values, @Nullable Type value)
     {
         Type oldValue = get(values);
         value = checkConstraints(values, oldValue, value);
@@ -119,7 +124,7 @@ public void validateAndSet(ContentSet values, Type value)
      * @param listener
      *         The {@link OnContentChangeListener} to register.
      */
-    public abstract void registerListener(ContentSet values, OnContentChangeListener listener, boolean initialNotification);
+    public abstract void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initialNotification);
 
     /**
      * Unregister a listener for the fields that this adapter adapts.
@@ -129,7 +134,7 @@ public void validateAndSet(ContentSet values, Type value)
      * @param listener
      *         The {@link OnContentChangeListener} to unregister.
      */
-    public abstract void unregisterListener(ContentSet values, OnContentChangeListener listener);
+    public abstract void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener);
 
 
     /**
@@ -138,11 +143,11 @@ public void validateAndSet(ContentSet values, Type value)
      * @param constraint
      *         The new constraint.
      */
-    public final FieldAdapter<Type> addContraint(AbstractConstraint<Type> constraint)
+    public final FieldAdapter<Type> addConstraint(@NonNull AbstractConstraint<Type> constraint)
     {
         if (mConstraints == null)
         {
-            mConstraints = new LinkedList<AbstractConstraint<Type>>();
+            mConstraints = new LinkedList<>();
         }
         mConstraints.add(constraint);
         return this;
@@ -153,14 +158,13 @@ public final FieldAdapter<Type> addContraint(AbstractConstraint<Type> constraint
      * Check all constraints and enforce them if possible.
      * <p>
      * TODO: Allow throwing an exception if any of the constraints could not be enforced. That requires some kind of transaction in {@link ContentSet}.
-     * </p>
      *
      * @param currentValues
      *         The current {@link ContentSet}.
      * @param newValue
      *         The new value to check.
      */
-    protected final Type checkConstraints(ContentSet currentValues, Type oldValue, Type newValue)
+    private Type checkConstraints(@NonNull ContentSet currentValues, @Nullable Type oldValue, @Nullable Type newValue)
     {
         if (mConstraints != null)
         {
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FloatFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FloatFieldAdapter.java
index 8012081a6..7a961185a 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FloatFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FloatFieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -78,8 +80,9 @@ public FloatFieldAdapter(String fieldName, Float defaultValue)
     }
 
 
+    @Nullable
     @Override
-    public Float get(ContentSet values)
+    public Float get(@NonNull ContentSet values)
     {
         // return the value as Float
         return values.getAsFloat(mFieldName);
@@ -87,7 +90,7 @@ public Float get(ContentSet values)
 
 
     @Override
-    public Float get(Cursor cursor)
+    public Float get(@NonNull Cursor cursor)
     {
         int columnIdx = cursor.getColumnIndex(mFieldName);
         if (columnIdx < 0)
@@ -99,35 +102,35 @@ public Float get(Cursor cursor)
 
 
     @Override
-    public Float getDefault(ContentSet values)
+    public Float getDefault(@NonNull ContentSet values)
     {
         return mDefaultValue;
     }
 
 
     @Override
-    public void set(ContentSet values, Float value)
+    public void set(@NonNull ContentSet values, @Nullable Float value)
     {
         values.put(mFieldName, value);
     }
 
 
     @Override
-    public void set(ContentValues values, Float value)
+    public void set(@NonNull ContentValues values, @Nullable Float value)
     {
         values.put(mFieldName, value);
     }
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mFieldName, initalNotification);
     }
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mFieldName);
     }
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FormattedStringFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FormattedStringFieldAdapter.java
index 3acea859f..0d28f3876 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FormattedStringFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/FormattedStringFieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -63,8 +65,9 @@ public FormattedStringFieldAdapter(String format, StringFieldAdapter... paramFie
     }
 
 
+    @Nullable
     @Override
-    public String get(ContentSet values)
+    public String get(@NonNull ContentSet values)
     {
         String[] params = new String[mParamFields.length];
         for (int i = 0, len = mParamFields.length; i < len; ++i)
@@ -75,8 +78,9 @@ public String get(ContentSet values)
     }
 
 
+    @Nullable
     @Override
-    public String get(Cursor cursor)
+    public String get(@NonNull Cursor cursor)
     {
         String[] params = new String[mParamFields.length];
         for (int i = 0, len = mParamFields.length; i < len; ++i)
@@ -87,8 +91,9 @@ public String get(Cursor cursor)
     }
 
 
+    @Nullable
     @Override
-    public String getDefault(ContentSet values)
+    public String getDefault(@NonNull ContentSet values)
     {
         String[] params = new String[mParamFields.length];
         for (int i = 0, len = mParamFields.length; i < len; ++i)
@@ -100,21 +105,21 @@ public String getDefault(ContentSet values)
 
 
     @Override
-    public void set(ContentSet values, String value)
+    public void set(@NonNull ContentSet values, @Nullable String value)
     {
         // setting values is not supported
     }
 
 
     @Override
-    public void set(ContentValues values, String value)
+    public void set(@NonNull ContentValues values, @Nullable String value)
     {
         // setting values is not supported
     }
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         for (FieldAdapter<?> adapter : mParamFields)
         {
@@ -124,7 +129,7 @@ public void registerListener(ContentSet values, OnContentChangeListener listener
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         for (FieldAdapter<?> adapter : mParamFields)
         {
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/IntegerFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/IntegerFieldAdapter.java
index 9b9dfe59c..dafd94e4b 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/IntegerFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/IntegerFieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -78,16 +80,18 @@ public IntegerFieldAdapter(String fieldName, Integer defaultValue)
     }
 
 
+    @Nullable
     @Override
-    public Integer get(ContentSet values)
+    public Integer get(@NonNull ContentSet values)
     {
         // return the value as Integer
         return values.getAsInteger(mFieldName);
     }
 
 
+    @Nullable
     @Override
-    public Integer get(Cursor cursor)
+    public Integer get(@NonNull Cursor cursor)
     {
         int columnIdx = cursor.getColumnIndex(mFieldName);
         if (columnIdx < 0)
@@ -98,36 +102,37 @@ public Integer get(Cursor cursor)
     }
 
 
+    @Nullable
     @Override
-    public Integer getDefault(ContentSet values)
+    public Integer getDefault(@NonNull ContentSet values)
     {
         return mDefaultValue;
     }
 
 
     @Override
-    public void set(ContentSet values, Integer value)
+    public void set(@NonNull ContentSet values, @Nullable Integer value)
     {
         values.put(mFieldName, value);
     }
 
 
     @Override
-    public void set(ContentValues values, Integer value)
+    public void set(@NonNull ContentValues values, @Nullable Integer value)
     {
         values.put(mFieldName, value);
     }
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mFieldName, initalNotification);
     }
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mFieldName);
     }
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/StringFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/StringFieldAdapter.java
index 8a68d6784..659fe6df8 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/StringFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/StringFieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -78,8 +80,9 @@ public StringFieldAdapter(String fieldName, String defaultValue)
     }
 
 
+    @Nullable
     @Override
-    public String get(ContentSet values)
+    public String get(@NonNull ContentSet values)
     {
         // return the value as String
         return values.getAsString(mFieldName);
@@ -87,7 +90,7 @@ public String get(ContentSet values)
 
 
     @Override
-    public String get(Cursor cursor)
+    public String get(@NonNull Cursor cursor)
     {
         int columnIdx = cursor.getColumnIndex(mFieldName);
         if (columnIdx < 0)
@@ -99,35 +102,35 @@ public String get(Cursor cursor)
 
 
     @Override
-    public String getDefault(ContentSet values)
+    public String getDefault(@NonNull ContentSet values)
     {
         return mDefaultValue;
     }
 
 
     @Override
-    public void set(ContentSet values, String value)
+    public void set(@NonNull ContentSet values, @Nullable String value)
     {
         values.put(mFieldName, value);
     }
 
 
     @Override
-    public void set(ContentValues values, String value)
+    public void set(@NonNull ContentValues values, @Nullable String value)
     {
         values.put(mFieldName, value);
     }
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mFieldName, initalNotification);
     }
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mFieldName);
     }
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimeFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimeFieldAdapter.java
index 3cf74a0cd..f586bbc53 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimeFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimeFieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.text.format.Time;
 
 import org.dmfs.tasks.model.ContentSet;
@@ -73,8 +75,9 @@ public TimeFieldAdapter(String timestampField, String tzField, String alldayFiel
     }
 
 
+    @Nullable
     @Override
-    public Time get(ContentSet values)
+    public Time get(@NonNull ContentSet values)
     {
         Long timestamp = values.getAsLong(mTimestampField);
         if (timestamp == null)
@@ -104,8 +107,9 @@ public Time get(ContentSet values)
     }
 
 
+    @Nullable
     @Override
-    public Time get(Cursor cursor)
+    public Time get(@NonNull Cursor cursor)
     {
         int tsIdx = cursor.getColumnIndex(mTimestampField);
         int tzIdx = mTzField == null ? -1 : cursor.getColumnIndex(mTzField);
@@ -142,7 +146,7 @@ public Time get(Cursor cursor)
 
 
     @Override
-    public Time getDefault(ContentSet values)
+    public Time getDefault(@NonNull ContentSet values)
     {
         // create a new Time for the given time zone, falling back to the default time zone if none is given
         String timezone = mTzField == null ? Time.TIMEZONE_UTC : values.getAsString(mTzField);
@@ -163,7 +167,7 @@ public Time getDefault(ContentSet values)
 
 
     @Override
-    public void set(ContentSet values, Time value)
+    public void set(@NonNull ContentSet values, @Nullable Time value)
     {
         values.startBulkUpdate();
         try
@@ -196,7 +200,7 @@ public void set(ContentSet values, Time value)
 
 
     @Override
-    public void set(ContentValues values, Time value)
+    public void set(@NonNull ContentValues values, @Nullable Time value)
     {
         if (value != null)
         {
@@ -221,7 +225,7 @@ public void set(ContentValues values, Time value)
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mTimestampField, initalNotification);
         if (mTzField != null)
@@ -236,7 +240,7 @@ public void registerListener(ContentSet values, OnContentChangeListener listener
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mTimestampField);
         if (mTzField != null)
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimezoneFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimezoneFieldAdapter.java
index 4f3300e3c..9a014c2e1 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimezoneFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/TimezoneFieldAdapter.java
@@ -18,6 +18,8 @@
 
 import android.content.ContentValues;
 import android.database.Cursor;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -86,8 +88,9 @@ public TimezoneFieldAdapter(String timezoneFieldName, String alldayFieldName, St
     }
 
 
+    @Nullable
     @Override
-    public TimeZone get(ContentSet values)
+    public TimeZone get(@NonNull ContentSet values)
     {
         String timezoneId = values.getAsString(mTzFieldName);
 
@@ -99,7 +102,7 @@ public TimeZone get(ContentSet values)
             isAllDay = allday != null && allday > 0;
         }
 
-        TimeZoneWrapper timeZone = isAllDay ? null : timezoneId == null ? getDefault(null) : new TimeZoneWrapper(timezoneId);
+        TimeZoneWrapper timeZone = isAllDay ? null : timezoneId == null ? getDefault() : new TimeZoneWrapper(timezoneId);
         if (timeZone != null && mReferenceTimeFieldName != null)
         {
             timeZone.setReferenceTimeStamp(values.getAsLong(mReferenceTimeFieldName));
@@ -108,8 +111,9 @@ public TimeZone get(ContentSet values)
     }
 
 
+    @Nullable
     @Override
-    public TimeZone get(Cursor cursor)
+    public TimeZone get(@NonNull Cursor cursor)
     {
         int tzColumnIdx = cursor.getColumnIndex(mTzFieldName);
 
@@ -133,7 +137,7 @@ public TimeZone get(Cursor cursor)
             isAllDay = !cursor.isNull(allDayColumnIdx) && cursor.getInt(allDayColumnIdx) > 0;
         }
 
-        TimeZoneWrapper timeZone = isAllDay ? null : timezoneId == null ? getDefault(null) : new TimeZoneWrapper(timezoneId);
+        TimeZoneWrapper timeZone = isAllDay ? null : timezoneId == null ? getDefault() : new TimeZoneWrapper(timezoneId);
         int refTimeCol;
         if (timeZone != null && mReferenceTimeFieldName != null && (refTimeCol = cursor.getColumnIndex(mReferenceTimeFieldName)) >= 0)
         {
@@ -147,12 +151,12 @@ public TimeZone get(Cursor cursor)
     /**
      * Returns whether this is an "all-day timezone".
      *
-     * @param cursor
-     *         The cursor to read from.
+     * @param values
+     *         The ContentSet to read from.
      *
      * @return <code>true</code> if the cursor points to an all-day date.
      */
-    public boolean isAllDay(ContentSet values)
+    private boolean isAllDay(ContentSet values)
     {
         if (mAllDayFieldName == null)
         {
@@ -167,12 +171,12 @@ public boolean isAllDay(ContentSet values)
     /**
      * Returns whether this is an "all-day timezone".
      *
-     * @param cursor
-     *         The cursor to read from.
+     * @param values
+     *         The ContentValues to read from.
      *
      * @return <code>true</code> if the cursor points to an all-day date.
      */
-    public boolean isAllDay(ContentValues values)
+    private boolean isAllDay(ContentValues values)
     {
         if (mAllDayFieldName == null)
         {
@@ -185,43 +189,25 @@ public boolean isAllDay(ContentValues values)
 
 
     /**
-     * Returns whether this is an "all-day timezone".
-     *
-     * @param cursor
-     *         The cursor to read from.
+     * Returns the local time zone.
      *
-     * @return <code>true</code> if the cursor points to an all-day date.
+     * @return The current time zone.
      */
-    public boolean isAllDay(Cursor cursor)
+    @Override
+    public TimeZoneWrapper getDefault(@NonNull ContentSet values)
     {
-        if (mAllDayFieldName == null)
-        {
-            return false;
-        }
-
-        int allDayColumnIdx = cursor.getColumnIndex(mAllDayFieldName);
-        if (allDayColumnIdx < 0)
-        {
-            throw new IllegalArgumentException("The allday column is missing in cursor.");
-        }
-        return !cursor.isNull(allDayColumnIdx) && cursor.getInt(allDayColumnIdx) > 0;
+        return getDefault();
     }
 
 
-    /**
-     * Returns the local time zone.
-     *
-     * @return The current time zone.
-     */
-    @Override
-    public TimeZoneWrapper getDefault(ContentSet values)
+    private TimeZoneWrapper getDefault()
     {
         return new TimeZoneWrapper();
     }
 
 
     @Override
-    public void set(ContentSet values, TimeZone value)
+    public void set(@NonNull ContentSet values, @Nullable TimeZone value)
     {
         if (!isAllDay(values))
         {
@@ -238,7 +224,7 @@ public void set(ContentSet values, TimeZone value)
 
 
     @Override
-    public void set(ContentValues values, TimeZone value)
+    public void set(@NonNull ContentValues values, TimeZone value)
     {
         if (!isAllDay(values))
         {
@@ -255,7 +241,7 @@ public void set(ContentValues values, TimeZone value)
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mTzFieldName, initalNotification);
         if (mAllDayFieldName != null)
@@ -270,7 +256,7 @@ public void registerListener(ContentSet values, OnContentChangeListener listener
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mTzFieldName);
         if (mAllDayFieldName != null)
diff --git a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/UriFieldAdapter.java b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/UriFieldAdapter.java
index d3d6c9451..1d8d80860 100644
--- a/opentasks/src/main/java/org/dmfs/tasks/model/adapters/UriFieldAdapter.java
+++ b/opentasks/src/main/java/org/dmfs/tasks/model/adapters/UriFieldAdapter.java
@@ -19,6 +19,8 @@
 import android.content.ContentValues;
 import android.database.Cursor;
 import android.net.Uri;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.dmfs.tasks.model.ContentSet;
 import org.dmfs.tasks.model.OnContentChangeListener;
@@ -76,8 +78,9 @@ public UriFieldAdapter(String urlField, Uri defaultValue)
     }
 
 
+    @Nullable
     @Override
-    public Uri get(ContentSet values)
+    public Uri get(@NonNull ContentSet values)
     {
         try
         {
@@ -90,8 +93,9 @@ public Uri get(ContentSet values)
     }
 
 
+    @Nullable
     @Override
-    public Uri get(Cursor cursor)
+    public Uri get(@NonNull Cursor cursor)
     {
         int columnIdx = cursor.getColumnIndex(mFieldName);
         if (columnIdx < 0)
@@ -109,15 +113,16 @@ public Uri get(Cursor cursor)
     }
 
 
+    @Nullable
     @Override
-    public Uri getDefault(ContentSet values)
+    public Uri getDefault(@NonNull ContentSet values)
     {
         return mDefaultValue;
     }
 
 
     @Override
-    public void set(ContentSet values, Uri value)
+    public void set(@NonNull ContentSet values, @Nullable Uri value)
     {
         if (value == null)
         {
@@ -131,7 +136,7 @@ public void set(ContentSet values, Uri value)
 
 
     @Override
-    public void set(ContentValues values, Uri value)
+    public void set(@NonNull ContentValues values, @Nullable Uri value)
     {
         if (value == null)
         {
@@ -145,14 +150,14 @@ public void set(ContentValues values, Uri value)
 
 
     @Override
-    public void registerListener(ContentSet values, OnContentChangeListener listener, boolean initalNotification)
+    public void registerListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener, boolean initalNotification)
     {
         values.addOnChangeListener(listener, mFieldName, initalNotification);
     }
 
 
     @Override
-    public void unregisterListener(ContentSet values, OnContentChangeListener listener)
+    public void unregisterListener(@NonNull ContentSet values, @NonNull OnContentChangeListener listener)
     {
         values.removeOnChangeListener(listener, mFieldName);
     }
diff --git a/opentasks/src/main/java/org/dmfs/tasks/utils/BooleanBinaryInt.java b/opentasks/src/main/java/org/dmfs/tasks/utils/BooleanBinaryInt.java
new file mode 100644
index 000000000..f96451d08
--- /dev/null
+++ b/opentasks/src/main/java/org/dmfs/tasks/utils/BooleanBinaryInt.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2018 dmfs GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.dmfs.tasks.utils;
+
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
+
+import org.dmfs.jems.optional.decorators.Mapped;
+import org.dmfs.jems.single.decorators.DelegatingSingle;
+import org.dmfs.optional.NullSafe;
+import org.dmfs.optional.Optional;
+
+
+/**
+ * Adapter for a {@link Boolean} to a 0, 1 binary representation in {@link Integer}.
+ * <p>
+ * If the {@link Boolean} is absent/null the value is 0.
+ *
+ * @author Gabor Keszthelyi
+ */
+public final class BooleanBinaryInt extends DelegatingSingle<Integer>
+{
+    public BooleanBinaryInt(@NonNull Optional<Boolean> booleanOptional)
+    {
+        super(() -> new Mapped<>(b -> b ? 1 : 0, booleanOptional).value(0));
+    }
+
+
+    public BooleanBinaryInt(@Nullable Boolean booleanValue)
+    {
+        this(new NullSafe<>(booleanValue));
+    }
+}
diff --git a/opentasks/src/test/java/org/dmfs/tasks/utils/BooleanBinaryIntTest.java b/opentasks/src/test/java/org/dmfs/tasks/utils/BooleanBinaryIntTest.java
new file mode 100644
index 000000000..2e166c379
--- /dev/null
+++ b/opentasks/src/test/java/org/dmfs/tasks/utils/BooleanBinaryIntTest.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2018 dmfs GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.dmfs.tasks.utils;
+
+import org.dmfs.optional.Present;
+import org.junit.Test;
+
+import static org.dmfs.optional.Absent.absent;
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+
+
+/**
+ * Unit test for {@link BooleanBinaryInt}.
+ *
+ * @author Gabor Keszthelyi
+ */
+public final class BooleanBinaryIntTest
+{
+
+    @Test
+    public void test()
+    {
+        assertThat(new BooleanBinaryInt(true).value(), is(1));
+        assertThat(new BooleanBinaryInt(false).value(), is(0));
+        assertThat(new BooleanBinaryInt((Boolean) null).value(), is(0));
+
+        assertThat(new BooleanBinaryInt(new Present<>(true)).value(), is(1));
+        assertThat(new BooleanBinaryInt(new Present<>(false)).value(), is(0));
+        assertThat(new BooleanBinaryInt(absent()).value(), is(0));
+    }
+
+}
\ No newline at end of file