diff --git a/src/org/frice/obj/Objects.kt b/src/org/frice/obj/Objects.kt
index dad15b9..f9057cf 100644
--- a/src/org/frice/obj/Objects.kt
+++ b/src/org/frice/obj/Objects.kt
@@ -30,13 +30,10 @@ interface AbstractObject {
  * @author ice1000
  * @since v0.4
  */
-interface FContainer {
+interface FContainer : FShapeQuad {
 
-	var x: Double
-	var y: Double
-
-	val width: Double
-	val height: Double
+	override var x: Double
+	override var y: Double
 
 	fun containsPoint(px: Int, py: Int) = px >= x && px <= x + width && py >= y && py <= y + height
 
diff --git a/src/org/frice/obj/sub/ImageObject.kt b/src/org/frice/obj/sub/ImageObject.kt
index 27fcd1e..364abb8 100644
--- a/src/org/frice/obj/sub/ImageObject.kt
+++ b/src/org/frice/obj/sub/ImageObject.kt
@@ -21,7 +21,7 @@ constructor(
 	var res: ImageResource,
 	override var x: Double = 0.0,
 	override var y: Double = 0.0,
-	id: Int = -1) : FShapeQuad, FObject(), FObject.ImageOwner {
+	id: Int = -1) : FObject(), FObject.ImageOwner {
 	constructor(res: FriceImage, x: Double, y: Double) : this(create(res), x, y, -1)
 
 	init {
diff --git a/src/org/frice/utils/QuadTree.kt b/src/org/frice/utils/QuadTree.kt
index 3ebbdef..6b3ee10 100644
--- a/src/org/frice/utils/QuadTree.kt
+++ b/src/org/frice/utils/QuadTree.kt
@@ -2,6 +2,7 @@ package org.frice.utils
 
 import org.frice.obj.PhysicalObject
 import org.frice.utils.shape.FQuad
+import org.frice.utils.shape.FShapeQuad
 import java.util.*
 
 /**
@@ -26,6 +27,13 @@ class QuadTree(private var level: Int, private var bounds: FQuad) {
 		for (i in nodes.indices) nodes[i] = null
 	}
 
+	private fun isInner(ob: PhysicalObject, quad: FShapeQuad): Boolean {
+		return ob.x >= quad.x
+			&& ob.x + ob.width <= quad.x + quad.width
+			&& ob.y >= quad.y
+			&& ob.y + ob.height <= quad.y + quad.height
+	}
+
 	private fun split() {
 		// width & height
 		val subWidth = bounds.width / 2
diff --git a/src/org/frice/utils/media/AudioPlayer.kt b/src/org/frice/utils/media/AudioPlayer.kt
index aa7e9bf..69ff3b5 100644
--- a/src/org/frice/utils/media/AudioPlayer.kt
+++ b/src/org/frice/utils/media/AudioPlayer.kt
@@ -31,7 +31,7 @@ class AudioPlayer internal constructor(file: File) : Thread() {
 	/**
 	 * @author ice1000
 	 * @since v1.7.6
-	 * @see AudioPlayer
+	 * @see org.frice.utils.media.AudioPlayer
 	 */
 	class AudioPlayerRunnable(file: File) : Runnable {
 		private var audioInputStream: AudioInputStream
diff --git a/src/org/frice/utils/shape/Quads.kt b/src/org/frice/utils/shape/Quads.kt
index d13e090..17a239e 100644
--- a/src/org/frice/utils/shape/Quads.kt
+++ b/src/org/frice/utils/shape/Quads.kt
@@ -1,6 +1,9 @@
 package org.frice.utils.shape
 
 /**
+ * It's just an abstract representation of an object
+ * with x, y, width and height.
+ *
  * @author ice1000
  * @since v1.6.7
  * @see org.frice.utils.shape.FQuad
diff --git a/src/org/frice/utils/shape/Shapes.kt b/src/org/frice/utils/shape/Shapes.kt
index 5dba45b..87821b3 100644
--- a/src/org/frice/utils/shape/Shapes.kt
+++ b/src/org/frice/utils/shape/Shapes.kt
@@ -2,19 +2,12 @@ package org.frice.utils.shape
 
 import java.awt.geom.Rectangle2D
 
-/**
- * Created by ice1000 on 2016/8/14.
- * @author ice1000
- * @since v0.1.1
- */
-interface FShape
-
-interface FShapeInt : FShape {
+interface FShapeInt {
 	var width: Int
 	var height: Int
 }
 
-interface FShapeDouble : FShape {
+interface FShapeDouble {
 	var width: Double
 	var height: Double
 }
@@ -42,7 +35,7 @@ open class FOval(rh: Double, rv: Double) : FShapeInt {
  * @author ice1000
  * @since v0.3
  */
-data class FPoint(var x: Int, var y: Int) : FShape
+data class FPoint(var x: Int, var y: Int)
 
 /**
  * Created by ice1000 on 2016/8/14.