Android : How can I change ImageButton size from xml in java

I have a some problem in my android project.

This is my xml code for ImageButton with fixed width and height.

<ImageButton
    android:layout_width="120dp"
    android:layout_height="120dp"
    android:id="@+id/ball1"
    android:background="@drawable/ball"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true" />

And here i called in java that ImageButton:

ImageButton ball1 = (ImageButton) findViewById(R.id.ball1);

My question is, what I need to do, to subtract my layout_width and layout_height by 10dp in java code?

Something like this:

int width = ball1.getWidth();
    int height = ball1.getHeight();
width = width - 10dp;
height = height - 10dp;

So how can I subtract Height and Width by 10dp?