OO diagrams - the object diagram
A single object diagram in OO gives you a snapshot of the data held in some object at any particular moment in time. For this reason, object diagrams are also known as instance diagrams. A sequence of object diagrams can show you how some actual objects might change as a sequence of events unfolds, or how they might change as time passes, or both.
For each object you are focusing on, you specify the data actually held in the attributes in the form of a diagram. The diagram itself is laid out in exactly the same way as a class diagram except there are usually two parts rather than three. This means that an object in an object diagram is represented as a box, which is split into two parts. The top part holds the name of the object. The second part holds the attributes that the object has along with the actual data in those attributes.
It is usual to copy across the definition of the class that an object was based on and simply replace the data type for each attribute in the class diagram with the actual piece of data being held to get the object diagram.