Thread: Abstract class

Well part of one of my assignments is to write an abstract class. I know an abstract class is used for inheritance and cannot be instantiated. I'm aware that all of the real coding is done in the subclasses. I just can't get a grip on what's suppose to be in the abstract class other than abstract methods. This is what I have so far

public abstract class DataItem {
 
	//abstract,method will determine if two objects contain the same data
	//Pre-condition: Object contains data is the object otherItem
	//Post-condition: Returns true 
	public abstract boolean equals(DataItem otherItem);
 
	//Method will compare two objects.
	//Pre-condition: Returns a value < 0 if this element is less than otherItem
	//Post-condition: Returns 0 if this element is the same as other item. 
	//returns a value > 0 if this element is greater than otherItem.
	public abstract int compareTo(DataItem otheritem);
 
 
	//Method to return a copy of this object.
	//Post-condition: A copy of this object is created and reference of the copy is returned.
	public abstract DataItem getCopy();	
 
}// End abstract class DataItem.