import javax.swing.table.AbstractTableModel;
import java.util.ArrayList;

class PatioProjectTableModel extends AbstractTableModel {
	private String[] columnNames = {"shape", "dimension", "material", "cost"};
	// each elemnt's property in separate arrays:
	private ArrayList<String> shapeList;
	private ArrayList<Double> widthList;
	private ArrayList<Double> lengthList;
	private ArrayList<String> materialList;
	private ArrayList<Double> costList;

	/**
	 * constructor
	 */
	public PatioProjectTableModel() {
		super();
		shapeList = new ArrayList<String>();
		widthList = new ArrayList<Double>();
		lengthList = new ArrayList<Double>();
		materialList = new ArrayList<String>();
		costList = new ArrayList<Double>();
	}

	/** 
	 * column count
	 */
	public int getColumnCount() {
		return columnNames.length;
	}

	/**
	 * row count
	 */
	public int getRowCount() {
		// just assume that all arrays are the same size
		return shapeList.size();
	}

	/**
	 * column name
	 */
	public String getColumnName(int col) {
		return columnNames[col];
	}

	... more code which isn't finished
}