What is the purpose / use of abstract classes? (Look for examples in the real world.)

Question

Can someone show me an example of an abstract class in Java? Something using the real world (rather than a sample textbook) would be desirable.

Thank!

Answer 1

Read this tutorial on Abstract Methods and Classes .

, GraphicObject, , , , moveTo . GraphicObject , , , -. GraphicObject - :

abstract class GraphicObject {
    int x, y;
    ...
    void moveTo(int newX, int newY) {
        ...
    }
    abstract void draw();
    abstract void resize();
}

GraphicObject, Circle :

class Circle extends GraphicObject {
    void draw() {
        ...
    }
    void resize() {
        ...
    }
}
class Rectangle extends GraphicObject {
    void draw() {
        ...
    }
    void resize() {
        ...
    }
}

Answer 2

, java.util.AbstractCollection?

Answer 3

AbstractTableModel. AbstractTableModel TableModel ( JTable). TableModel :

public boolean isCellEditable(int rowIndex, int columnIndex);
public Class<?> getColumnClass(int columnIndex);
public int getColumnCount();
public int getRowCount();
public Object getValueAt(int rowIndex, int columnIndex);
public String getColumnName(int columnIndex);
public void addTableModelListener(TableModelListener l);
public void removeTableModelListener(TableModelListener l);
public void setValueAt(Object aValue, int rowIndex, int columnIndex);

AbstractTableModel , (getRowCount, getColumnCount getValueAt), exteding . , .

Answer 4

Java abstract classes , .
, ( ). . , ( ). , .

.

abstract class A {
         public abstract abs_value();

         void show() 
                   {
                   System.out.println("This is an abstract class");
                   }
               }

Answer 5

, :

, , , ORM, ​​ Hibernate ( , , , / ).

, , (DAO) , , DAO , . AbstractBaseDao, , DAO:

public class AbstractBaseDao {
    public void saveOrUpdate(Object o){
         //save or update object from database
    }

    public void loadAll(Class clazz){
        //Load all from database
    }

    public Object loadById(Class clazz, Long id){
       // Retrieve object by Id from database
    }

    // Additional common methods
}

, , , , DAO , .

BaseDao DAO , - :

public class PersonDao extends AbstractBaseDao{
     public List<People> listAllPersonsByLastName(String lastName){
         //Load people by last names from database
     }

     public List<People> listAllPersonsByBirthYear(Date date){
        //List people by birth year from database
     }
}

PersonDao , , , . .

, , , , .