共有オブジェクト

Main
---
public class Main {

    public static void main(String[] args) {
        ModelA a = new ModelA( new Integer(10) );
        ModelB b = new ModelB( "name" );
		
        ModelFactory factory = ModelFactory.getInstance();
        factory.setModel( a );
		
        ModelA m = (ModelA) factory.getModel();
		
        factory.setModel( b );

　　　　// mの参照先って？？
    }
}

AbstractModel
---
public abstract class AbstractModel {
}

ModelA
---
public class ModelA extends AbstractModel {
    private Integer age;
	
    public ModelA( Integer age ) {
    	this.age = age;
    }
    
    public Integer getAge() {
        return age;
    }

    public void setAge( Integer integer ) {
        age = integer;
    }
}

ModelB
---
public class ModelB extends AbstractModel {
    private String name;
	
    public ModelB( String name ) {
    	this.name = name;
    }
    
    public String getName() {
        return name;
    }

    public void setName( String string ) {
        name = string;
    }
}

ModelFactory
---
public class ModelFactory {
    private static ModelFactory factory;
    private AbstractModel model;
	
    private ModelFactory() {}
	
    public static synchronized ModelFactory getInstance() {
        if ( factory == null ) factory = new ModelFactory();
        return factory;
    }
	
    public AbstractModel getModel() {
        return model;
    }

    public void setModel( AbstractModel model ) {
        this.model = model;
    }
}

	ModelA a = new ModelA( new Integer(10) );
	ModelB b = new ModelB( "name" );

	ModelFactory factory = ModelFactory.getInstance();

factory.setModel( a );

	ModelA m = (ModelA) factory.getModel();

	factory.setModel( b );