牛仔褲的夏天

我對decorate模式的理解：
1-首先要有一個Interface，提供一個通用的方法；
2-其次有n多個class來實現這個接口中的方法，這些實現方法在功能上應該是并列的，但不是必須的，從而可以根據需要decorate那個需要被修飾的初始對象。
3-最后這些class都有一個帶有Interface作為參數的構造方法和一個Interface的屬性，這樣在構造的時候就可以取得那個需要修飾的對象的實例，然后在實現Interface的方法里面，首先實現修飾對象實例的修飾方法，然后再進行自己的decorate方法。

根據這個想法來畫一張畫：

package ?test;

public ? interface ?Painting? {
???? public ? void ?paint();
}

class ?SunPainting? implements ?Painting? {
????Painting?painting? = ? null ;

???? public ?SunPainting()? {
????}

???? public ?SunPainting(Painting?painting)? {
???????? this .painting? = ?painting;
????}

???? public ? void ?paint()? {
???????? if ?(painting? != ? null )? {
????????????painting.paint();
????????}
????????System.out.println( " 畫了一個太陽 " );
????}
}

class ?CloudPainting? implements ?Painting? {
????Painting?painting? = ? null ;

???? public ?CloudPainting()? {
????}

???? public ?CloudPainting(Painting?painting)? {
???????? this .painting? = ?painting;
????}

???? public ? void ?paint()? {
???????? if ?(painting? != ? null )? {
????????????painting.paint();
????????}
????????System.out.println( " 畫了一朵白云 " );
????}
}

class ?LawnPainting? implements ?Painting? {
????Painting?painting? = ? null ;

???? public ?LawnPainting()? {
????}

???? public ?LawnPainting(Painting?painting)? {
???????? this .painting? = ?painting;
????}

???? public ? void ?paint()? {
???????? if ?(painting? != ? null )? {
????????????painting.paint();
????????}
????????System.out.println( " 畫了一片草地 " );
????}
}

再寫段測試畫畫的代碼:

package ?test;

public ? class ?TestPainting? {
???? public ? static ? void ?main(String[]?args) {
???????? new ?SunPainting( new ?CloudPainting( new ?LawnPainting())).paint();
????}
}

測試結果:

畫了一片草地
畫了一朵白云
畫了一個太陽

?最后有個問題：為什么不直接定義一個畫畫的類，然后把這些該怎么畫的事情分別通過方法來實現呢？ 那么什么情況下使用decorate模式更為合適呢？