?1  class ?Component1 {
 ?2 ? public ?Component1( int ?i) {
 ?3 ?System.out.println( " Component1 " );
 ?4 ?}
?5 }
?6 
 ?7  class ?Component2 {
 ?8 ? public ?Component2( int ?i) {
 ?9 ?System.out.println( " Compunent2 " );
 10 ?}
11 }
12 
 13  class ?Component3 {
 14 ? public ?Component3( int ?i) {
 15 ?System.out.println( " Compunent3 " );
 16 ?}
17 }
18 
 19  class ?Root {
 20 ? private ? int ?i;
 21 ?Component1?com1 = new ?Component1(i);
 22 ?Component2?com2 = new ?Component2(i);
 23 ?Component3?com3 = new ?Component3(i);
 24 ? public ?Root( int ?i) {
 25 ? this .i = i;
 26 ?System.out.println(com1);? // (2)
27 ?System.out.println( " Root " );
 28 ?}
29 ? public ?String?toString() {? return ? " Root " ;}
30 ? public ? void ?print() {System.out.println( " Root " );}
31 }
32 
 33  public ? class ?Stem? extends ?Root {
 34 ? private ? int ?i;
 35 ?Component1?com1 = new ?Component1(i);
 36 ?Component2?com2 = new ?Component2(i);
 37 ?Component3?com3 = new ?Component3(i);
 38 ? public ?Stem( int ?i) {
 39 ? super (i);
 40 ? this .i = i;
 41 ?System.out.println(com1);? // (1)
42 ?System.out.println( " Stem " );
 43 ?}
44 ? public ?String?toString() {? return ? " Stem " ;}
45 ? public ? void ?print() {System.out.println( " Stem " );}
46 ? public ? static ? void ?main(String[]?args) {
 47 ? // Stem?st=new?Stem(10);? // (1)
48 ?Root?st = new ?Stem( 10 );? // (2)
49 ?System.out.println(st.com1); // (1)?這里的com1是Stem中的Component1對(duì)象
50 ? // (2)?這里的com1是Root中的Component1對(duì)象
51 ?
 52 ?System.out.println(st); // 向下轉(zhuǎn)型了
53 ?st.print(); // 向下轉(zhuǎn)型了
54 ?}
55 }
56
這里可以看出，就st對(duì)象的類型來(lái)取得相應(yīng)類的成員變量！
好像只有在用到成員方法才會(huì)自動(dòng)向下轉(zhuǎn)型，而成員變量卻不會(huì)！