Java 多型的“缺陷”
阿新 • • 發佈:2018-11-08
四種“缺陷”
私有方法
程式碼:
public class PrivateOverride { private void f() { System.out.println("private f()"); } public static void main(String[] args) { PrivateOverride po = new Derived(); po.f(); } } class Derived extends PrivateOverride { public void f() { System.out.println("public f()"); } }
輸出結果:
private f()
總結:
只有非 private 方法才可以被覆蓋,因此在匯出類中,對於基類的中的 private 方法,最好採用不同的名字。
類的屬性
程式碼:
class Super { public int field = 0; public int getField() { return field; } } class Sub extends Super { public int field = 1; @Override public int getField() { return field; } public int getSuperField() { return super.field; } } public class FieldAccess { public static void main(String[] args) { Super sup = new Sub(); System.out.println("sup.field = " + sup.field + ", sup.getField() = " + sup.getField()); Sub sub = new Sub(); System.out.println("sub.field = " + sub.field + ", sub.getField() = " + sub.getField() + ", sub.getSuperField() = " + sub.getSuperField()); } }
輸出結果:
sup.field = 0, sup.getField() = 1
sub.field = 1, sub.getField() = 1, sub.getSuperField() = 0
總結:
當進行向上轉型時,任何訪問類的屬性操作都是由編譯器解析,因此不是多型。因此,我們一般建議把類的屬性設定為 private,然後通過方法來訪問屬性值,這樣就避免來因為多型而帶來的屬性取值問題。
靜態方法
程式碼:
class StaticSuper { public static String staticGet() { return "base staticGet()"; } public String get() { return "base get()"; } } class StaticSub extends StaticSuper { public static String staticGet() { return "sub staticGet()"; } @Override public String get() { return "sub get()"; } } public class StaticAccess { public static void main(String[] args) { StaticSuper ss = new StaticSub(); System.out.println(ss.staticGet()); System.out.println(ss.get()); } }
輸出結果:
base staticGet()
sub get()
總結:
靜態方法不具有多型行為。
構造器和多型
程式碼:
class Glyph {
public Glyph() {
System.out.println("Glyph() before draw()");
draw();
System.out.println("Glyph() after draw()");
}
void draw() {
System.out.println("Glyph.draw()");
}
}
class RoundGlyph extends Glyph {
private int radius = 1;
RoundGlyph(int r) {
radius = r;
System.out.println("RoundGlyph(i).radius = " + radius);
}
void draw() {
System.out.println("RoundGlyph.draw(), radius = " + radius);
}
}
public class PolyConstructor {
public static void main(String[] args) {
new RoundGlyph(5);
}
}
輸出結果:
Glyph() before draw()
RoundGlyph.draw(), radius = 0
Glyph() after draw()
RoundGlyph(i).radius = 5
總結
基類構造器裡面的 draw() 方法實際執行的是子類裡面的,這就導致子類的屬性沒有被正確初始化,因此我們一定要避免這種寫法。