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                合規國際互聯網加速 OSASE為企業客戶提供高速穩定SD-WAN國際加速解決方案。 廣告
                ## [協變返回類型](https://lingcoder.gitee.io/onjava8/#/book/09-Polymorphism?id=%e5%8d%8f%e5%8f%98%e8%bf%94%e5%9b%9e%e7%b1%bb%e5%9e%8b) Java 5 中引入了協變返回類型,這表示派生類的被重寫方法可以返回基類方法返回類型的派生類型: ~~~ // polymorphism/CovariantReturn.java class Grain { @Override public String toString() { return "Grain"; } } class Wheat extends Grain { @Override public String toString() { return "Wheat"; } } class Mill { Grain process() { return new Grain(); } } class WheatMill extends Mill { @Override Wheat process() { return new Wheat(); } } public class CovariantReturn { public static void main(String[] args) { Mill m = new Mill(); Grain g = m.process(); System.out.println(g); m = new WheatMill(); g = m.process(); System.out.println(g); } } ~~~ 輸出: ~~~ Grain Wheat ~~~ 關鍵區別在于 Java 5 之前的版本強制要求被重寫的`process()`方法必須返回**Grain**而不是**Wheat**,即使**Wheat**派生自**Grain**,因而也應該是一種合法的返回類型。協變返回類型允許返回更具體的**Wheat**類型。
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