重写的方法可以在返回类型上有所不同吗? [英] Can overridden methods differ in return type?

问题描述

被覆盖的方法可以有不同的返回类型吗?

Can overridden methods have different return types?

推荐答案

Java 支持覆盖方法的^* 协变返回类型.这意味着被覆盖的方法可能具有 更多 特定的返回类型.也就是说，只要新的返回类型可以分配给您要覆盖的方法的返回类型，就可以使用.

Java supports^* covariant return types for overridden methods. This means an overridden method may have a more specific return type. That is, as long as the new return type is assignable to the return type of the method you are overriding, it's allowed.

例如:

class ShapeBuilder {
    ...
    public Shape build() {
    ....
}

class CircleBuilder extends ShapeBuilder{
    ...
    @Override
    public Circle build() {
    ....
}

这在 中指定Java 语言规范的第 8.4.5 节:

如果返回类型是引用类型，则返回类型可能因相互覆盖的方法而异.返回类型可替换性的概念支持协变返回，即将返回类型特化为一个子类型.

Return types may vary among methods that override each other if the return types are reference types. The notion of return-type-substitutability supports covariant returns, that is, the specialization of the return type to a subtype.

返回类型为 R1 的方法声明 d1 可以返回类型替换为另一个返回类型为 R2 的方法 d2，当且仅当以下条件成立:

A method declaration d1 with return type R1 is return-type-substitutable for another method d2 with return type R2, if and only if the following conditions hold:

• 如果 R1 为空，则 R2 为空.

• If R1 is void then R2 is void.

如果 R1 是原始类型，则 R2 与 R1 相同.

If R1 is a primitive type, then R2 is identical to R1.

如果 R1 是引用类型，则:

If R1 is a reference type then:

• R1 要么是 R2 的子类型，要么 R1 可以通过未经检查的转换(第 5.1.9 节)转换为 R2 的子类型，或者

• R1 is either a subtype of R2 or R1 can be converted to a subtype of R2 by unchecked conversion (§5.1.9), or

R1 = |R2|

("|R2|"指的是R2的擦除，定义在JLS §4.6.)

("|R2|" refers to the erasure of R2, as defined in §4.6 of the JLS.)

^{* 在 Java 5 之前，Java 具有不变返回类型，这意味着方法覆盖的返回类型需要与被覆盖的方法完全匹配.}

^{* Prior to Java 5, Java had invariant return types, which meant the return type of a method override needed to exactly match the method being overridden.}

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