重写方法可以不同的返回类型吗？
解决方案Java支持 * 重写方法的协变返回类型。这意味着重写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你要覆盖的方法的返回类型，就允许这样做。
例如：
class ShapeBuilder { ... public Shape build（）{ .... } 类CircleBuilder扩展ShapeBuilder { ... @Override public Circle build（）{ .... }这是在 Java语言规范的第8.4.5节：
如果返回类型是引用类型，则返回类型可能会因覆盖彼此的方法而异。 return-type-substitutability的概念支持协变返回，即返回类型到子类型的特化。
返回类型为R1的方法声明d1返回-type-可替代另一个返回类型为R2的方法d2，当且仅当以下条件成立时：
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如果R1是如果R2是基本类型，则R2与R1相同。
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如果R1是引用类型，则：
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R1是R2的子类型或R1可以是通过未经检查的转换（第5.1.9节）转换为R2的子类型，或
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R1 = | R2 |
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（| R2 |指的是R2的擦除，如 JLS的§4.6中所定义。）
*在Java 5之前，Ja va有不变返回类型，这意味着需要方法覆盖的返回类型才能与被覆盖的方法完全匹配。
Can overridden methods have different return types?
解决方案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.
For example:
class ShapeBuilder { ... public Shape build() { .... } class CircleBuilder extends ShapeBuilder{ ... @Override public Circle build() { .... }This is specified in section 8.4.5 of the Java Language Specification:
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.
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:
If R1 is void then R2 is void.
If R1 is a primitive type, then R2 is identical to R1.
If R1 is a reference type then:
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|" refers to the erasure of R2, as defined in §4.6 of the JLS.)
* 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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被覆盖的方法可以在返回类型上有所不同吗？
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