我理解如果一个类实现了包含相同名称的默认方法的多个接口,那么我们需要在子类中重写该方法,以便明确定义我的方法将执行的操作.问题是,请参阅下面的代码：
interface A {
default void print() {
System.out.println(" In interface A ");
}
}
interface B {
default String print() {
return "In interface B";
}
}
public class C implements A,B {
@Override
public String print() {
return "In class C";
}
public static void main(String arg[]) {
// Other funny things
}
}
现在,接口A和B都有一个名为’print’的默认方法,但我想覆盖接口B的print方法 – 返回字符串并按原样保留A的打印方式.但是这段代码不能编译给出：
Overrides A.print
The return type is incompatible with A.print()
很明显,编译器试图覆盖A的打印方法,我不明白为什么！
这是不可能的.
8.4.8.3：
If a method declaration d1 with return type R1 overrides or hides the declaration of another method d2 with return type R2,then d1 must be return-type-substitutable for d2,or a compile-time error occurs.
8.4.5：
A method declaration d1 with return type R1 is return-type-substitutable for another method d2 with return type R2 iff any of the following is true:
-
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 one of the following is true:
-
R1,adapted to the type parameters of d2,is a subtype of R2.
-
R1 can be converted to a subtype of R2 by unchecked conversion.
-
d1 does not have the same signature as d2,and R1 = |R2|.
换句话说,void,primitive和reference-returns方法可能只会被相同的相应类别的方法覆盖和覆盖. void方法可能只覆盖另一个void方法,引用返回方法可能只覆盖另一个引用返回方法,依此类推.
您遇到的问题的一种可能解决方案可能是使用组合而不是继承：
class C {
private A a = ...;
private B b = ...;
public A getA() { return a; }
public B getB() { return b; }
}
