Overriding and return type compatibility

Question

The following compiles without any problem

boolean flag = true;
Boolean flagObj = flag; 

Now imaging the following scenario

interface ITest{

     Boolean getStatus();

}

 class TestImpl implements ITest{

     public boolean getStatus(){ // Compile error: return type is incompatible
         return true;
     }
 }

My question is about the compile error at the mentioned line. My Interface mentions return type as Boolean but the implemented method returns boolean ( the literal )

My question is, if Boolean and boolean are compatible then why the compiler is complaining ? Doesn't the autoboxing apply here ?

Answer 1

You can only return a sub-class of the parent's return type.

The compile lets you auto-box and unbox between primitives and wrappers but this doesn't make one a sub-class of the other. Primitives are not classes and cannot be used in the way you suggest.

I would just have the getStatus() return Boolean or make the parent return boolean

In theory, auto-boxing could be extended to allow what you suggest, but I don't imagine much use for it.

In theory you could also write this

class A {
    int method() { ... }
}

class B extends A {
    short method() { .... }
}

As the compiler supports implicit upcasting. However again, I suspect there is not much use for this either.

Answer 2

The methods have different signatures on the prototype and the implementation. The primitive, not being a class, cannot subclass the Boolean of the prototype. Even with autoboxing, the implementation violates the general prototype. Auto-unboxing is performed after a return so if getStatus was implemented like so:

public Boolean getStatus(){ // Compile error: return type is incompatible
     return Boolean.TRUE;
}

it could be unboxed after returning as:

if(getStatus()) doSomething();

Answer 3

我们知道，我们只能返回父类返回类型的子类.Here Boolean是包装类，而boolean是原始数据类型。简而言之，两者作为包装类和原语不同。因此它给出了不兼容的错误。