Why is lambda return type not checked at compile time?

Question

The used method reference has return type Integer. But an incompatible String is allowed in the following example.

How to fix the method <

Answer 1

Its the type inference that is playing its role here. Consider the generic R in the method signature:

 Builder with(Function getter, R returnValue)

In the case as listed:

Builder.of(MyInterface.class).with(MyInterface::getLength, "I am NOT an Integer");

the type of R is successfully inferred as

Serializable, Comparable>

and a String does imply by this type, hence the compilation succeeds.

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To explicitly specify the type of R and find out the incompatibility, one can simply change the line of code as :

Builder.of(MyInterface.class).with(MyInterface::getLength, "not valid");