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

In the first example, MyInterface::getLength and "I am NOT an Integer" helped to resolve the generic parameters T and R to MyInterface and Serializable & Comparable>respectively.

// it compiles since String is a Serializable
Function function = MyInterface::getLength;
Builder.of(MyInterface.class).with(function, "I am NOT an Integer");

MyInterface::getLength is not always a Function unless you explicitly say so, which would lead to a compile-time error as the second example showed.

// it doesn't compile since String isn't an Integer
Function function = MyInterface::getLength;
Builder.of(MyInterface.class).with(function, "I am NOT an Integer");