A lambda expression is an anonymous Java function-like expression that supplies an implementation compatible with a functional interface. Its parameter list makes argument flow explicit, including arguments that the body ignores. section 2.13
Avoiding an accidental constructor binding
In contents.computeIfAbsent(fileName, StringBuilder::new), the method reference receives the map key and can select StringBuilder(String), producing content such as numbers.txt3.1415. Convert it to s -> new StringBuilder() when the key should be ignored.
Use lambda syntax when an overloaded method or constructor makes a compact method reference ambiguous. Naming the parameter and writing the target call exposes which declaration receives it; an IDE conversion can help reveal the binding. section 2.13 section 2.15
