The idea of the unBound receiver such as String::length is you’re referring to a
method of an object that will be supplied as one of the lambda’s parameters. For example,
the lambda expression (String s) -> s.toUpperCase() can be rewritten as String::toUpperCase.
But Bounded refers to a situation when you’re calling a method in a
lambda to an external object that already exists. For example, the lambda expression () -> expensiveTransaction.getValue() can be rewritten as expensiveTransaction::getValue.
Situations for three different ways of method reference
(args) -> ClassName.staticMethod(args)
can be ClassName::staticMethod // This is static (you can think as unBound also)
(arg0, rest) -> arg0.instanceMethod(rest)
can be ClassName::instanceMethod (arg0 is of type ClassName) // This is unBound
(args) -> expr.instanceMethod(args)
can be expr::instanceMethod // This is Bound
Answer retrieved from Java 8 in Action book