Difference between method reference Bound Receiver and Unbound Receiver

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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

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