Java How to Program, Early Objects, 11th Edition

17.9 Lambdas: A Deeper Look

Type Inference and a Lambda’s Target Type

Lambda expressions can be used anywhere functional interfaces are expected. The Java compiler can usually infer the types of a lambda’s parameters and the type returned by a lambda from the context in which the lambda is used. This is determined by the lambda’s target type—the functional-interface type that’s expected where the lambda appears in the code. For example, in the call to IntStream method map from stream pipeline in Fig. 17.4

IntStream.rangeClosed(1, 10)
         .map((int x) -> {return x * 2;})
         .sum()

the target type is IntUnaryOperator, which represents a method that takes one int parameter and returns an int result. In this case, the lambda parameter’s type is explicitly ...