Given import java.util.List; import java.util.function.BinaryOperator; public class Main { public static void main(String... args) { List<Employee> list = List.of(new Employee("John", 80000.0), new…

Question

Options

• Adouble totalSalary = list.stream().map(e -> e.getSalary() * ratio).reduce(bo).ifPresent(p -> p. doubleValue());
• Bdouble totalSalary = list.stream().mapToDouble(e -> e.getSalary() * ratio).sum();
• Cdouble totalSalary = list.stream().mapToDouble(e -> e.getSalary() * ratio).reduce(0.0,bo).orElse(0.0);
• Ddouble totalSalary = list.stream().mapToDouble(e -> e.getSalary() * ratio).reduce(starts, bo);

• A
  4% (1)
• B
  11% (3)
• C
  78% (21)
• D
  7% (2)

Explanation

Option C is correct because mapToDouble() returns a DoubleStream (not Stream<Double>), and DoubleStream's single-argument reduce(DoubleBinaryOperator) returns an OptionalDouble - meaning .orElse(0.0) is required to safely unbox the result into a double, mirroring line 1's behavior of returning a plain double with a guaranteed identity of 0.0.

Why the distractors fail:

• A - reduce(bo) on Stream<Double> returns Optional<Double>, and ifPresent() returns void, so the assignment to double totalSalary won't compile.
• B - .sum() is hardcoded summation that doesn't delegate to bo at all; it's numerically equivalent for this input, but "equivalent to line 1" means using the same accumulator contract, not just producing the same number.
• D - After mapToDouble() you're on DoubleStream, which expects a DoubleBinaryOperator, not a BinaryOperator<Double> - a type mismatch that causes a compile error, and there's no .orElse() to handle the OptionalDouble return.

Memory tip: When you convert a Stream<Double> to a DoubleStream via mapToDouble(), remember the "primitive stream tax" - reduce without an identity returns OptionalDouble, so you always need .orElse() or .getAsDouble() to land back on a primitive double.

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