Java 8 lambda expression evaluation

Question

I have a method similar to the following:

public double[] foo(double[] doubleArray) { 
    DoubleStream stream = Arrays.stream(doubleArray);

    return stream.map(s -> s / stream.sum()).toArray();
}

What is the complexity of this method? How many times will the method be executed DoubleStream sum? Once or O(n)once, s n = doubleArray.length?

Answer 1

This code throws an exception because you cannot use the same thread more than once. You can only perform one terminal operation per thread.

If you change the code to:

public double[] foo(double[] doubleArray) { 
    return Arrays.stream(doubleArray).map(s -> s / Arrays.stream(doubleArray).sum()).toArray();
}

it will work, but the runtime will be quadratic ( O(n^2)), since the sum will be calculated nonce.

A better approach would be to calculate the sum only once:

public double[] foo(double[] doubleArray) { 
    double sum = Arrays.stream(doubleArray).sum();
    return Arrays.stream(doubleArray).map(s -> s / sum).toArray();
}

This will run in linear time.