Java 8 lambda表达式求值

Question

I have a method similar to the following one:

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 DoubleStream 's sum method be executed? Once or O(n) times, with n = doubleArray.length ?

Answer 1

This code will throw an exception, since you can't consume the same Stream more than once. You can only execute one terminal operation on a Stream.

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 running time will be quadratic ( O(n^2) ), since the sum will be computed n times.

A better approach would be to compute the sum just 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.