Java 8 Stream API allows to do this:
double[] doubleValues = Arrays.stream(guaranteedOutput)
.mapToDouble(Double::parseDouble)
.toArray();
Double colon is used as a method reference. Read more here.
Before using the code don’t forget to import java.util.Arrays;
UPD: If you want to cast your array to Double[], not double[], you can use the following code:
Double[] doubleValues = Arrays.stream(guaranteedOutput)
.map(Double::valueOf)
.toArray(Double[]::new);