Java8 variable in for loop

Question

int sum = a;
int pow = 1;

for (int i = 0; i < n ; i++) {
    sum += b*pow;
    System.out.print(sum+" ");
    pow *= 2;
}

In Java-8 on using Stream gives errors for sum and pow variable that the variable should be final.

You could use a AtomicInteger instead of regular int to circumvent that issue. — Edwin Dalorzo
– Edwin Dalorzo, Commented Jan 7, 2020 at 20:03
@EdwinDalorzo: It's a workaround as well as a trick with an array, however, wrong by principle. The java-stream provides mechanisms to process such operations. — Nikolas
– Nikolas, Commented Jan 7, 2020 at 20:45
Seeing AtomicInteger is going to make readers wonder how a simple pipeline is concurrent, and impose another unnecessary performance penalty beyond converting the loop to a stream. I strongly discourage this abuse of the type. — erickson
– erickson, Commented Jan 7, 2020 at 23:58

Nikolas · Accepted Answer · 2020-01-07 20:44:16Z

You can use the generated Stream using IntStream and process the numbers in the same way. Note that Math::pow returns Double therefore the pipeline results in DoubleStream.

IntStream.range(0, n).mapToDouble(i -> b * Math.pow(2, i)).reduce(Double::sum);

The only disadvantage is no consumer is available during the reducing, therefore you have to amend it a bit:

IntStream.range(0, n).mapToDouble(i -> b * Math.pow(2, i)).reduce((left, right) -> {
    double s = left + right;
    System.out.println(s);
    return s;
});

To answer this:

In java8 on using stream give the errors for sum and pow variable that the variable should be final.

One of the java-stream conditions is that the variables used within lambda expressions must be either final or effectively-final, therefore the mutable operations you used in the for-loop are not allowed and have to be replaced with mapping feature of Streams. This issue is nicely explained at http://ilkinulas.github.io.

Remember, you cannot use java-stream the same way you use for-loops.

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