Decompose a String into Array of Long or List of Long without Loop in JAVA

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I want to decompose a String array into Long array or List. I don't want to use Loop.

Is there any Java Method to do this.

There is no O(1) operation to "convert" a String[] (with numeric strings) to a long[]. It will always be O(n), if the loop visible or hidden in some thirdparty method.

If you don't want to "see" the loop, simply implement a method

Long[] pseudoOneStepConversion(numbers);

and implement

privat Long[] pseudoOneStepConversion(String[] numbers) {
  Long[] result = new long[numbers.length];
  for (int i = 0; i < numbers.length; i++)
     result[i] = Long.parseLong(numbers[i]);
  return result;

We can do it recursively too - it is still O(n), less performant and doesn't look like a loop:

public static void main(String[] args) {
    List<Long> target = new ArrayList<Long>();
    copy(new String[]{"1", "2", "3"}, target, 0);

private static void copy(String[] source, List<Long> target, int index) {
    if (index == source.length)
    copy(source, target, index+1);

Note - because I start getting downvotes for the recursion example: It is purely academic and not inteded for use in production code - thought, that was clear ;)

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Simplified Eugene answer with Guava library. Since Guava 16.0.

List<Long> longList = Lists.transform(Arrays.asList(stringArray), Longs.stringConverter());

Update: Solution with Java 8, without 3th party libraries:

List<Long> longList = Stream.of(stringArray).map(Long::valueOf).collect(Collectors.toList());

Split String Into Array Of Integers in Java, In order to split string into an array of integers, it's necessary to invoke the for ( int i = 0; i < strArr.length; i++) { String num = strArr[i]; intArr[i] = Integer. There should be a single loop to go through each element to perform the Unfortunately, there seems no one function call in current Java api to do this,� Because of the dichotomy between primitives and objects in Java, you can't use the generic list List<Long>.toArray(Long[]) to build a primitive array as the result. There are primitive collections which can be used, but either way - using a list or working over the groups - you're copying data from a temporary storage to a primitive array.

With a little help of 3rd party libraries you can avoid coding loops in your own code, but there would be a loop somewhere under the hood. For example:

List<String> stringList = Arrays.asList(stringArray);
List<Long> longList = Lists.transform(stringList, new Function<String, Long>() {
   public Long apply(String s) {
      return Long.valueOf(s);

Classes Lists and Function are from Guava library.

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No there is no way to do this without a loop (even if you don't code explicitly a loop, the method you will call will use one), unless you now the number of long values contained in the String and add them manually to your List.

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Your best option is Guavas Lists.transform function.

String[] stringArray = {"1999", "20000"};
List<String> stringList = Arrays.asList(stringArray);
List<Long> longList = Lists.transform(stringList, 
                                      new Function<String, Long>() {
                                              Long apply(String value) {
                                                return Long.valueOf(value);

Note: Guava will of course have to do a loop to achieve this, but there are no loops in your code.

Java Split String Into ArrayList Examples, Float To String Without Exponential Scientific Notation � Long Max Value � String The split() method of the String class splits a String into an array of the output of the Java String Split, which is a String array, to an ArrayList is to The code manually loops the array list and add the items to the ArrayList. Note that in bozho's answer, the array is not copied twice, but only once : Arrays.asList doesn't return a copy, but returns a wrapper around the array (the wrapper implements the List interface). The first copy is needed because Arrays.asList doesn't work with primitive types, hence the first conversion from primitive (long) to object type (Long).

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  • So the array contains string representations of longs? You need to be a lot more specific and include both desired in-/output and your attempt at solving it yourself.
  • Duplicate of this question asked recently.
  • @milkplusvellocet: No, the other question is about parsing a String into an array of ints. This question is about "converting" a String array into a Long array (without defining the "converting" part)
  • Perhaps OP should clarify what they mean. This popped up a very short time after a very similarly worded question which is why it stood out to me as a duplicate. Anyway the essence of the question (not wanting to use a loop) is the same.
  • @milkplusvellocet maybe a bunch of people taking an online exam or something
  • I think this is not about time complexity, but about code conciseness. As I read it, OP needs something like map in functional programming (I think Guava has something like that).
  • Unforunately in Java, loops are the most concise/readable form in many of these cases. Perhaps closures will change this in Java 8.
  • Down voting, because recursion performs much worse then plain loop in Java. There are optimizations in the JVM JIT compilers targeting loops, but currently there is no optimization for tail recursion in the JVM (unlike Lisp and few other VMs).
  • Err, Eugene, please read question and answer carefully: (a) Lalit asked for a solution without loops and (b) I already warned, that the recursion is not advisable ("less performant").
  • This is the best answer if you are looking to minimize no of lines of code.
  • Of course there is: Tail recursion. When it comes down to the processor level, all control structures are goto. Also, I don't think it would be a problem for OP if the helper function uses a loop, as long as he doesn't have to polute his/her code with repetitive iteration.