Find numbers which begins with a given number in a Map in Java

I'd like to count all keys in a HashMap which begin with a given number.
The size of each key is not always the same.
Example:

given number(long):

long l = 9988776655


find the keys (long) which begin with that number like:

9988776655xxxxxxxxxxxxxxx


in which x stands for any integer.

How do I approach this problem? Since the length of the keys is not always the same I cannot do it with multiple modulo operations. (or can I?)

String.valueOf(veryLongNumber).startsWith(String.valueOf(smallerNumber))
– Lino
Jun 29 at 11:10

String.valueOf(veryLongNumber).startsWith(String.valueOf(smallerNumber))

Thanks for the quick answer!
– notsosmart.nk
Jun 29 at 11:13

3 Answers
3

I'd just convert the keys to strings:

public static long keysStartingWith(Map<Long, ?> map, long toSearch) {
String searchStr = String.valueOf(toSearch);
return map.keySet().stream().filter(k -> k.toString().startsWith(searchStr)).count();
}


You could also use an intermediate .map(Object::toString) but this is likely opinion based. Else, good answer!
– Lino
Jun 29 at 11:15

.map(Object::toString)

Try converting the long l and the keys to strings. Then compare the beginning of the strings. Something like this:

l

long l = 1234L;
Map<Long, Object> hashMap = new HashMap<>();
hashMap.put(1234567L, 1);
hashMap.put(1334567L, 2);
String longString = ""+l;
for(Map.Entry entry: hashMap.entrySet()) {
String keyString = ""+entry.getKey();
if(keyString.startsWith(longString)) {
System.out.println(entry.getValue());
}
}


the constructor call new String(); should not be used
– Lino
Jun 29 at 11:21

new String();

I agree. I wrote this out as an example for an approach, though it is far from being the best
– SelketDaly
Jun 29 at 11:23

Also instead of "" + somevalue always prefer ? String.valueOf(somevalue)
– Lino
Jun 29 at 11:25

"" + somevalue

String.valueOf(somevalue)

and BTW that is not casting - its creating a new String (or converting into a String)
– Carlos Heuberger
Jun 29 at 11:27

You could implement a general method, like this:

public static int numberOfStartsWith(Map mp, String start) {
int count = 0;
for (String key : map.keySet()) {
if (key.startsWith(start) count++;
}
return count;
}


and overload it, like this:

public static int numberOfStartsWith(Map mp, int start) {
return MyClass.numberOfStartsWith(mp, String.valueOf(start));
}


and

public static int numberOfStartsWith(Map mp, long start) {
return MyClass.numberOfStartsWith(mp, String.valueOf(start));
}


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