Java Solutions: Stream

Showing posts with label Stream. Show all posts

Thursday, December 21, 2017

Java 8: Calculate average, sum, maximum, minimum and count of elements of a list

The easiest way probably is to get the summary statistics of the list through stream first. Get all the values, then, from the summary statistics. Below is an example for a integer list. The same logic applies to all other type of numbers.

List<Integer> intList = Arrays.asList(123, 886, 43, 25, 33, 89);

IntSummaryStatistics intStat = intList.stream()
.mapToInt ( i -> i)
.summaryStatistics();

double average = intStat.getAverage();
long sum = intStat.getSum();
long count = intStat.getCount();
int max = intStat.getMax();
int min = intStat.getMin();

Of course, if you just want to get the sum or find the maximum, you can do it like below.

int sum = intList.stream()
.mapToInt(i -> i)
.sum();

int max = intList.stream()
.mapToInt(i -> i)
.max()
.orElse(0);

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Friday, December 15, 2017

java 8: Convert a long array to String array

//Convert a primitive long[] to String[]
long[] pla = {1155L, 22334L, 559876L};

String[] spla = Arrays.stream(pla)
.mapToObj(String :: valueOf)
.toArray(String[] :: new);

//Convert a boxed Long[] to String[]
Long[] bla = LongStream.of(pla)
.boxed().toArray(Long[] :: new);

String[] sbla = Arrays.stream(bla)
.mapToLong(Long::longValue)
.mapToObj(String::valueOf)
.toArray(String[] :: new);

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java 8: Convert a Long number list to a boxed and unboxed primitive array / convert a String list to String array by Stream

To convert a Long list to array, while Long[] ar = list.stream().toArray(size -> new Long[size]) is perfectly fine, long[] par = list.stream().toArray(size -> new long[size]) does not work. The argument of the toArray method, the IntFunction, takes an parameter type that is bound to Object, and long is not an Object while Long is.

The workaround is to map each element in the Stream to a Long so that when the toArray method without argument is called, it returns an long[] instead of an Object[].

import java.util.List;
import java.util.Arrays;

public class StreamListToArray {
public static void main(String[] args){
List<Long> list = Arrays.asList(new Long(11238), new Long(55444), new Long(1099886));

Long[] ar = list.stream().toArray(size -> new Long[size]);
for(Long lg : ar){
System.out.println(lg);
}
System.out.println();

//Method 1
long[] ars = list.stream().mapToLong(Long :: new).toArray();
for(Long lg : ars){
System.out.println(lg);
}
System.out.println();

//Method 2
long[] par = list.stream().mapToLong(Long :: longValue).toArray();
for(long lg : par){
System.out.println(lg);
}

System.out.println();
List<String> slist = Arrays.asList("String line 1", "String line 2", "String line 3");

String[] sar = slist.stream().toArray(size -> new String[size]);
for(String s : sar){
System.out.println(s);
}
}
}

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