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[New] Added CollectionsHelper.singletonCollector(boolean allowNull)

This commit is contained in:
Robert von Burg 2018-12-05 15:47:19 +01:00
parent 55b762f2f1
commit d9cdda2157
1 changed files with 44 additions and 2 deletions
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46
li.strolch.utils/src/main/java/li/strolch/utils/collections/CollectionsHelper.java
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@ -56,7 +56,25 @@ public class CollectionsHelper {
* if not 1 and only 1 element is in the stream
*/
public static <T> Collector<T, List<T>, T> singletonCollector() throws IllegalStateException {
return singletonCollector(() -> null);
return singletonCollector(false, () -> null);
}
/**
* Returns a collector which returns exactly one element from a stream, and throws an exception if not exactly one
* element is in the stream
*
* @param allowNull
* if true, then if the stream is empty, null is returned, instead of an exception is thrown
* @param <T>
* the type of element to return
*
* @return the singleton collector
*
* @throws IllegalStateException
* if not 1 and only 1 element is in the stream
*/
public static <T> Collector<T, List<T>, T> singletonCollector(boolean allowNull) throws IllegalStateException {
return singletonCollector(allowNull, () -> null);
}
/**
@ -74,7 +92,7 @@ public class CollectionsHelper {
* if not 1 and only 1 element is in the stream
*/
public static <T> Collector<T, List<T>, T> singletonCollector(String errorMsg) throws IllegalStateException {
return singletonCollector(() -> errorMsg);
return singletonCollector(false, () -> errorMsg);
}
/**
@ -93,12 +111,36 @@ public class CollectionsHelper {
*/
public static <T> Collector<T, List<T>, T> singletonCollector(Supplier<String> errorMsgSupplier)
throws IllegalStateException {
return singletonCollector(false, errorMsgSupplier);
}
/**
* Returns a collector which returns exactly one element from a stream, and throws an exception if not exactly one
* element is in the stream
*
* @param allowNull
* if true, then if the stream is empty, null is returned, instead of an exception is thrown
* @param errorMsgSupplier
* the supplier for an error message to use if not 1 and only 1 element is in the collection
* @param <T>
* the type of element to return
*
* @return the singleton collector
*
* @throws IllegalStateException
* if not 1 and only 1 element is in the stream
*/
public static <T> Collector<T, List<T>, T> singletonCollector(boolean allowNull, Supplier<String> errorMsgSupplier)
throws IllegalStateException {
return Collector.of(ArrayList::new, List::add, (left, right) -> {
left.addAll(right);
return left;
}, list -> {
if (list.isEmpty()) {
if (allowNull)
return null;
String errorMsg = errorMsgSupplier.get();
if (errorMsg == null) {
throw new IllegalStateException("Expect one element, but received no elements");