[New] Added CollectionsHelper.singletonCollector(boolean allowNull)
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@ -56,7 +56,25 @@ public class CollectionsHelper {
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* if not 1 and only 1 element is in the stream
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*/
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public static <T> Collector<T, List<T>, T> singletonCollector() throws IllegalStateException {
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return singletonCollector(() -> null);
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return singletonCollector(false, () -> null);
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}
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/**
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* Returns a collector which returns exactly one element from a stream, and throws an exception if not exactly one
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* element is in the stream
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*
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* @param allowNull
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* if true, then if the stream is empty, null is returned, instead of an exception is thrown
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* @param <T>
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* the type of element to return
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*
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* @return the singleton collector
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*
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* @throws IllegalStateException
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* if not 1 and only 1 element is in the stream
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*/
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public static <T> Collector<T, List<T>, T> singletonCollector(boolean allowNull) throws IllegalStateException {
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return singletonCollector(allowNull, () -> null);
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}
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/**
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@ -74,7 +92,7 @@ public class CollectionsHelper {
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* if not 1 and only 1 element is in the stream
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*/
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public static <T> Collector<T, List<T>, T> singletonCollector(String errorMsg) throws IllegalStateException {
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return singletonCollector(() -> errorMsg);
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return singletonCollector(false, () -> errorMsg);
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}
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/**
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@ -93,12 +111,36 @@ public class CollectionsHelper {
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*/
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public static <T> Collector<T, List<T>, T> singletonCollector(Supplier<String> errorMsgSupplier)
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throws IllegalStateException {
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return singletonCollector(false, errorMsgSupplier);
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}
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/**
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* Returns a collector which returns exactly one element from a stream, and throws an exception if not exactly one
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* element is in the stream
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*
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* @param allowNull
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* if true, then if the stream is empty, null is returned, instead of an exception is thrown
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* @param errorMsgSupplier
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* the supplier for an error message to use if not 1 and only 1 element is in the collection
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* @param <T>
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* the type of element to return
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*
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* @return the singleton collector
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*
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* @throws IllegalStateException
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* if not 1 and only 1 element is in the stream
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*/
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public static <T> Collector<T, List<T>, T> singletonCollector(boolean allowNull, Supplier<String> errorMsgSupplier)
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throws IllegalStateException {
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return Collector.of(ArrayList::new, List::add, (left, right) -> {
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left.addAll(right);
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return left;
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}, list -> {
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if (list.isEmpty()) {
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if (allowNull)
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return null;
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String errorMsg = errorMsgSupplier.get();
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if (errorMsg == null) {
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throw new IllegalStateException("Expect one element, but received no elements");
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