GroupingByCollector.java
package net.morimekta.collect.collectors;
import net.morimekta.collect.UnmodifiableList;
import net.morimekta.collect.UnmodifiableMap;
import net.morimekta.collect.UnmodifiableSet;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;
import static java.util.Objects.requireNonNull;
public class GroupingByCollector<E, K, V> implements Collector<E, Map<K, List<V>>, UnmodifiableMap<K, List<V>>> {
private final Function<E, K> toKey;
private final Function<E, V> toValue;
public GroupingByCollector(Function<E, K> toKey, Function<E, V> toValue) {
this.toKey = requireNonNull(toKey);
this.toValue = requireNonNull(toValue);
}
@Override
public Supplier<Map<K, List<V>>> supplier() {
return HashMap::new;
}
@Override
public BiConsumer<Map<K, List<V>>, E> accumulator() {
return (b, e) -> b
.computeIfAbsent(toKey.apply(e), k -> new ArrayList<>())
.add(toValue.apply(e));
}
@Override
public BinaryOperator<Map<K, List<V>>> combiner() {
return (b1, b2) -> {
for (Map.Entry<K, List<V>> entry : b2.entrySet()) {
b1.computeIfAbsent(entry.getKey(), k -> new ArrayList<>(entry.getValue().size()))
.addAll(entry.getValue());
}
return b1;
};
}
@Override
public Function<Map<K, List<V>>, UnmodifiableMap<K, List<V>>> finisher() {
return map -> {
UnmodifiableMap.Builder<K, List<V>> builder = UnmodifiableMap.newBuilder();
for (Map.Entry<K, List<V>> entry : map.entrySet()) {
builder.put(entry.getKey(), UnmodifiableList.asList(entry.getValue()));
}
return builder.build();
};
}
@Override
public Set<Characteristics> characteristics() {
return UnmodifiableSet.setOf();
}
}