我在考慮用更現代的東西替換一些舊的比較器代碼。通過函數列表構建鏈式比較器
public int compare(String o1, String o2) {
if (null == o1 && null == o2)
return 0;
if (null == o1)
return -1;
if (null == o2)
return 1;
if (0 == o1.length() && 0 == o2.length())
return 0;
if (0 == o1.length())
return -1;
if (0 == o2.length())
return 1;
int result = Integer.compare(o1.length(), o2.length());
if (0 != result)
return result;
result = Character.compare(o1.charAt(0), o2.charAt(0));
if (0 != result)
return result;
return o1.compareTo(o2);
}
我知道關於Java 8的比較方法,因此,上述能夠成爲:
Comparator<String> chained = Comparator.nullsFirst(
Comparator.comparingInt(String::length)).thenComparing(
s -> s.length() == 0 ? -1 : s.charAt(0)).thenComparing(
Function.identity());
但是,這是一個有點討厭,我需要寫出來comparing
和thenComparing
每次比較。理想情況下,我只需傳入一個List或多個lambda表達式。
喜歡的東西:
Comparator.comparing(
String::length,
s -> s.length() == 0 ? 0 : s.charAt(0),
Function.identity());
以下是我已經得到了最近,一些評論部分,指明哪些是無效的,由於泛型不匹配。困難點似乎是我想通過多個Function<T, ?>
通配符不被固定爲一次捕獲。我猜這是不可能的。
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.function.Function;
public class MultiComparator<T> implements Comparator<T> {
private Comparator<T> comparator;
public MultiComparator() {
this.comparator = Comparator.nullsFirst(Comparator.comparing(o -> 0));
}
/*
public MultiComparator(List<Function<T, ? extends Comparable<?>>> extractors) {
this();
extractors.stream().forEach(this::add);
}
public MultiComparator(Function<T, ? extends Comparable<?>>... extractors) {
this(Arrays.asList(extractors));
}
*/
public static void main(String[] args) {
List<String> strings = Arrays.asList("1", "11", "111", "2", "22", "222", "3", "123", "432", "aaa", "1234",
"", "", null, null, null);
MultiComparator<String> multi = new MultiComparator<>();
multi.add(String::length);
multi.add(s -> s.length() == 0 ? 0 : s.charAt(0));
multi.add(Function.identity());
Collections.shuffle(strings);
Collections.sort(strings, multi);
System.err.println(strings);
/*
multi = new MultiComparator<>(String::length, s -> s.length() == 0 ? -1 : s.charAt(0));
Collections.shuffle(strings);
Collections.sort(strings, multi);
System.err.println(strings);
*/
Comparator<String> chained = Comparator.nullsFirst(Comparator.comparingInt(String::length)).thenComparing(s
-> s.length() == 0 ? -1 : s.charAt(0)).thenComparing(Function.identity());
Collections.shuffle(strings);
Collections.sort(strings, chained);
System.err.println(strings);
Collections.shuffle(strings);
Collections.sort(strings, (o1, o2) -> {
if (null == o1 && null == o2) return 0;
if (null == o1) return -1;
if (null == o2) return 1;
if (0 == o1.length() && 0 == o2.length()) return 0;
if (0 == o1.length()) return -1;
if (0 == o2.length()) return 1;
int result = Integer.compare(o1.length(), o2.length());
if (0 != result) return result;
result = Character.compare(o1.charAt(0), o2.charAt(0));
if (0 != result) return result;
return o1.compareTo(o2);
});
System.err.println(strings);
}
public <U extends Comparable<U>> void add(Function<T, U> fieldExtractor) {
this.comparator = this.comparator.thenComparing(Comparator.nullsFirst(Comparator.comparing(fieldExtractor)));
}
@Override
public int compare(T o1, T o2) {
return this.comparator.compare(o1, o2);
}
}
積極的一面,我可以只使用thenComparing,而無需編寫自己的實用工具類。
多虧了Tagir Valeev的解決方案,最終我放棄了自定義的比較級,只是有兩個方法返回一個新的比較。
@SuppressWarnings("unchecked")
public static <T> Comparator<T> create(List<Function<T, ? extends Comparable<?>>> extractors) {
return extractors.stream().map(e -> Comparator.comparing((Function<T, Comparable<Object>>) e)).reduce
(Comparator::thenComparing).orElse(Comparator.comparing(o -> 0));
}
@SafeVarargs
public static <T> Comparator<T> create(Function<T, ? extends Comparable<?>>... extractors) {
return create(Arrays.asList(extractors));
}
當然!不知道爲什麼我沒有想到鑄造。非常感謝。 – fracai