Lambda表示式排序，出現空指標異常

問題描述：物件集合中，根據物件的某個屬性進行公升降排序時，未考慮對應屬性的null值，導致出現空指標異常。

解決方法：comparator.nullsfirst和comparator.nullslast

listnlist = list.stream().sorted(

comparator.comparing(user::getcode, comparator.nullsfirst(string::compareto)))

.collect(collectors.tolist());12

3下見原始碼：

/*** returns a null-friendly comparator that considers to be

* less than non-null. when both are , they are considered

* equal. if both are non-null, the specified is used

* to determine the order. if the specified comparator is ,

* then the returned comparator considers all non-null values to be equal.

** the returned comparator is serializable if the specified comparator

* is serializable.

** @param  the type of the elements to be compared

* @param  comparator a for comparing non-null values

* @return a comparator that considers to be less than

*         non-null, and compares non-null objects with the supplied

*         .

* @since 1.8

*/public static comparatornullsfirst(comparator<? super t> comparator)

/*** null-friendly comparators

*/final static class nullcomparatorimplements comparator, serializable

@override

public int compare(t a, t b) else if (b == null) else

}@override

public comparatorthencomparing(comparator<? super t> other)

@override

public comparatorreversed()

}

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