我要
java.lang.NullPointerException：尝试调用虚拟方法 空对象引用上的'float java.lang.Number.floatValue（）'
使用以下代码：
val localHistory: ArrayList<Float> = ArrayList<Float>()
...
val strHistory = localHistory.map { value -> decoration.decoratedValue(ref.format, value) }
我刚刚了解到ArrayList可能包含空值（oookay）。这意味着映射转换闭包中的 value 可能为NULL，不是吗？
但这不能是因为 value 不是可选类型，并且编译器说if（value！= null）始终为true。
所以问题是在这种情况下如何避免NPE？
答案 0 :(得分：6)
我建议将您从Java收到的 [[[[-0.00476191 -0.00361736 0. 0.25361738 -0.07576995
0.03405379 0.40910327]
[ 0.21594621 0.04544836 0. 0.28788495 0.30689242
-0.13025634 0.05074273]
[ 0.46358964 0.19925728 0. -0.09778295 0.26563603
0.34778297 -0.02014329]
[-0.01515752 0.3534766 0. 0.32857144 -0.00361736
0.67142856 0.25361738]
[ 0.25756338 0.03405379 0. 0.21594621 0.3787817
-0.05689242 0.6212183 ]
[ 0.30689242 0.203077 0. 0.796923 0.19925728
0.40103063 -0.09778295]
[ 0.5989694 0.34778297 0. -0.01515752 0.68680996
0.26515752 0.66190475]
[-0.00361736 1.0047619 0. 0.59089667 0.03405379
1.0757699 0.21594621]
[ 0.712115 -0.05689242 0. 0.30689242 0.53641033
0.05074273 1.1302563 ]
[ 0.19925728 0.7343639 0. 0.93230265 0.34778297
0.64652336 -0.01515752]
[ 1.0201433 0.26515752 0. 0.24638264 0.33809522
0.50361735 -0.07576995]
[ 0.2840538 0.40910327 0. 0.04544836 0.19310758
0.28788495 0.5568924 ]
[-0.13025634 0.30074272 0. 0.44925728 0.06769729
0.15221705 0.26563603]
[ 0.59778297 -0.02014329 0. 0.3534766 0.5151575
0.32857144 0.24638264]
[ 0.67142856 0.50361735 0. 0.2840538 0.7424366
0.4659462 0.3787817 ]
[ 0.19310758 0.6212183 0. 0.203077 0.30074272
0.796923 0.44925728]
[ 0.40103063 0.15221705 0. 0.59778297 0.31319004
0.23484248 0.68680996]
[ 0.5151575 0.66190475 0. 1.0047619 0.50361735
0.59089667 0.2840538 ]
[ 1.0757699 0.4659462 0. 0.19310758 0.95455164
0.5568924 0.53641033]
[ 0.30074272 1.1302563 0. 0.7343639 0.15221705
0.93230265 0.59778297]
[ 0.64652336 0.23484248 0. 0.5151575 -0.00476191
0.49638262 0.33809522]
[ 0.75361735 -0.07576995 0. 0.40910327 0.7159462
0.04544836 0.44310758]
[ 0.28788495 0.8068924 0. 0.55074275 0.46358964
0.69925725 0.06769729]
[ 0.40221703 0.26563603 0. -0.02014329 0.48484248
0.3534766 0.7651575 ]
[ 0.32857144 0.49638262 0. 0.75361735 0.25756338
0.5340538 0.7424366 ]
[ 0.7159462 0.3787817 0. 0.6212183 0.8068924
0.203077 0.55074275]
[ 0.796923 0.69925725 0. 0.40221703 0.5989694
0.84778297 0.31319004]
[ 0.48484248 0.68680996 0. 0.66190475 0.49638262
1.0047619 0.75361735]
[ 0.59089667 0.5340538 0. 0.7159462 0.712115
0.44310758 0.95455164]
[ 0.8068924 0.53641033 0. 1.1302563 0.69925725
0.7343639 0.40221703]
[ 0.93230265 0.84778297 0. 0.48484248 1.0201433
0.7651575 -0.00476191]
[ 0.74638265 0.33809522 0. -0.07576995 0.7840538
0.40910327 0.9659462 ]
[ 0.04544836 0.6931076 0. 1.0568924 -0.13025634
0.80074275 0.46358964]
[ 0.94925725 0.06769729 0. 0.26563603 1.0977829
-0.02014329 0.7348425 ]
[ 0.3534766 1.0151576 0. 0.74638265 0.67142856
1.0036174 0.25756338]
[ 0.7840538 0.7424366 0. 0.3787817 0.6931076
0.6212183 1.0568924 ]
[ 0.203077 0.80074275 0. 0.94925725 0.40103063
0.65221703 0.5989694 ]
[ 1.0977829 0.31319004 0. 0.68680996 1.0151576
0.66190475 0.74638265]
[ 1.0047619 1.0036174 0. 0.7840538 1.0757699
0.9659462 0.712115 ]
[ 0.6931076 0.95455164 0. 0.53641033 0.80074275
1.1302563 0.94925725]
[ 0.7343639 0.65221703 0. 1.0977829 0.64652336
0.7348425 1.0201433 ]
[ 1.0151576 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.1 0.2
0.2 0.1 ]
[ 0.1 0.2 0. 0.1 0.1
0.2 0.2 ]
[ 0.1 0.1 0. 0.2 0.1
0.1 0.2 ]
[ 0.2 0.1 0. 0.2 0.2
0.1 0.1 ]
[ 0.2 0.2 0. 0.8479438 0.67317617
0.5581815 0.1778345 ]
[-0.9215721 1.5896183 0. 0.6099795 0.5955366
-0.46569395 -0.8461083 ]
[ 1.6129647 1.4244858 0. 0.5209342 0.17585325
-0.8687666 1.7872683 ]
[ 1.3389692 0.8533131 0. -0.00590521 -0.7195761
1.6236191 1.1828533 ]
[ 1.1838211 0.6728102 0. -0.785988 1.2751837
1.1616383 0.933811 ]
[ 0.4684658 0.2719049 0. 1.2093123 0.66612804
0.66964823 0.55971766]
[ 0.17104894 -1.0688283 0. 0.6494252 0.6844874
0.66586125 0.01329695]
[-1.2607187 -0.22749203 0. -0.8741171 -0.9443728
-0.9659323 -0.03422031]
[-0.0364061 0.54829746 0. 0.6263525 0.66758543
0.04167109 -0.11780822]
[ 0.48400337 0.4685324 0. -0.04594427 0.02469592
-0.3487326 0.08831279]
[ 0.4161314 0.23332608 0. -0.13553022 -0.31008872
0.04969648 0.5674252 ]
[ 0.36492363 -0.07475745 0. -0.03859219 0.2016789
-0.39845943 -0.07058203]
[-0.08173721 0.1720942 0. 0.02323131 0.07122216
0.07469177 0.12792486]
[-0.24689877 0.196296 0. 0.5564647 0.535513
0.22528338 -0.37152448]
[-1.7235181 -1.8204601 0. -1.5040898 -1.8099409
-1.8550183 -1.1855855 ]
[-1.6341007 -1.3448519 0. -1.6656716 -1.6564709
-1.2735447 -1.3357594 ]
[-1.2829769 -1.2869868 0. -1.6657944 -1.4066424
-1.4230443 -1.4196167 ]
[-1.3691044 -1.656098 0. -1.4339573 -1.5685135
-1.633306 -1.4437945 ]]]]
中的每个参数标记为可为空（除非您当然知道永远不能为Collection：
null然后，您可以过滤出val floats: List<Float?> = someJavaClass.getFloats()
并执行地图操作：
null请记住，每个val newFloats: List<Float> = floats
.filter { it != null }
.map { it + 1 }
，filter（等等）操作都将迭代整个集合并每次都创建一个新集合。最好使用map，它们是懒惰地迭代的。
如评论中所述，可以使用Sequence直接过滤掉所有空值：
filterNotNull并且val newFloats: List<Float> = floats
.filterNotNull()
.map { it + 1 }
仅保留map函数不返回mapNotNull的值，这也消除了null：
filterNotNull另一个我可以想到的结构可以与您的示例一起使用，尽管它非常难以理解并且非常复杂。但是这里是：
val newFloats: List<Float> = floats
.mapNotNull { it?.plus(1) }
答案 1 :(得分：2)
如果您只想排除ArrayList中的空值，请调用mapNotNull而不是map。有关详细信息，请参见here。
然后，您还需要处理value在lambda中为空的情况，您可以使用?.let进行如下操作：
localHistory.mapNotNull { value -> value?.let { decoration.decoratedValue(ref.format, it) } }
答案 2 :(得分：0)
再一次使用mapNotNull：
val strHistory = localHistory.mapNotNull { it ?: return@mapNotNull null
decoration.decoratedValue(ref.format, it) }