英文:
Java8: change the reference of the variable outside of forEach
问题
我尝试同时做两件事情：
1. 对列表中对象的特定字段进行求和
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
});
2. 从列表中过滤出一个对象
DoRequestDetailModel originProductRequestDetail = models.stream()
.filter(m -> m.getIsOriginProduct())
.reduce((a, b) -> {
throw new IllegalStateException();
})
.get();
我希望这段代码能够工作，但实际上它并不能：
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
DoRequestDetailModel originProductRequestDetail = new DoRequestDetailModel();
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
if(detail.getIsOriginProduct()) {
originProductRequestDetail = detail;
}
});
下面的代码可以完成任务：
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
List<DoRequestDetailModel> tempList = new ArrayList<>();
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
if(detail.getIsOriginProduct()) {
tempList.add(detail);
}
});
是否有更好的解决方案？
英文:
I try to do two things at once：<br/>
1.Sum values from specific field of the objects in a list
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
});
2.Filter a object from list
DoRequestDetailModel originProductRequestDetail = models.stream()
.filter(m -> m.getIsOriginProduct())
.reduce((a, b) -> {
throw new IllegalStateException();
})
.get();
I would like this code, but it dosen't work:
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
DoRequestDetailModel originProductRequestDetail = new DoRequestDetailModel();
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
if(detail.getIsOriginProduct()) {
originProductRequestDetail = detail;
}
});
The next code could be done
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
List<DoRequestDetailModel> tempList = new ArrayList<>();
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
if(detail.getIsOriginProduct()) {
tempList.add(detail);
}
});
Is there a better solution ?
答案1
得分: 1
你还需要使用`AtomicReference`：
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<DoRequestDetailModel> originProductRequestDetail = new AtomicReference<>(new DoRequestDetailModel());
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
if(detail.getIsOriginProduct()) {
if(originProductRequestDetail.get()) throw new IllegalStateException();
originProductRequestDetail.set(detail);
}
});
这是因为你在`forEach`内部调用的函数范围中更改了外部变量的引用。[这里][1]有一个相关的问题。
[1]: https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value
英文:
You would have to use AtomicReference as well:
AtomicReference<BigDecimal> doReqQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<BigDecimal> boQtySum = new AtomicReference<>(BigDecimal.ZERO);
AtomicReference<DoRequestDetailModel> originProductRequestDetail = new AtomicReference<>(new DoRequestDetailModel());
models.forEach(detail -> {
doReqQtySum.accumulateAndGet(detail.getDoReqQty(), (bg1, bg2) -> bg1.add(bg2));
boQtySum.accumulateAndGet(detail.getBoQty(), (bg1, bg2) -> bg1.add(bg2));
if(detail.getIsOriginProduct()) {
if(originProductRequestDetail.get()) throw new IllegalStateException();
originProductRequestDetail.set(detail);
}
});
It's because you're changing a reference of an outside variable in the scope of the function invoked inside forEach. Here is a related question.
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