前些天發現了一個巨牛的人工智能學習網站，通俗易懂，風趣幽默，忍不住分享一下給大家。點擊跳轉到教程。

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一、情況描述

報錯如題：

java.lang.UnsupportedOperationException: nullat java.util.Collections$UnmodifiableCollection.removeAll(Collections.java:1068)

報錯行定位到下圖紅框中這行：

代碼如下（只是單純的想 2 個集合求差集）：

List<WorkWeight> removeList = Lists.newArrayList();workWeightsList.forEach((workWeight -> {if (!employeeId.equals(workWeight.getApproverId()) && workWeight.getEmployeeId().equals(employeeId)) { // 當前人提交的不由自己審批的數據removeList.add(workWeight);}}));if (!CollectionUtils.isEmpty(removeList)) {             workWeightsList.removeAll(removeList);  // 這行報錯}

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二、解決辦法

用 ArrayList 把集合?workWeightsList?再組裝，如下圖：

? ?代碼：

List<WorkWeight> removeList = Lists.newArrayList();workWeightsList.forEach((workWeight -> {if (!employeeId.equals(workWeight.getApproverId()) && workWeight.getEmployeeId().equals(employeeId)) { // 當前人提交的不由自己審批的數據removeList.add(workWeight);}}));if (!CollectionUtils.isEmpty(removeList)) {workWeightsList = new ArrayList<WorkWeight>(workWeightsList);  // 改寫這一行workWeightsList.removeAll(removeList);}return new Payload(new PageImpl(workWeightsList, pageable, result.getTotalElements()));

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三、這樣改寫的原因參見：

UnsupportedOperationException報錯原因解說

參考：https://www.jianshu.com/p/7f718d900111

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