小编Era*_*ran的帖子

我试图从我的一个JComboBoxes中删除一个项目监听器.下面是代码.

clientNameCombo.removeItemListener(clientNameCombo.getItemListeners()[clientNameCombo.getItemListeners().length-1]);

我动态添加ItemListener到上面的组合框.但是,有时当上面的代码运行时,它会给出错误:

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: -1
    at UI.MainUIHolder$CoreClientOtherPaymentTypeComboAction.itemStateChanged(MainUIHolder.java:10031)
    at javax.swing.JComboBox.fireItemStateChanged(JComboBox.java:1223)
    at javax.swing.JComboBox.selectedItemChanged(JComboBox.java:1280)
    at javax.swing.JComboBox.contentsChanged(JComboBox.java:1329)
    at javax.swing.AbstractListModel.fireContentsChanged(AbstractListModel.java:118)
    at javax.swing.DefaultComboBoxModel.setSelectedItem(DefaultComboBoxModel.java:93)
    at javax.swing.JComboBox.setSelectedItem(JComboBox.java:576)
    at javax.swing.JComboBox.setSelectedIndex(JComboBox.java:622)
    at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:834)
    at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290)
    at java.awt.Component.processMouseEvent(Component.java:6525)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(BasicComboPopup.java:498)
    at java.awt.Component.processEvent(Component.java:6290)
    at java.awt.Container.processEvent(Container.java:2234)
    at java.awt.Component.dispatchEventImpl(Component.java:4881)
    at java.awt.Container.dispatchEventImpl(Container.java:2292)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
    at java.awt.Container.dispatchEventImpl(Container.java:2278)
    at java.awt.Window.dispatchEventImpl(Window.java:2739)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
    at java.awt.EventQueue.access$400(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:697)
    at java.awt.EventQueue$3.run(EventQueue.java:691)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.awt.EventQueue$4.run(EventQueue.java:719)
    at java.awt.EventQueue$4.run(EventQueue.java:717) …

我想知道是否可以使用新的Streams API在一行中执行以下操作:

List<MyItem> arr = new ArrayList<>();

// MyItem has a single field, which is a value
arr.add(new MyItem(3));
arr.add(new MyItem(5));

// the following operation is the one I want to do without iterating over the array
int sum = 0;
for(MyItem item : arr){
  sum += item.getValue();
}

如果数组只包含ints,我可以做到这样的事情:

int sum = array.stream().mapToInt(Integer::intValue).sum();

但是,我可以将相同的想法应用于任意对象列表吗？

我知道HashMap不允许插入重复值，它用最新条目替换最后一个重复值。有没有办法打印在该put方法中发现的重复项？

我有以下代码片段：

for( int i = 0; i <= elements.length - 1; i++) {
    nodeDBList = (NodeList) xPath.compile(elements[i]).evaluate(dbDocument, XPathConstants.NODESET);
    for (int j = 0; j < nodeDBList.getLength(); j++) {
        if(nodeDBList.item(j).getFirstChild() != null)
            dbList.put(nodeDBList.item(j).getFirstChild().getNodeValue().toLowerCase().trim(), 
                       nodeDBList.item(j).getNodeName().toLowerCase().trim());

    }
} 

这是我的Test样子:

@Test
    public void testFunctional() 
    {
        final Member member = new Member("email", "memberExternalId", "clientId", "clientSecret");
        final Category foodCategory = new Category("Groceries", "Food");
        final Category utilityCategory = new Category("Internet", "utility");

        final List<Transaction> transactions = new ArrayList<>();
        {
            transactions.add(new Transaction("food1", new BigDecimal("10.00"), true, LocalDateTime.now(), member, foodCategory));
            transactions.add(new Transaction("food2", new BigDecimal("20.00"), true, LocalDateTime.now(), member, foodCategory));
        }
        {
            transactions.add(new Transaction("utility1", new BigDecimal("100.00"), true, LocalDateTime.now(), member, utilityCategory));
            transactions.add(new Transaction("utility2", new BigDecimal("200.00"), true, LocalDateTime.now(), member, utilityCategory));
        }

        final Map<String, List<BigDecimal>> groupByCategory = transactions.stream().collect(Collectors.groupingBy(Category::getGroupName));
    } …

给定以下代码：

@Test
public void test7() {
    Map<String, Integer> sortedData = new HashMap<>();
    sortedData.put("One", 1);
    sortedData.put("Two", 2);
    sortedData.put("Three", 3);

    Stream<Map.Entry<String, Integer>> stream = sortedData.entrySet().stream();
    List<String> sorted = stream
            .sorted(Comparator.comparing(Map.Entry::getValue))
            .map(Map.Entry::getKey)
            .collect(Collectors.toList());
} 

它可以成功编译，但是当我更改时

.sorted(Comparator.comparing(Map.Entry::getValue))

至

.sorted(Comparator.comparing(Map.Entry::getValue).reversed())

编译器抱怨 Non static method can't be referenced in static context

我可以想象这是因为getValue不是的静态方法Map.Entry，但是我在这里无法解释问题。

我有以下对象的列表.使用StreamAPI,我可以得到User它出现的频率数List吗？

public class User {
    string name;
    int age;
}

考虑到下面的功能,我注意到编译器没有发出任何问题:

private static int returnTwoTypes() {
    int a = 1;
    if (a == 1) {
        return -1;
    }
    return 'a';
}

为什么我可以在函数返回类型时返回a character和a ？intint

我试图找到有关它的信息但失败了.

如何size()进行HashMap或HashSet实施？它是如何工作的？是手术O(1)还是O(n)手术？

我有这段代码.我该如何改造它？功能风格？事实上,我有List<X>.每个X包含List<V>.此列表中的每个V都有List<M>一个参数.我需要构建Map<X,Y>,其中Y是存储在对象X中聚合的所有V对象中的所有M个对象的数量.

HashMap<Country, Integer> modelsPerCountryMap = new HashMap<>();
int count;
for (Country country : CountryDataSingleton.getCountryDataCollection()) {
    count = 0;
    for (CarMaker cm : country.getListOfMakers()) {
        count += cm.getModels().size();
    }
    modelsPerCountryMap.put(country, count);
}

每当我尝试从List使用list.removeIf(condition)中删除一个元素时,它会抛出UnsupportedOperationException:

public class Test 
{
    public static void main(final String[] args) 
    {
        String[] stringArray = new String[]{"A","B","C","D"}; 
        List<String> stringList = Arrays.asList(stringArray); 
        stringList.forEach(System.out::println); 
        stringList.removeIf((String string) -> string.equals("B")); 
        stringList.forEach(System.out::println); 
    }
}

为什么不起作用？