java比较方法违反了它的一般约定，但我可能想这样做？
我正在尝试编写一个表排序器，它总是将空值排序到底部。所以我写了一个implements Comparable的“包装器”类：
public class WrappedBigDecimal implements Comparable<WrappedBigDecimal> {
public final BigDecimal value;
public WrappedBigDecimal(BigDecimal value) {
this.value = value;
}
@Override
public int compareTo(WrappedBigDecimal o) {
if (value == null && (o == null || o.value == null)) { // both are null, thus equal
return 0;
} else if (value == null && (o != null && o.value != null)) { // value is null and compared value isn't
return -1;
} else if (value != null && (o == null || o.value == null)) {
return 1;
} else {
return value.compareTo(o.value);
}
}
@Override
public String toString() {
return String.valueOf(value);
}
}
您会注意到compareTo方法只执行空检查，然后遵从包装值类的compareTo方法
然后我写了一个RowSorter，它的Comparator检查SortOrder
public class WrappedNumberSorter extends TableRowSorter<TableModel> {
public WrappedNumberSorter(TableModel model) {
super(model);
}
@Override
public Comparator<?> getComparator(final int column) {
Comparator c = new Comparator() {
@Override
public int compare(Object o1, Object o2) {
boolean ascending = getSortKeys().get(0).getSortOrder() == SortOrder.ASCENDING;
if (o1 instanceof WrappedBigDecimal && ((WrappedBigDecimal)o1).value == null) {
if(ascending)
return 1;
else
return -1;
} else if (o2 instanceof WrappedBigDecimal && ((WrappedBigDecimal)o2).value == null) {
if(ascending)
return -1;
else
return 1;
} else {
return ((Comparable<Object>) o1).compareTo(o2);
}
}
};
return c;
}
}
但这会引发一个错误（无法找出原因，无法复制——请继续阅读）：
java.lang.IllegalArgumentException: Comparison method violates its general contract!
幸运的是，这个错误似乎没有影响任何事情，因为一切都按预期进行
我看到了这个问题（java.lang.IllegalArgumentException: Comparison method violates its general contract），我认为我的问题是我的比较是不可传递的。虽然我不确定，因为如果A==B和B=C，那么我想我的也会返回A=C，但我正在努力思考这个问题
不管怎样，我的问题是：
- 以这种方式故意使行分拣机排序为空到底部是否存在潜在危险
- 这对我来说似乎是过渡性的。我真的违反了
compareTo在这里的可传递契约吗？或者，它是否必须遵守另一份合同，而我不知道，这就是导致错误的原因
我写了一个方法来测试我的行分类器，但我无法重现错误：
public static void main(String[] args) {
JFrame frame = new JFrame();
JTable table = new JTable();
JScrollPane jsp = new JScrollPane(table);
String[] headers = new String[] {"h1", "h2"};
Object[][] data = new Object[10][2];
for(int i = 0; i < 7; i++) {
data[i][0] = new WrappedBigDecimal(BigDecimal.TEN.multiply(BigDecimal.valueOf(i % 3 == 0 ? (i*i*-1) : (i*i))));
}
data[7][0] = new WrappedBigDecimal(null);
data[8][0] = new WrappedBigDecimal(null);
data[9][0] = new WrappedBigDecimal(null);
for(int i = 10; i < 17; i++) {
data[i-10][1] = new WrappedBigDecimal(BigDecimal.TEN.multiply(BigDecimal.valueOf(i % 3 == 0 ? (i*i*-1) : (i*i))));
}
data[7][1] = new WrappedBigDecimal(null);
data[8][1] = new WrappedBigDecimal(null);
data[9][1] = new WrappedBigDecimal(null);
table.setModel(new DefaultTableModel(data, headers) {
@Override
public boolean isCellEditable(int row, int column) {
return false;
}
@Override
public Class<?> getColumnClass(int columnIndex) {
return BigDecimal.class;
}
});
table.setRowSorter(new WrappedNumberSorter(table.getModel()));
frame.add(jsp);
frame.setSize(200,400);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
一切似乎都很顺利
我错过了什么
编辑（7/11/18）：试图摆脱WrappedBigDecimal解决方案并实现Matt McHenry的解决方案带来了另一个问题。我将RowSorter简化为：
static class NullsLastSorter extends TableRowSorter<TableModel> {
public NullsLastSorter(TableModel model) {
super(model);
}
@Override
public Comparator<?> getComparator(int column) {
return Comparator.<Optional<BigDecimal>, Boolean>comparing(Optional::isPresent).reversed().thenComparing(o -> o.orElse(BigDecimal.ONE));
}
}
并用以下方法进行了测试：
public static void main(String[] args) {
JFrame frame = new JFrame();
JTable table = new JTable();
JScrollPane jsp = new JScrollPane(table);
String[] headers = new String[] {"h1", "h2"};
Object[][] data = new Object[10][2];
for(int i = 0; i < 7; i++) {
data[i][0] = BigDecimal.TEN.multiply(BigDecimal.valueOf(i % 3 == 0 ? (i*i*-1) : (i*i)));
}
data[7][0] = null;
data[8][0] = null;
data[9][0] = null;
for(int i = 10; i < 17; i++) {
data[i-10][1] = BigDecimal.TEN.multiply(BigDecimal.valueOf(i % 3 == 0 ? (i*i*-1) : (i*i)));
}
data[7][1] = null;
data[8][1] = null;
data[9][1] = null;
table.setModel(new DefaultTableModel(data, headers) {
@Override
public boolean isCellEditable(int row, int column) {
return false;
}
@Override
public Class<?> getColumnClass(int columnIndex) {
return BigDecimal.class;
}
});
table.setRowSorter(new NullsLastSorter(table.getModel()));
frame.add(jsp);
frame.setSize(200,400);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
现在当我尝试排序时，我得到一个错误：
Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: java.math.BigDecimal cannot be cast to java.util.Optional
at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469)
at java.util.Collections$ReverseComparator2.compare(Collections.java:5178)
at java.util.Comparator.lambda$thenComparing$36697e65$1(Comparator.java:216)
at javax.swing.DefaultRowSorter.compare(DefaultRowSorter.java:968)
at javax.swing.DefaultRowSorter.access$100(DefaultRowSorter.java:112)
at javax.swing.DefaultRowSorter$Row.compareTo(DefaultRowSorter.java:1376)
at javax.swing.DefaultRowSorter$Row.compareTo(DefaultRowSorter.java:1366)
at java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:320)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:188)
at java.util.Arrays.sort(Arrays.java:1246)
at javax.swing.DefaultRowSorter.sort(DefaultRowSorter.java:607)
at javax.swing.DefaultRowSorter.setSortKeys(DefaultRowSorter.java:319)
at javax.swing.DefaultRowSorter.toggleSortOrder(DefaultRowSorter.java:480)
at javax.swing.plaf.basic.BasicTableHeaderUI$MouseInputHandler.mouseClicked(BasicTableHeaderUI.java:112)
at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:270)
at java.awt.Component.processMouseEvent(Component.java:6536)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4534)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
共 (0) 个答案