比较法在比较java.util.Date时违反其一般契约

Question

以下是我的错误：

Caused by: javax.faces.el.EvaluationException: java.lang.IllegalArgumentException: Comparison method violates its general contract!
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:101) [jboss-jsf-api_2.1_spec-2.1.28.SP1-redhat-1.jar:2.1.28.SP1-redhat-1]
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:101) [jsf-impl-2.1.28.redhat-10.jar:2.1.28.redhat-10]
    at javax.faces.component.UICommand.broadcast(UICommand.java:315) [jboss-jsf-api_2.1_spec-2.1.28.SP1-redhat-1.jar:2.1.28.SP1-redhat-1]
    at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:786) [jboss-jsf-api_2.1_spec-2.1.28.SP1-redhat-1.jar:2.1.28.SP1-redhat-1]
    at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:1251) [jboss-jsf-api_2.1_spec-2.1.28.SP1-redhat-1.jar:2.1.28.SP1-redhat-1]
    at com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:81) [jsf-impl-2.1.28.redhat-10.jar:2.1.28.redhat-10]
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101) [jsf-impl-2.1.28.redhat-10.jar:2.1.28.redhat-10]
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118) [jsf-impl-2.1.28.redhat-10.jar:2.1.28.redhat-10]
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:593) [jboss-jsf-api_2.1_spec-2.1.28.SP1-redhat-1.jar:2.1.28.SP1-redhat-1]
    ... 29 more
Caused by: java.lang.IllegalArgumentException: Comparison method violates its general contract!
    at java.util.TimSort.mergeHi(TimSort.java:899) [rt.jar:1.8.0_65]
    at java.util.TimSort.mergeAt(TimSort.java:516) [rt.jar:1.8.0_65]
    at java.util.TimSort.mergeForceCollapse(TimSort.java:457) [rt.jar:1.8.0_65]
    at java.util.TimSort.sort(TimSort.java:254) [rt.jar:1.8.0_65]
    at java.util.Arrays.sort(Arrays.java:1512) [rt.jar:1.8.0_65]
    at java.util.ArrayList.sort(ArrayList.java:1454) [rt.jar:1.8.0_65]
    at java.util.Collections.sort(Collections.java:175) [rt.jar:1.8.0_65]

下面是我的比较方法代码。vo1.getAttribute()返回java.util.Date对象。

    @Override
    public int compare(DateComparableVO vo1, DateComparableVO vo2) {
        if (vo1 != null && vo1.getAttribute() != null && vo2 != null && vo2.getAttribute() != null) {
            return vo1.getAttribute().compareTo(vo2.getAttribute());
        }
        return -1;
    }

比较方法的实现有什么问题吗？

在空的情况下。

为什么下面的代码工作没有任何问题。

package test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
import java.util.List;

public class TestMain {

    public static void main(String[] args) {
        List<Employee> employees = new ArrayList<Employee>();
        employees.add(new Employee(new Date()));
        employees.add(null);
        employees.add(new Employee(new Date()));
        employees.add(new Employee(new Date()));
        employees.add(null);
        employees.add(new Employee(new Date()));
        employees.add(null);
        System.out.println(employees.size());
        Collections.sort(employees, new EmployeeComparator());

    }

}

class Employee {


    private Date attribute;

    public Employee() {
        // TODO Auto-generated constructor stub
    }

    public Employee(Date attribute) {
        this.attribute = attribute;
    }


    public Date getAttribute() {
        return attribute;
    }

    public void setAttribute(Date attribute) {
        this.attribute = attribute;
    }

    @Override
    public String toString() {
        return "Employee [attribute=" + attribute + "]";
    }
}

class EmployeeComparator implements Comparator<Employee>{

    @Override
    public int compare(Employee vo1, Employee vo2) {
        System.out.println("VO1 : " + vo1 + " VO2 : " + vo2);

        if (vo1 != null && vo1.getAttribute() != null && vo2 != null && vo2.getAttribute() != null) {
            return vo1.getAttribute().compareTo(vo2.getAttribute());
        }
        return -1;
    }

}

输出

7
VO1 : null VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017] VO2 : null
VO1 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017] VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017] VO2 : null
VO1 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017] VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : null VO2 : null
VO1 : null VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : null VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017] VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017] VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : null VO2 : null
VO1 : null VO2 : Employee [attribute=Fri Aug 18 11:51:11 SGT 2017]
VO1 : null VO2 : null

我是不是在理解比较的过程中遗漏了一些重要的部分？

Answer 1

最后的盲-1意味着null值没有得到稳定的处理(并且破坏了合并排序算法)。你可以做这样的事

@Override
public int compare(DateComparableVO vo1, DateComparableVO vo2) {
    Date l = null, r = null;
    if (vo1 != null) {
        l = vo1.getAttribute();
    }
    if (vo2 != null) {
        r = vo2.getAttribute();
    }
    if (l == null && r == null) {
        return 0;
    } else if (l == null) {
        return -1;
    } else if (r == null) {
        return 1;
    }
    return l.compareTo(r);
}

Answer 2

除了其他答案已经指出的之外，在java 8中实现这样一个Comparator的另一种方法是使用静态工厂方法组合它，例如：

Comparator<DateComparableVO> c = Comparator.nullsLast(
    Comparator.comparing(DateComparableVO::getAttribute,
        Comparator.nullsLast(Date::compareTo)));

或nullsFirst，取决于您希望如何处理空值。这是否更易读可能是一个讨论的问题，但它是自我记录什么是关于空值的预期行为。

如果您知道不能有任何空值，无论是对象本身还是属性，都可以简化为

Comparator<DateComparableVO> c = Comparator.comparing(DateComparableVO::getAttribute);

它将在遇到空值时抛出NullPointerException。