java - 使用具有多个比较器的比较器

Question

我可以使用此代码中的所有简单比较器进行排序，但不能使用ComplexComparator。我不知道如何编码才能使其正常工作。任何建议/解释将不胜感激。

这是我的主程序:

package pkgTest;

import java.util.Arrays;

public class Main {

    public static void main(String[] args) {
        Student[] students = new Student[6];
        students[0] = new Student("Pete", 1989, 3.6);
        students[1] = new Student("Tomas", 1989, 3.9);
        students[2] = new Student("Helen", 1990, 3.6);
        students[3] = new Student("Steve", 1991, 3.7);
        students[4] = new Student("Natalie", 1993, 3.7);
        students[5] = new Student("John", 1992, 4.0);

        NameComparator byName
                = new NameComparator();
        BirthDateComparator byBirthDate
                = new BirthDateComparator();
        AverageComparator byAverage
                = new AverageComparator();

        ComplexComparator complexSorting
                = new ComplexComparator(byName,
                        byAverage);

        System.out.println("===============");
        System.out.println("Before sorting:");
        System.out.println("===============");
        for (Student student : students) {
            System.out.println(student.getName()
                    + " // " + student.getBirthDate()
                    + " // " + student.getAverage());
        }

        Arrays.sort(students, complexSorting);

        System.out.println("==============");
        System.out.println("After sorting:");
        System.out.println("==============");
        for (Student student : students) {
            System.out.println(student.getName()
                    + " // " + student.getBirthDate()
                    + " // " + student.getAverage());
        }
    }
}

以下是其余类(class):

package pkgTest;

public class Student {

    private String name;
    private int birthDate;
    private double average;

    public Student(String name, int birthDate,
            double average) {
        this.name = name;
        this.birthDate = birthDate;
        this.average = average;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getBirthDate() {
        return this.birthDate;
    }

    public void setBirthDate(int birthDate) {
        this.birthDate = birthDate;
    }

    public double getAverage() {
        return this.average;
    }

    public void setAverage(double average) {
        this.average = average;
    }
}


package pkgTest;

import java.util.Comparator;

public class ComplexComparator implements Comparator<Student> {

    public ComplexComparator(Comparator<Student> one,
            Comparator<Student> another) {
    }

    @Override
    public int compare(Student one, Student another) {
        /*This is the part that
        I just couldn't figure
        it out to get it work.

        It has to work no matter
        which 2 of the 3 comparators
        I use to set the input
        parameters of ComplexComparator.

        I have to make it work by
        modifying only this part of
        the code.*/
    }
}


package pkgTest;

import java.util.Comparator;

public class AverageComparator implements Comparator<Student> {

    @Override
    public int compare(Student one, Student another) {
        if (one.getAverage()
                < another.getAverage()) {
            return -1;
        } else if (one.getAverage()
                == another.getAverage()) {
            return 0;
        } else {
            return +1;
        }
    }
}

package pkgTest;

import java.util.Comparator;

public class BirthDateComparator implements Comparator<Student> {

    @Override
    public int compare(Student one, Student another) {
        if (one.getBirthDate()
                < another.getBirthDate()) {
            return -1;
        } else if (one.getBirthDate()
                == another.getBirthDate()) {
            return 0;
        } else {
            return +1;
        }
    }
}


package pkgTest;

import java.util.Comparator;

public class NameComparator implements Comparator<Student> {

    @Override
    public int compare(Student one, Student another) {
        return one.getName().
                compareToIgnoreCase(another.getName());
    }
}

Answer 1

您必须像下面这样修改类ComplexComparator，至少......

import java.util.Comparator;

public class ComplexComparator implements Comparator<Student> {

    private Comparator<Student> comparatorOne;
    private Comparator<Student> comparatorTwo;

    public ComplexComparator(Comparator<Student> one,
            Comparator<Student> another) {
        this.comparatorOne = one;
        this.comparatorTwo = another;
    }

    @Override
    public int compare(Student one, Student another) {
        // make a first comparison using comparator one
        int comparisonByOne = comparatorOne.compare(one, another);

        // check if it was 0 (items equal in that attribute)
        if (comparisonByOne == 0) {
            // if yes, return the result of the next comparison
            return comparatorTwo.compare(one, another);
        } else {
            // otherwise return the result of the first comparison
            return comparisonByOne;
        }
    }
}

对于两个以上的Comparator，您将需要它们的List(或另一个重载的构造函数)和一个保持特定比较顺序的循环。

编辑

对于您有关排序顺序的额外要求，这可能会有所帮助:

    public class ComplexComparator implements Comparator<Student> {

    private Comparator<Student> comparatorOne;
    private Comparator<Student> comparatorTwo;
    private boolean orderOneAscending = true;
    private boolean orderTwoAscending = true;

    /**
     * Constructor without any sort orders
     * @param one   a comparator
     * @param another   another comparator
     */
    public ComplexComparator(Comparator<Student> one, Comparator<Student> another) {
        this.comparatorOne = one;
        this.comparatorTwo = another;
    }

    /**
     * Constructor that provides the possibility of setting sort orders 
     * @param one   a comparator
     * @param orderOneAscending sort order for comparator one 
     *      (true = ascending, false = descending)
     * @param another   another comparator
     * @param orderTwoAscending sort order for comparator two
     *      (true = ascending, false = descending)
     */
    public ComplexComparator(Comparator<Student> one, boolean orderOneAscending,
            Comparator<Student> another, boolean orderTwoAscending) {
        this.comparatorOne = one;
        this.comparatorTwo = another;
        this.orderOneAscending = orderOneAscending;
        this.orderTwoAscending = orderTwoAscending;
    }

    @Override
    public int compare(Student one, Student another) {
        int comparisonByOne;
        int comparisonByAnother;

        if (orderOneAscending) {
            /*  note that your lexicographical comparison in NameComparator 
                returns a negative integer if the String is greater!
                If you take two numerical Comparators, the order will
                turn into the opposite direction! */
            comparisonByOne = comparatorOne.compare(another, one);
        } else {
            comparisonByOne = comparatorOne.compare(one, another);
        }

        if (orderTwoAscending) {
            comparisonByAnother = comparatorTwo.compare(one, another);
        } else {
            comparisonByAnother = comparatorTwo.compare(another, one);
        }

        if (comparisonByOne == 0) {
            return comparisonByAnother;
        } else {
            return comparisonByOne;
        }
    }
}

只需尝试一下这些值并尝试一些修改即可熟悉有关比较和排序的常见问题。 我希望这会有所帮助...