java如何创建比较器

Question

假设我有泛型类

public class MyClass<T extends Comparable<T>> {
    T a;

    MyClass(T x) {
        a = x;
    }

    public String toString() {
        return a.toString();
    }
}

我想让Comparator能够对充满MyClass元素的集合进行排序

如果我这样写

public class MyComparator implements Comparator<MyClass> {
    public int compare(MyClass o1, MyClass o2) {
        return o1.a.compareTo(o2.a);
    }
}

我接到一个电话

这样写比较器对吗

import java.util.Comparator;

public class MyComparator<T extends Comparable<T>> implements Comparator<MyClass<T>> {
    public int compare(MyClass<T> o1, MyClass<T> o2) {
        return o1.a.compareTo(o2.a);
    }
}

Answer 1

如果您希望MyClass成为Comparable，那么您可以在不传递Comparator的情况下进行排序，这可以通过以下方式完成：

public class MyClass<T extends Comparable<T>> implements
        Comparable<MyClass<T>> {
    T a;

    MyClass(T x) {
        a = x;
    }

    public String toString() {
        if (a == null) return "null";
        return a.toString();
    }

    @Override
    public int compareTo(MyClass<T> o) {
        if (this.a == null) {
            if (o == null || o.a == null) return 0;
            return -1;
        } else if (o == null || o.a == null) return 1;
        return this.a.compareTo(o.a);
    }
}

如果你真的想创建一个MyClassComparator，那么它应该看起来像

public class MyClass<T extends Comparable<T>> {
    T a;

    MyClass(T x) {
        a = x;
    }

    public String toString() {
        if (a == null) return "null";
        return a.toString();
    }
}

public class MyClassComparator<T extends Comparable<T>> implements
        Comparator<MyClass<T>> {
    @Override
    public int compare(MyClass<T> left, MyClass<T> right) {
        if (left == null) {
            if (right == null) {
                return 0;
            }
            return -1;
        } else if (right == null) {
            return 1;
        }
        if (left.a == null) {
            if (right.a == null) {
                return 0;
            }
            return -1;
        } else if (right.a == null) {
            return 1;
        }
        return left.a.compareTo(right.a);
    }
}