【编程基础】Java Comparator接口的使用

public interface Comparator<T> { /** * Compares the two specified objects to determine their relative ordering. The ordering * implied by the return value of this method for all possible pairs of * {@code (lhs, rhs)} should form an <i>equivalence relation</i>. * This means that * <ul> * <li>{@code compare(a,a)} returns zero for all {@code a}</li> * <li>the sign of {@code compare(a,b)} must be the opposite of the sign of {@code * compare(b,a)} for all pairs of (a,b)</li> * <li>From {@code compare(a,b) > 0} and {@code compare(b,c) > 0} it must * follow {@code compare(a,c) > 0} for all possible combinations of {@code * (a,b,c)}</li> * </ul> * * @param lhs * an {@code Object}. * @param rhs * a second {@code Object} to compare with {@code lhs}. * @return an integer < 0 if {@code lhs} is less than {@code rhs}, 0 if they are * equal, and > 0 if {@code lhs} is greater than {@code rhs}. * @throws ClassCastException * if objects are not of the correct type. */ public int compare(T lhs, T rhs); /** * Compares this {@code Comparator} with the specified {@code Object} and indicates whether they * are equal. In order to be equal, {@code object} must represent the same object * as this instance using a class-specific comparison. * <p> * A {@code Comparator} never needs to override this method, but may choose so for * performance reasons. * * @param object * the {@code Object} to compare with this comparator. * @return boolean {@code true} if specified {@code Object} is the same as this * {@code Object}, and {@code false} otherwise. * @see Object#hashCode * @see Object#equals */ public boolean equals(Object object); }

函数说明：

1、若一个类要实现Comparator接口，那么这个类一定要实现它的两个方法compareTo(T o1, T o2)和equals(Object obj);

2、int compareTo(T o1, T o2)方法的返回值决定了比较的顺序，看你具体是怎么实现的，o1大于o2返回正数,o1等于o2返回0，o1小于o2返回负数；

3、equals(Object obj)方法可以空着，因为任何类默认已经实现了equals(Object obj)方法；

如果我们要对某个对象进行排序我们可以建一个该类的比较器，比较的规则可以自己制定，比如：

public class Student { int age; String name; Student(int age, String name) { this.age = age; this.name = name; } public static void main(String[] arg) { ArrayList<Student> students = new ArrayList<Student>(); students.add(new Student(23, "dd")); students.add(new Student(22, "cc")); students.add(new Student(22, "bb")); students.add(new Student(25, "aa")); Collections.sort(students, new StudentComparator()); System.out.print(students); } @Override public String toString() { return " age = " + age + " name = " + name; } } class StudentComparator implements Comparator<Student> { @Override public int compare(Student o1, Student o2) { return (o1.age - o2.age); } }

上面的比较器是按照学生的年龄大小进行比较，这个比较的规则你可以自定义，你也可以按照名字来比较，上面的程序运行结果是：

[ age = 22 name = cc, age = 22 name = bb, age = 23 name = dd, age = 25 name = aa]