主要方法体：
int ret = 0;
Object a,b;
try {
Field f = a.getClass().getDeclaredField(field);
f.setAccessible(true);
Class<?> type = f.getType();
if (type == int.class) {
ret = ((Integer) f.getInt(a)).compareTo((Integer) f
.getInt(b));
}
}
//SortListUtil.sort(list, "id", SortListUtil.DESC);
int ret = 0;
try {
Method m = a.getClass().getMethod(method, null);
m.setAccessible(true);
Class<?> type = m.getReturnType();
if (type == int.class) {
ret = ((Integer) m.invoke(a, null))
.compareTo((Integer) m.invoke(b, null));
}
}
//SortListUtil.sortByMethod(list, "getId", null);
需求：
做一个对象排序的功能，需要按不同规则排序
Compareable接口和Comparator接口都是用来实现集合排序的，只是Comparable是在集合内部定义的方法实现的排序，而Comparator是在集合外部实现的排序 ，所以，如想实现排序，就需在集合外定义Comparator接口的方法或在集合内实现Compareable接口的方法。
Comparable 是一个对象本身就已经支持自比较所需要实现的接口（如 String、Integer 自己就可以完成比较大小操作）;
Comparator 是一个专用的比较器，当这个对象不支持自比较或者自比较函数不能满足你的要求时，你可以写一个比较器来完成两个对象之间大小的比较。
可以说一个是类自已完成比较，一个是外部程序实现比较的差别而已。
Compareable接口
要对对象进行比较大小则需要实现Compareable接口
Compareable接口只有一个方法int compareTo(Object obj) 实现了此接口的 对象列表(或数组) 可以通过Collections.sort(Arrays.sort)来排序
class Paper implements Comparable<Paper>
{
String pcolor;
public Paper(String pcolor)
{
this.pcolor=pcolor;
}
public int compareTo(Object o)//public int compareTo(Paper o)
{
if (o instanceof TestComparable) {
Paper p=(Paper)o; //未使用泛型不转换的话不能访问pcolor
int cmp=this.pcolor.compareTo(p.pcolor);
return cmp;
}
}
//重载toString方法定义输出
public String toString()
{
return this.pcolor;
}
}
public class TestComparable
{
public static void main(String []args)
{
Paper p1=new Paper("red");
Paper p2=new Paper("blue");
Paper p3=new Paper("green");
List l=new LinkedList();
l.add(p1);
l.add(p2);
l.add(p3);
Collections.sort(l);
System.out.println(l);
}
}
Comparator接口：
在Comparator接口里，只有一个方法是需要实现的：
int compare(Object o1,Object o2);
提示: 如果o1小于o2,返回一个负数;如果o1大于o2，返回一个正数;如果他们相等，则返回0;
注意: compare 方法一定要是对称的，意思是compare(a,b) 返回的结果要跟compare(b,a) 相反。相反的结果意味着，要么返回的值带有不同的正负号，要么都是0。注意，象compare(a,b) 返回4而compare(b,a) 返回-2这样的情况是合法的。方法里常常可能抛出异常，在这个方法里抛出异常也要是对称的。如果调用compare(a,b) 时抛出了一个ClassCastException 异常,那么调用compare(b,a) 时也必须抛出一个ClassCastException 异常。
另外: 任何你准备重用的Comparator 都必须是可序列化的。TreeSet 和TreeMap 类存储Comparator 以便进行比较，因此为了这两个类能被序列化，它们使用的Comparator 也必须能被序列化。
Comparator 接口，通常被Collections.sort 方法使用，它是JAVA中需要了解的一个很重要的部分
public class Person {
private String name;
private int age;
public Person() {
}
public Person(String name, int age) {
this.name = name;
this.age = age;
}
get.set...
@Override
public String toString() {
return "Person [age=" + age + ", name=" + name + "]";
}
}
新建一个实现Comparator的实现类PersonComparator：
import java.util.Comparator;
public class PersonComparator implements Comparator<Person> {
@Override
public int compare(Person p1, Person p2) {
if (p1.getAge() > p2.getAge()) {
return 1;
} else if (p1.getAge() < p2.getAge()) {
return -1;
}
return 0;
}
}
CompareTest做测试：
import java.util.Arrays;
public class CompareTest {
public static void main(String[] args) {
Person[] persons = {
new Person("P1", 60),
new Person("P2", 20),
new Person("P3", 40)
};
System.out.println(Arrays.toString(persons));
Arrays.sort(persons, new PersonComparator());
System.out.println(Arrays.toString(persons));
}
}
public class RunDemoMain {
public static void main(String[] args) {
List<DataBean> dataBeans = new ArrayList<DataBean>();
dataBeans.add(new DataBean(23, "b", "w"));
dataBeans.add(new DataBean(34, "e", "w"));
dataBeans.add(new DataBean(12, "s", "w"));
dataBeans.add(new DataBean(10, "c", "w"));
System.out.println("==========排序前==========");
for (DataBean bean : dataBeans) {
System.out.println(bean);
}
System.out.println("==========排序后==========");
Collections.sort(dataBeans, new DemoComparator());
for (DataBean bean : dataBeans) {
System.out.println(bean);
}
}
}
参考：
http://kentkwan.iteye.com/blog/739505
http://baomw.iteye.com/blog/197905
实际项目应用：
if(hotelForm.get("ascOrDesc").equals("ASC")){
Collections.sort(list, new MyComparator.HotelComparator());
}else{
Collections.sort(list, new MyComparator.HotelComparator());
Collections.reverse(list);
}
public class MyComparator {
public static class HotelComparator implements Comparator {
public int compare(Object first, Object second) {
return ((Hotel)first).getHotelNameEn().compareToIgnoreCase(((Hotel)second).getHotelNameEn());
}
}
}
公用排序方法：
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
import java.util.List;
/**
* 通用工具类之按对象中某属性排序
* @author 李坤
* 交流博客:http://blog.csdn.net/lk_blog
*/
public class SortListUtil {
public static final String DESC = "desc";
public static final String ASC = "asc";
/**
* 对list中的元素按升序排列.
*
* @param list
* 排序集合
* @param field
* 排序字段
* @return
*/
public static List<?> sort(List<?> list, final String field) {
return sort(list, field, null);
}
/**
* 对list中的元素进行排序.
*
* @param list
* 排序集合
* @param field
* 排序字段
* @param sort
* 排序方式: SortList.DESC(降序) SortList.ASC(升序).
* @return
*/
@SuppressWarnings("unchecked")
public static List<?> sort(List<?> list, final String field,
final String sort) {
Collections.sort(list, new Comparator() {
public int compare(Object a, Object b) {
int ret = 0;
try {
Field f = a.getClass().getDeclaredField(field);
f.setAccessible(true);
Class<?> type = f.getType();
if (type == int.class) {
ret = ((Integer) f.getInt(a)).compareTo((Integer) f
.getInt(b));
} else if (type == double.class) {
ret = ((Double) f.getDouble(a)).compareTo((Double) f
.getDouble(b));
} else if (type == long.class) {
ret = ((Long) f.getLong(a)).compareTo((Long) f
.getLong(b));
} else if (type == float.class) {
ret = ((Float) f.getFloat(a)).compareTo((Float) f
.getFloat(b));
} else if (type == Date.class) {
ret = ((Date) f.get(a)).compareTo((Date) f.get(b));
} else if (isImplementsOf(type, Comparable.class)) {
ret = ((Comparable) f.get(a)).compareTo((Comparable) f
.get(b));
} else {
ret = String.valueOf(f.get(a)).compareTo(
String.valueOf(f.get(b)));
}
} catch (SecurityException e) {
e.printStackTrace();
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
if (sort != null && sort.toLowerCase().equals(DESC)) {
return -ret;
} else {
return ret;
}
}
});
return list;
}
/**
* 对list中的元素按fields和sorts进行排序,
* fields[i]指定排序字段,sorts[i]指定排序方式.如果sorts[i]为空则默认按升序排列.
*
* @param list
* @param fields
* @param sorts
* @return
*/
@SuppressWarnings("unchecked")
public static List<?> sort(List<?> list, String[] fields, String[] sorts) {
if (fields != null && fields.length > 0) {
for (int i = fields.length - 1; i >= 0; i--) {
final String field = fields[i];
String tmpSort = ASC;
if (sorts != null && sorts.length > i && sorts[i] != null) {
tmpSort = sorts[i];
}
final String sort = tmpSort;
Collections.sort(list, new Comparator() {
public int compare(Object a, Object b) {
int ret = 0;
try {
Field f = a.getClass().getDeclaredField(field);
f.setAccessible(true);
Class<?> type = f.getType();
if (type == int.class) {
ret = ((Integer) f.getInt(a))
.compareTo((Integer) f.getInt(b));
} else if (type == double.class) {
ret = ((Double) f.getDouble(a))
.compareTo((Double) f.getDouble(b));
} else if (type == long.class) {
ret = ((Long) f.getLong(a)).compareTo((Long) f
.getLong(b));
} else if (type == float.class) {
ret = ((Float) f.getFloat(a))
.compareTo((Float) f.getFloat(b));
} else if (type == Date.class) {
ret = ((Date) f.get(a)).compareTo((Date) f
.get(b));
} else if (isImplementsOf(type, Comparable.class)) {
ret = ((Comparable) f.get(a))
.compareTo((Comparable) f.get(b));
} else {
ret = String.valueOf(f.get(a)).compareTo(
String.valueOf(f.get(b)));
}
} catch (SecurityException e) {
e.printStackTrace();
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
if (sort != null && sort.toLowerCase().equals(DESC)) {
return -ret;
} else {
return ret;
}
}
});
}
}
return list;
}
/**
* 默认按正序排列
*
* @param list
* @param method
* @return
*/
public static List<?> sortByMethod(List<?> list, final String method) {
return sortByMethod(list, method, null);
}
@SuppressWarnings("unchecked")
public static List<?> sortByMethod(List<?> list, final String method,
final String sort) {
Collections.sort(list, new Comparator() {
public int compare(Object a, Object b) {
int ret = 0;
try {
Method m = a.getClass().getMethod(method, null);
m.setAccessible(true);
Class<?> type = m.getReturnType();
if (type == int.class) {
ret = ((Integer) m.invoke(a, null))
.compareTo((Integer) m.invoke(b, null));
} else if (type == double.class) {
ret = ((Double) m.invoke(a, null)).compareTo((Double) m
.invoke(b, null));
} else if (type == long.class) {
ret = ((Long) m.invoke(a, null)).compareTo((Long) m
.invoke(b, null));
} else if (type == float.class) {
ret = ((Float) m.invoke(a, null)).compareTo((Float) m
.invoke(b, null));
} else if (type == Date.class) {
ret = ((Date) m.invoke(a, null)).compareTo((Date) m
.invoke(b, null));
} else if (isImplementsOf(type, Comparable.class)) {
ret = ((Comparable) m.invoke(a, null))
.compareTo((Comparable) m.invoke(b, null));
} else {
ret = String.valueOf(m.invoke(a, null)).compareTo(
String.valueOf(m.invoke(b, null)));
}
if (isImplementsOf(type, Comparable.class)) {
ret = ((Comparable) m.invoke(a, null))
.compareTo((Comparable) m.invoke(b, null));
} else {
ret = String.valueOf(m.invoke(a, null)).compareTo(
String.valueOf(m.invoke(b, null)));
}
} catch (NoSuchMethodException ne) {
System.out.println(ne);
} catch (IllegalAccessException ie) {
System.out.println(ie);
} catch (InvocationTargetException it) {
System.out.println(it);
}
if (sort != null && sort.toLowerCase().equals(DESC)) {
return -ret;
} else {
return ret;
}
}
});
return list;
}
@SuppressWarnings("unchecked")
public static List<?> sortByMethod(List<?> list, final String methods[],
final String sorts[]) {
if (methods != null && methods.length > 0) {
for (int i = methods.length - 1; i >= 0; i--) {
final String method = methods[i];
String tmpSort = ASC;
if (sorts != null && sorts.length > i && sorts[i] != null) {
tmpSort = sorts[i];
}
final String sort = tmpSort;
Collections.sort(list, new Comparator() {
public int compare(Object a, Object b) {
int ret = 0;
try {
Method m = a.getClass().getMethod(method, null);
m.setAccessible(true);
Class<?> type = m.getReturnType();
if (type == int.class) {
ret = ((Integer) m.invoke(a, null))
.compareTo((Integer) m.invoke(b, null));
} else if (type == double.class) {
ret = ((Double) m.invoke(a, null))
.compareTo((Double) m.invoke(b, null));
} else if (type == long.class) {
ret = ((Long) m.invoke(a, null))
.compareTo((Long) m.invoke(b, null));
} else if (type == float.class) {
ret = ((Float) m.invoke(a, null))
.compareTo((Float) m.invoke(b, null));
} else if (type == Date.class) {
ret = ((Date) m.invoke(a, null))
.compareTo((Date) m.invoke(b, null));
} else if (isImplementsOf(type, Comparable.class)) {
ret = ((Comparable) m.invoke(a, null))
.compareTo((Comparable) m.invoke(b,
null));
} else {
ret = String.valueOf(m.invoke(a, null))
.compareTo(
String.valueOf(m
.invoke(b, null)));
}
} catch (NoSuchMethodException ne) {
System.out.println(ne);
} catch (IllegalAccessException ie) {
System.out.println(ie);
} catch (InvocationTargetException it) {
System.out.println(it);
}
if (sort != null && sort.toLowerCase().equals(DESC)) {
return -ret;
} else {
return ret;
}
}
});
}
}
return list;
}
/**
* 判断对象实现的所有接口中是否包含szInterface
*
* @param clazz
* @param szInterface
* @return
*/
public static boolean isImplementsOf(Class<?> clazz, Class<?> szInterface) {
boolean flag = false;
Class<?>[] face = clazz.getInterfaces();
for (Class<?> c : face) {
if (c == szInterface) {
flag = true;
} else {
flag = isImplementsOf(c, szInterface);
}
}
if (!flag && null != clazz.getSuperclass()) {
return isImplementsOf(clazz.getSuperclass(), szInterface);
}
return flag;
}
public static void main(String[] args) throws Exception {
List<Student> list = new ArrayList<Student>();
list.add(new Student(3, "b", 1, new Date(11110000)));
list.add(new Student(1, "c", 3, new Date(44440000)));
list.add(new Student(2, "a", 2, new Date(22210000)));
list.add(new Student(4, "a", 11, new Date(33330000)));
System.out.println("-------原来序列-------------------");
for (Student stu : list) {
System.out.println(stu.toString());
}
// 按age正序排序,注意结果排完后是1,2,3,11. 不是1,11,2,3(如果是String类型正序排序是这样)
SortListUtil.sort(list, "age", null);
System.out.println("---------测试Integer和正序,按age正序排序-----------------");
for (Student stu : list) {
System.out.println(stu.toString());
}
// 按id倒序
SortListUtil.sort(list, "id", SortListUtil.DESC);
System.out.println("--------测试int和倒序,按id倒序------------------");
for (Student stu : list) {
System.out.println(stu.toString());
}
// 先按name正序排序,再按id正序排序
SortListUtil.sort(list, new String[] { "name", "id" }, new String[] {});
System.out
.println("---------测试多个排序字段,先按name正序,name相同时再按id正序-----------------");
for (Student stu : list) {
System.out.println(stu.toString());
}
// 先按name正序排序,再按id倒序排序
SortListUtil.sort(list, new String[] { "name", "id" }, new String[] {
SortListUtil.ASC, SortListUtil.DESC });
System.out
.println("---------测试多个排序字段,先按name正序,name相同时再按id倒序-----------------");
for (Student stu : list) {
System.out.println(stu.toString());
}
// 按birthday排序
SortListUtil.sort(list, "birthday");
System.out
.println("---------测试实现了Comparable接口的对象排序,按birthday正序-----------------");
for (Student stu : list) {
System.out.println(stu.toString());
}
// sortByMethod
SortListUtil.sortByMethod(list, "getId", null);
System.out
.println("---------测试sortByMethod,按getId方法正序-----------------");
for (Student stu : list) {
System.out.println(stu.toString());
}
}
}
排序方法来源：http://blog.csdn.net/lk_blog/article/details/12804873