Compareable接口和Comparator + 公用排序方法

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