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ollections對List集合中的數據進行排序
有時候需要對集合中的元素按照一定的規則進行排序，這就需要用到
Java中提供的對集合進行操作的工具類Collections，其中的sort方法
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先看一個簡單的例子：
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[java]?view plain?copy
- public?static?void?main(String[]?args)?{??
- ????List<Integer>?nums?=?new?ArrayList<Integer>();??
- ????????nums.add(3);??
- ????????nums.add(5);??
- ????????nums.add(1);??
- ????????nums.add(0);??
- ????????System.out.println(nums);??
- ????????Collections.sort(nums);??
- ????????System.out.println(nums);??
- }??
輸出結果：
[3, 5, 1, 0]
[0, 1, 3, 5]
稍微復雜的List里面放一個復雜的對象
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[java]?view plain?copy
- package?core.java.collection.collections;??
- ??
- public?class?User?implements?Comparable<User>{??
- ??????
- ????private?int?score;??
- ??????
- ????private?int?age;??
- ??????
- ????public?User(int?score,?int?age){??
- ????????super();??
- ????????this.score?=?score;??
- ????????this.age?=?age;??
- ????}??
- ??
- ????public?int?getScore()?{??
- ????????return?score;??
- ????}??
- ??
- ????public?void?setScore(int?score)?{??
- ????????this.score?=?score;??
- ????}??
- ??
- ????public?int?getAge()?{??
- ????????return?age;??
- ????}??
- ??
- ????public?void?setAge(int?age)?{??
- ????????this.age?=?age;??
- ????}??
- ??
- ????@Override??
- ????public?int?compareTo(User?o)?{??
- ????????int?i?=?this.getAge()?-?o.getAge();//先按照年齡排序??
- ????????if(i?==?0){??
- ????????????return?this.score?-?o.getScore();//如果年齡相等了再用分數進行排序??
- ????????}??
- ????????return?i;??
- ????}??
- ??????
- }??
- ??
- public?static?void?main(String[]?args)?{??
- ????????List<User>?users?=?new?ArrayList<User>();??
- ????????users.add(new?User(78,?26));??
- ????????users.add(new?User(67,?23));??
- ????????users.add(new?User(34,?56));??
- ????????users.add(new?User(55,?23));??
- ????????Collections.sort(users);??
- ????????for(User?user?:?users){??
- ????????????System.out.println(user.getScore()?+?","?+?user.getAge());??
- ????????}??
- }??
輸出結果：
55,23
67,23
78,26
34,56
我們會發現sort(List<T>)方法中List中的T必須實現Comparable<T>接口，然后實現
compareTo（）方法，該方法的返回值0代表相等，1表示大于，-1表示小于；為什么
在簡單例子中沒有看到實現Comparable接口呢？是因為Integer類其實自己已經實現
了Comparable接口，Java已經給我們做好了。
Collections提供的第二種排序方法sort(List<T> list, Comparator<? super T> c)
先看例子：
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[java]?view plain?copy
- package?core.java.collection.collections;??
- ??
- public?class?Students?{??
- ??????
- ????private?int?age;??
- ????private?int?score;??
- ??????
- ????public?Students(int?age,?int?score){??
- ????????super();??
- ????????this.age?=?age;??
- ????????this.score?=?score;??
- ????}??
- ??????
- ????public?int?getAge()?{??
- ????????return?age;??
- ????}??
- ????public?void?setAge(int?age)?{??
- ????????this.age?=?age;??
- ????}??
- ????public?int?getScore()?{??
- ????????return?score;??
- ????}??
- ????public?void?setScore(int?score)?{??
- ????????this.score?=?score;??
- ????}??
- }??
- public?static?void?main(String[]?args)?{??
- ????????List<Students>?students?=?new?ArrayList<Students>();??
- ????????students.add(new?Students(23,?100));??
- ????????students.add(new?Students(27,?98));??
- ????????students.add(new?Students(29,?99));??
- ????????students.add(new?Students(29,?98));??
- ????????students.add(new?Students(22,?89));??
- ????????Collections.sort(students,?new?Comparator<Students>()?{??
- ??
- ????????????@Override??
- ????????????public?int?compare(Students?o1,?Students?o2)?{??
- ????????????????int?i?=?o1.getScore()?-?o2.getScore();??
- ????????????????if(i?==?0){??
- ????????????????????return?o1.getAge()?-?o2.getAge();??
- ????????????????}??
- ????????????????return?i;??
- ????????????}??
- ????????});??
- ????????for(Students?stu?:?students){??
- ????????????System.out.println("score:"?+?stu.getScore()?+?":age"?+?stu.getAge());??
- ????????}??
- }??
輸出結果：
score:89:age22
score:98:age27
score:98:age29
score:99:age29
score:100:age23
從上面的例子我們可以看出Students類沒有實現Comparable<T>接口，只是在sort()方法
中多傳入一個參數，只不過該參數是一個接口我們需要實現其compare方法。
以上就是是Java中Colelctions工具類為我們提供的兩種集合排序方法。
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插件補充)
