Java Collections的sort方法的使用

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java.util.Collections.sort(List<T> list)方法的声明

   public static <T extends Comparable<? super T>> void sort(List<T> list) {        list.sort(null);    }

java.util.Collections.sort(List<T> list,Comparator<? super T> c)方法的声明

  public static <T> void sort(List<T> list, Comparator<? super T> c) {        list.sort(c);    }

(1)如果是单一的数据存放在集合中,是可以直接排序的

private static void sort1() {List list = new ArrayList();list.add(9);list.add(29);list.add(100);list.add(3);// sort(集合)Collections.sort(list);for (Object object : list) {System.out.println(object);}}

(2)如果是对象数据存放在集合中,是不可以直接排序的!

(a)实体类必须要实现这个接口Comparable

/** * 实体类  必须实现这个接口Comparable *  * @author Administrator * */public class Student implements Comparable<Student> {// 属性private String name;private int age;private String sex;public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}/** * 带参数的构造方法 *  * @param name * @param age * @param sex */public Student(String name, int age, String sex) {super();this.name = name;this.age = age;this.sex = sex;}/** * 无参的构造方法 */public Student() {super();}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + ", sex=" + sex + "]";}/** * 重写比较方法 * 按照此比较的东西从小到大排列 */@Overridepublic int compareTo(Student o) {if (this.age < o.age)return -1;if (this.age > o.age) {return 1;}return 0;}}

测试类:

private static void sort2() {// 1实例化对象Student stu1 = new Student("qq", 22, "F");Student stu2 = new Student("rose", 16, "M");Student stu3 = new Student("boy", 25, "F");Student stu4 = new Student("fei", 18, "M");List<Student> list1 = new ArrayList<Student>();list1.add(stu1);list1.add(stu2);list1.add(stu3);list1.add(stu4);// Exception in thread "main" java.lang.ClassCastException:// com.hlx.Student cannot be cast to java.lang.ComparableCollections.sort(list1);for (Student student : list1) {System.out.println(student);}}

效果:从小到大的顺序排序

(b)用Comparator接口

注意:实体类不需要实现这个接口

测试类:

private static void mysort1() {// 1实例化对象Student stu1 = new Student("qq", 22, "F");Student stu2 = new Student("rose", 16, "M");Student stu3 = new Student("boy", 25, "F");Student stu4 = new Student("fei", 18, "M");List<Student> list1 = new ArrayList<Student>();list1.add(stu1);list1.add(stu2);list1.add(stu3);list1.add(stu4);// 这样就可以实现自定义降序或者升序排列list了Collections.sort(list1, new Comparator<Student>() {@Overridepublic int compare(Student o1, Student o2) {if (o1.getAge() > o2.getAge()) {return -1;} else if (o1.getAge() < o2.getAge()) {return 1;}return 0;}});for (Student student : list1) {System.out.println(student);}}

效果: 从大到小的顺序排序

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