JAVA中Collections工具类sort()排序方法

时间:2022-09-06 19:43:26

本问介绍了Collections工具类两种sort()方法,具体如下:

一、Collections工具类两种sort()方法

格式一: public static <T extends Comparable<? super T>> void sort(List<T> list)

说明:该方法中的泛型<T>都是Comparable接口的子类,即只有是Comparable接口子类类型的数据,才能进行比较排序。如果其他类型的数据要进行比较排序,必须继承Comparable接口并

覆写equals()和compareTo()方法。其中如String类、Integer类都是Comparable接口子类,可以进行排序,而基本类型不能进行sort排序。比较项目在类内指定

格式二:public static <T> void sort(List<T> list, Comparator<? super T> c)

说明:该方法中指定比较方式Comparator<? super T> c,即c必须实现Comparator<? super T>接口,覆写compareTo()方法指定比较项目。比较项目在类外指定,比较灵活

二、示例

示例中获取字符串和数字的公用方法:

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/**
   * 生成随机 不重复的字符串 : number 生成字符串个数
   */
  public static List<String> generateString(int number) {
    List<String> listString = new ArrayList<>(); // 用于存放返回值
    List<Integer> length = null; // 字符串长度
    StringBuffer sb = new StringBuffer(); // 中间变量
    int control = 0; // 控制个数
    String[] chars = new String[] { "a", "b", "c", "d", "e", "f", "g", "h",
        "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t",
        "u", "v", "w", "x", "y", "z", "0", "1", "2", "3", "4", "5",
        "6", "7", "8", "9", "A", "B", "C", "D", "E", "F", "G", "H",
        "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T",
        "U", "V", "W", "X", "Y", "Z" };
    while (true) {
      // 控制结束
      if ( control==number ) {
        break;
      }
      // 生成随机数,生成36位的2aaab761-4341-4968-aceb-3861ee3824b2 UUID类型数据
      String uuid = UUID.randomUUID().toString().replace("-", "");
      sb.setLength(0);
      // 获得随机字符串长度,长度不为0
      do {
        length = getDiffNo(1, 11);
      } while ( length.get(0)==0 );
      // 拼凑字符串
      for (int i=0; i<length.get(0); i++) {
        String str = uuid.substring(i*3, (i*3+3));
        //将str字符串转换为16进制,获得其值
        int x = Integer.parseInt(str, 16);
        //取余:x % 0x3E--0x3E = 3*16 + 14 = 62, 其中chars有62个字符
        sb.append(chars[x % 0x3E]);
      }
      listString.add(sb.toString());
      control++;
    }
    return listString;
  }
 
  /**
   * 生成随机不重复的数字 :n生成个数 max生成范围
   */
  public static List<Integer> getDiffNo(int n, int max) {
    // 生成 [0-n] 个不重复的随机数
    // list 用来保存这些随机数
    List<Integer> list = new ArrayList<>();
    Random random = new Random();
    Integer k;
    for (int i=0; i<n; i++) {
      do {
        k = random.nextInt(max);
      } while (list.contains(k));
      list.add(k);
    }
    return list;
  }

1、对Integer泛型的List进行排序

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/**
   * 1.通过Collections.sort()方法,对Integer泛型的List进行排序;
   * 创建一个Integer泛型的List,插入十个100以内的不重复随机整数, 调用Collections.sort()方法对其进行排序
   * 2.排序规则:先数字后字母,数字0-9,字母A-Z a-z的顺序
   */
  public void listIntegerSort() {
    // 插入十个100以内的不重复随机整数
    List<Integer> integerList = getDiffNo(10, 100);
    System.out.println("-------------排序前--------------");
    for (Integer integer : integerList) {
      System.out.println("元素:" + integer);
    }
    Collections.sort(integerList);
    System.out.println("----------------排序后-------------------");
    for (Integer integer : integerList) {
      System.out.println("元素:" + integer);
    }
  }

2、对String泛型的List进行排序

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/**
   * 1.对String泛型的List进行排序; 创建String泛型的List,添加乱序的String元素,
   * 调用sort方法,再次输出排序后的顺序
   */
  public void listStringSort() {
    List<String> stringList = new ArrayList<String>();
    stringList.add("eipJlcx");
    stringList.add("WvQRufC");
    stringList.add("J");
    stringList.add("HdaU2G");
    stringList.add("M0WswHD3");
    System.out.println("------------排序前-------------");
    for (String string : stringList) {
      System.out.println("元素:" + string);
    }
    Collections.sort(stringList);
    System.out.println("--------------排序后---------------");
    for (String string : stringList) {
      System.out.println("元素:" + string);
    }
  }
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/**
   * 对String泛型的List进行排序,要求随机生成10个的不重复字符串,字符串的长度在10以内
   */
  public void listStringRandomSort() {
    // 生成随机字符串
    List<String> listString = generateString(10);
    System.out.println("--------------排序前---------------");
    for (String integer : listString) {
      System.out.println("元素:" + integer);
    }
    // 排序
    Collections.sort(listString);
    System.out.println("----------------排序后------------------");
    for (String integer : listString) {
      System.out.println("元素:" + integer);
    }
  }

3、对其他类型泛型的List进行排序

Course类实现

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/**
 * 课程类
 * @author Administrator
 *
 */
public class Course {
  public String id;
  public String name;
  public Course(String id, String name) {
    this.id = id ;
    this.name = name;
  }
  public Course() {
  }
  
  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((name == null) ? 0 : name.hashCode());
    return result;
  }
 
  @Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (obj == null)
      return false;
    if (!(obj instanceof Course))
      return false;
    Course other = (Course) obj;
    if (name == null) {
      if (other.name != null)
        return false;
    } else if (!name.equals(other.name))
      return false;
    return true;
  }
}

Student类实现Comparable接口,类内设置比较项

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import java.util.HashSet;
import java.util.Set;
 
/**
 * 学生类
 * @author Administrator
 *
 */
public class Student implements Comparable<Student> {
  public String id;
  public String name;
  public Set<Course> courses;
  
  public Student(String id, String name) {
    this.id = id;
    this.name = name;
    this.courses = new HashSet<Course>();
  }
 
  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((name == null) ? 0 : name.hashCode());
    return result;
  }
 
  @Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (obj == null)
      return false;
    if (!(obj instanceof Student))
      return false;
    Student other = (Student) obj;
    if (name == null) {
      if (other.name != null)
        return false;
    } else if (!name.equals(other.name))
      return false;
    return true;
  }
 
  @Override
  public int compareTo(Student o) { // 设置ID为比较项
    // TODO Auto-generated method stub
    return this.id.compareTo(o.id);
  }
}

实现Comparator接口,类外设置比较项

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import java.util.Comparator;
public class StudentComparator implements Comparator<Student> {
 
  @Override
  public int compare(Student o1, Student o2) {
    // TODO Auto-generated method stub
    return o1.name.compareTo(o2.name);
  }
}

比较Student类

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/**
   * 对其他类型泛型的List进行排序,以Student为例。
   */
  public void listComparatorSort() {
    List<Student> studentList = new ArrayList<Student>();
    List<Integer> list = getDiffNo(4, 1000);
 
    studentList.add(new Student(list.get(0) + "", "Mike"));
    studentList.add(new Student(list.get(1) + "", "Angela"));
    studentList.add(new Student(list.get(2) + "", "Lucy"));
    studentList.add(new Student(1000 + "", "Beyonce"));
    System.out.println("--------------排序前---------------");
    for (Student student : studentList) {
      System.out.println("学生:" + student.id + ":" + student.name);
    }
    // 实现Comparator<T>接口,设置ID比较方式
    Collections.sort(studentList);
    System.out.println("----------------按照ID排序后------------------");
    for (Student student : studentList) {
      System.out.println("学生:" + student.id + ":" + student.name);
    }
 
    // 实现Comparator<T>接口,设置特定比较方式,以name比较排序
    Collections.sort(studentList, new StudentComparator());
    System.out.println("----------------按照姓名排序后-----------------");
    for (Student student : studentList) {
      System.out.println("学生:" + student.id + ":" + student.name);
    }
  }

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