java Collections.sort sorting function usage details

Comparator is an interface that overrides compare() and equals() for the price comparison function. If is null, is to use the default order of elements, such as a b, c, d, e, f, g, is a b, c, d, e, f, g so, of course number also is such.

The compare (a,b) method returns a negative integer, a zero, or a positive integer, depending on whether the first parameter is less than, equal to, or greater than the second parameter.

equals (obj) method: returns true only if the specified object is also an Comparator and the same sort is enforced as this Comparator.

Collections. sort (list, new PriceComparator ()); The second parameter of the int method returns a value of type int, which is equivalent to a flag telling the sort method in what order to sort list.

The specific implementation code method is as follows:

Book entity class:

package com.tjcyjd.comparator; 
 
import java.text.DecimalFormat; 
import java.text.SimpleDateFormat; 
import java.util.GregorianCalendar; 
import java.util.Iterator; 
import java.util.TreeMap; 
 
/** 
 *  The book entity class  
 * 
 * @author yjd 
 * 
 */ 
public class Book implements Comparable { //  Definition, Book Class, inherited by default Object class  
  public int id;//  Serial number  
  public String name;//  The name of the  
  public double price; //  The price  
  private String author;//  The author  
  public GregorianCalendar calendar;//  Date of publication  
 
  public Book() { 
    this(0, "X", 0.0, new GregorianCalendar(), ""); 
  } 
 
  public Book(int id, String name, double price, GregorianCalendar calender, 
      String author) { 
    this.id = id; 
    this.name = name; 
    this.price = price; 
    this.calendar = calender; 
    this.author = author; 
  } 
 
  //  Overrides inherit from the parent class Object The way to meet Book Class information description requirements  
  public String toString() { 
    String showStr = id + "\t" + name; //  Defines a string that displays class information  
    DecimalFormat formatPrice = new DecimalFormat("0.00");//  Format the price to two decimal places  
    showStr += "\t" + formatPrice.format(price);//  Formatted price  
    showStr += "\t" + author; 
    SimpleDateFormat formatDate = new SimpleDateFormat("yyyy years MM month dd day "); 
    showStr += "\t" + formatDate.format(calendar.getTime()); //  Formatting time  
    return showStr; //  Returns a string of class information  
  } 
 
  public int compareTo(Object obj) {// Comparable Methods in an interface  
    Book b = (Book) obj; 
    return this.id - b.id; //  According to the book id Compare size for default sort  
  } 
 
  public static void main(String[] args) { 
    Book b1 = new Book(10000, " A dream of red mansions ", 150.86, new GregorianCalendar(2009, 
        01, 25), " Cao xueqin, gao e "); 
    Book b2 = new Book(10001, "3 The historical novel ", 99.68, new GregorianCalendar(2008, 7, 
        8), " Luo guanzhong  "); 
    Book b3 = new Book(10002, " Water margin ", 100.8, new GregorianCalendar(2009, 6, 
        28), " Shy naih-an  "); 
    Book b4 = new Book(10003, " Journey to the west ", 120.8, new GregorianCalendar(2011, 6, 
        8), " Wu chengen "); 
    Book b5 = new Book(10004, " Draco 8 The ministry of ", 10.4, new GregorianCalendar(2011, 9, 
        23), " sohu "); 
    TreeMap tm = new TreeMap(); 
    tm.put(b1, new Integer(255)); 
    tm.put(b2, new Integer(122)); 
    tm.put(b3, new Integer(688)); 
    tm.put(b4, new Integer(453)); 
    tm.put(b5, new Integer(40)); 
    Iterator it = tm.keySet().iterator(); 
    Object key = null, value = null; 
    Book bb = null; 
    while (it.hasNext()) { 
      key = it.next(); 
      bb = (Book) key; 
      value = tm.get(key); 
      System.out.println(bb.toString() + "\t Inventory: " + tm.get(key)); 
    } 
  } 
} 

Custom comparator and test class:

package com.tjcyjd.comparator; 
 
import java.util.ArrayList; 
import java.util.Collections; 
import java.util.Comparator; 
import java.util.GregorianCalendar; 
import java.util.Iterator; 
import java.util.List; 
 
public class UseComparator { 
  public static void main(String args[]) { 
    List<Book> list = new ArrayList<Book>(); //  Array sequence  
    Book b1 = new Book(10000, " A dream of red mansions ", 150.86, new GregorianCalendar(2009, 
        01, 25), " Cao xueqin, gao e "); 
    Book b2 = new Book(10001, "3 The historical novel ", 99.68, new GregorianCalendar(2008, 7, 
        8), " Luo guanzhong  "); 
    Book b3 = new Book(10002, " Water margin ", 100.8, new GregorianCalendar(2009, 6, 
        28), " Shy naih-an  "); 
    Book b4 = new Book(10003, " Journey to the west ", 120.8, new GregorianCalendar(2011, 6, 
        8), " Wu chengen "); 
    Book b5 = new Book(10004, " Draco 8 The ministry of ", 10.4, new GregorianCalendar(2011, 9, 
        23), " sohu "); 
    list.add(b1); 
    list.add(b2); 
    list.add(b3); 
    list.add(b4); 
    list.add(b5); 
    // Collections.sort(list); // There is no default comparator and cannot sort  
    System.out.println(" The elements in an array sequence :"); 
    myprint(list); 
    Collections.sort(list, new PriceComparator()); //  Sort by price  
    System.out.println(" Sort by the price of the book :"); 
    myprint(list); 
    Collections.sort(list, new CalendarComparator()); //  Sort by time  
    System.out.println(" Sort the books by the time they were published :"); 
    myprint(list); 
  } 
 
  //  Custom method: print out the branch list The elements in the  
  public static void myprint(List<Book> list) { 
    Iterator it = list.iterator(); //  Get the iterator for traversing list All the elements in  
    while (it.hasNext()) {//  Returns if there is an element in the iterator true 
      System.out.println("\t" + it.next());//  Display the element  
    } 
  } 
 
  //  Custom comparator: sort by book price  
  static class PriceComparator implements Comparator { 
    public int compare(Object object1, Object object2) {//  Implement the methods in the interface  
      Book p1 = (Book) object1; //  Casts.  
      Book p2 = (Book) object2; 
      return new Double(p1.price).compareTo(new Double(p2.price)); 
    } 
  } 
 
  //  Custom comparator: sort the book by the time it was published  
  static class CalendarComparator implements Comparator { 
    public int compare(Object object1, Object object2) {//  Implement the methods in the interface  
      Book p1 = (Book) object1; //  Casts.  
      Book p2 = (Book) object2; 
      return p2.calendar.compareTo(p1.calendar); 
    } 
  } 
}