Comparable和Comparator以及equals和HashCode方法的覆写

本文主要是介绍Comparable和Comparator以及equals和HashCode方法的覆写，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1. Comparable和Comparator的使用

• 当两个元素进行比较时，属于自然顺序比较，不需要第三方介入比较时，使用Comparable比较，当需要第三方进行比较时，使用Comparator进行比较。
• 使用Comparable时需要覆写comparaTo方法，而使用Comparator接口时需要覆写compara方法。
  举一个例子，比如说以下代码：

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.List;
import java.util.Vector;class authorComparator implements Comparator<Book> {@Overridepublic int compare(Book o1, Book o2) {return o1.author.compareTo(o2.author);}
}
class nameComparator implements Comparator<Book> {@Overridepublic int compare(Book o1, Book o2) {return o1.name.compareTo(o2.name);}
}
class pComparator implements Comparator<Book> {boolean asc;public pComparator(boolean asc) {this.asc = asc;}@Overridepublic int compare(Book o1, Book o2) {{if (asc) {return o1.p - o2.p;} else {return o2.p - o1.p;}}}
}
class saleComparator implements Comparator<Book> {@Overridepublic int compare(Book o1, Book o2) {return o1.sale-o2.sale;}
}
class commentComparator implements Comparator<Book> {@Overridepublic int compare(Book o1, Book o2) {return o1.comment-o2.comment;}
}//以上的比较都需要第三方对象的介入才能进行比较
public class Book implements Comparable<Book> {public String isbn;public String author;public String name;public int p;public int sale;public int comment;public Book(String isbn, String author, String name, int p, int sale, int comment) {this.isbn = isbn;this.author = author;this.name = name;this.p = p;this.sale = sale;this.comment = comment;}@Overridepublic String toString() {return "Book{" +"isbn=" + isbn +", author='" + author + '\'' +", name='" + name + '\'' +", p=" + p +", sale=" + sale +", comment=" + comment +'}';}@Overridepublic int compareTo(Book o) {return isbn.compareTo(o.isbn);}
}                                 //图书编号按照自然顺序比较class bookSort {public static void main(String[] args) {List<Book> a = new ArrayList<>();a.add(new Book("123456","小军","我和我的祖国",1,20,456));a.add(new Book("124456","小明","你",1,30,45));a.add(new Book("125456","小红","我们",1,80,457));a.add(new Book("126456","小峰","应物兄",1,50,452));a.add(new Book("127456","小蓝","我们三",1,60,458));a.add(new Book("128456","小紫","更狂日记",1,90,457));List<Book> copy;// 按自然顺序（ISBN）排序System.out.println("按 ISBN 排序：");copy = new ArrayList<>(a);sort(copy);System.out.println(copy);// 按书名排序System.out.println("按 书名 排序：");copy = new ArrayList<>(a);sort(copy, new nameComparator());System.out.println(copy);// 按价格排序-从小到大System.out.println("按 价格-从小到大 排序：");copy = new ArrayList<>(a);sort(copy, new pComparator(true));System.out.println(copy);// 按价格排序-从大到小System.out.println("按 价格-从大到小 排序：");copy = new ArrayList<>(a);sort(copy, new pComparator(false));System.out.println(copy);System.out.println("按 销量 排序：");copy = new ArrayList<>(a);sort(copy,new saleComparator());System.out.println(copy);System.out.println("按 评价 排序：");copy = new ArrayList<>(a);sort(copy,new commentComparator());System.out.println(copy);}public static void sort(List<Book> books) {for (int i = 1; i < books.size(); i++) {Book k = books.get(i);int j = i - 1;for (; j >= 0 && (books.get(j).compareTo(k)) > 0; j--) {books.set(j + 1, books.get(j));}books.set(j + 1, k);}}public static void sort(List<Book> books, Comparator<Book> cmp) {for (int i = 1; i < books.size(); i++) {Book k = books.get(i);int j = i - 1;for (; j >= 0 && cmp.compare(books.get(j), k) > 0; j--) {books.set(j + 1, books.get(j));}books.set(j + 1, k);}}
}

2.eauals的覆写

比如说比较两个人的学号

@Override
public boolean equals (Object o){
if(this==obj) return true;
if(!(o instanceof Person)) return false;
Person person=(Person) o;
return sn.equals(person.sn);

3.hashCode的覆写

在写一个哈希表时，必须对equals和hashCode进行覆写

@ Override
public int hashCode(){
return Objects.hash(sn);
}

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