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  1. import java.util.*;
  2.  
  3. class Book implements Comparable<Book>, Cloneable {
  4.  /* private */ String title;
  5.  /* private */ Date publishDate;
  6.  /* private */ String comment;
  7.  
  8.  public boolean equals(Object obj) {
  9.   if(obj == this) return true;
  10.   if(obj == null) return false;
  11.   if(!(obj instanceof Book)) return false;
  12.   Book result = (Book) obj;
  13.   if(!result.title.equals(this.title)) return false;
  14.   if(!result.publishDate.equals(this.publishDate)) return false;
  15.   return true;
  16.  }
  17.  
  18.  public int hashCode() {
  19.   int hash = 1;
  20.   hash = hash * 31 + title.hashCode();
  21.   hash = hash * 31 + publishDate.hashCode();
  22.   return hash;
  23.  }
  24.  
  25.  public int compareTo(Book obj) {
  26.   return this.publishDate.compareTo(obj.publishDate);
  27.  }
  28.  
  29.  public Book clone() {
  30.   Book result = new Book();
  31.   result.title = this.title;
  32.   result.comment = this.comment;
  33.   result.publishDate = (Date) this.publishDate.clone();
  34.   return result;
  35.  }
  36. }
  37.  
  38. public class Main {
  39.   public static void main(String[] args) {
  40.     Book book1 = new Book();
  41.     book1.title = "本1";
  42.     book1.publishDate = new Date(1);
  43.     Book book2 = new Book();
  44.     book2.title = "本1";
  45.     book2.publishDate = new Date(1);
  46.     boolean bool1 = book1.equals(book2);
  47.     System.out.println(bool1);
  48.  
  49.     Book book3 = new Book();
  50.     book3.title = "本2";
  51.     book3.publishDate = new Date();
  52.     Book book4 = new Book();
  53.     book4.title = "本2";
  54.     boolean bool2 = book3.equals(book4);
  55.     System.out.println(bool2);
  56. //    System.out.println(book4.publishDate);
  57.   }
  58. }
Runtime error #stdin #stdout #stderr 0.1s 27876KB
comments (?)
stdin
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Standard input is empty
stdout
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true
stderr
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Exception in thread "main" java.lang.NullPointerException
	at Book.equals(Main.java:14)
	at Main.main(Main.java:54)
https://ideone.com/QhI9r8
language:
Java (HotSpot 12)
created:
6 years ago
visibility:
public

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