Comprehensive use of Singleton model and static loading (Youku interview questions), static model

Comprehensive use of Singleton model and static loading (Youku interview questions), static model
I. original question

1/* 2 * running result 3*2 4*1 5 */6 public class BookTest {7 static BookTest book = new BookTest (); 8 static int; 9 static int B = 0; 10 11 private BookTest () {12 a ++; 13 B ++; 14} 15 16 public static void main (String [] args) {17 BookTest book = new BookTest (); 18 System. out. println (book. a); 19 System. out. println (book. b); 20} 21 22}

 

Ii. original question explanation

1 package com. heima. when the test; 2 3/* 4 * program is executed, jvm first loads the Book class, then new Book (), and then static variables a and B; but 5 * at this time, no value is assigned to a and B (using the default value corresponding to its data type, that is, two static variables a = 0, B = 0 6 * loaded into memory), so the new Book () calls its private constructor to output a = 1, B = 1; and then assigns the initial value to the static variable, 7 * That is, the value a given by US remains unchanged, and B changes to 0 (now a = 1, B = 0). Then, in the main method, a new Book () object is created, call the constructor eight times, that is, a is changed to 2, B is changed to 1. 9 */10 11/* 12 * final execution result: 13 the constructor is called. Before calling: 14 a = 0 B = 015 after the constructor is called: 16 a = 1 B = 117 static code block called: 18 a = 5; B = 019 constructor called. Before calling: 20 a = 5 B = 021 after calling the constructor: 22 a = 6 B = 123 624 125 */26 public class Book {27 static Book book = new Book (); 28 static int a = 5; 29 static int B = 0; 30 static {31 System. out. println ("static code block called: \ na =" + a + "; \ tb =" + B); // you can also use Book. a and Book. b32} 33 34 private Book () {35 System. out. println ("constructor called, before calling:"); 36 System. out. print ("a =" + a + "\ tb =" + B); // you can also use Book. a and Book. b37 a ++; 38 B ++; 39 System. out. println ("\ n constructor called: \ na =" + a + "\ tb =" + B); 40} 41 42 public static void main (String [] args) {43 Book book = new Book (); 44 System. out. println ("a last equals:" + book. a); 45 System. out. println ("B is equal to:" + book. b); 46} 47 48}