問(wèn)題描述

在驗(yàn)證《Core Java》第9版4-5代碼時(shí)，發(fā)現(xiàn)程序輸出結(jié)果和自己理解的不太一樣。

import java.util.Random;class Employee { private static int nextId; private int id; private String name = ''; private double salary; static {Random generator = new Random();nextId = generator.nextInt(10000); } {id = nextId;nextId++; } public Employee(String name, double salary) {this.name = name;this.salary = salary; } public Employee(double salary) {this('Employee #' + nextId, salary); } public Employee() { } public String getName() {return name; } public double getSalary() {return salary; } public int getId() {return id; }}public class ConstructorTest { public static void main(String[] args) {Employee[] staff = new Employee[3];staff[0] = new Employee('Harry', 40000);staff[1] = new Employee(60000);staff[2] = new Employee();for (Employee e : staff) { System.out.println('id = ' + e.getId() + ', name = ' + e.getName() + ', salary = ' + e.getSalary());} }}

以下是輸出結(jié)果：

id = 6943, name = Harry, salary = 40000.0id = 6944, name = Employee #6944, salary = 60000.0id = 6945, name = , salary = 0.0

根據(jù)第一條語(yǔ)句得出靜態(tài)初始化塊生成的nextId為6943，然后在初始化塊中id被賦值為6943，nextId自增后為6944。再執(zhí)行第一個(gè)構(gòu)造函數(shù)；

那么對(duì)于第二個(gè)對(duì)象來(lái)說(shuō)，就應(yīng)該直接執(zhí)行初始化塊，此時(shí)id為6944，nextId自增為6945。再執(zhí)行第二個(gè)構(gòu)造函數(shù)，此時(shí)this('Employee #' + nextId, salary);語(yǔ)句中的nextId應(yīng)該為6945，為什么輸出結(jié)果為6944呢？

問(wèn)題解答

這個(gè)類(lèi)初始化的順序確實(shí)是個(gè)神奇的問(wèn)題，只可根據(jù)結(jié)果去理解。我打了個(gè)斷點(diǎn)去測(cè)試，staff[0] = new Employee('Harry', 40000);和staff[2] = new Employee();都是代碼塊先于構(gòu)造方法執(zhí)行，但staff[1] = new Employee(60000);卻先執(zhí)行走到this('Employee #' + nextId, salary);，然后代碼塊，然后public Employee(String name, double salary)構(gòu)造函數(shù)。如果你使用2，則按你的預(yù)期，代碼塊先于構(gòu)造方法。

public Employee(double salary) { // 1 this('Employee #' + nextId, salary); // 2// this.name = 'Employee #' + nextId; // this.salary = salary;}回答2：

正常來(lái)說(shuō)，java 編譯器會(huì)把實(shí)例初始化塊復(fù)制構(gòu)造方法中，具體位置在調(diào)用父類(lèi)的構(gòu)造方法以后，構(gòu)造方法里面的語(yǔ)句之前，但是存在例外情況。Java 官方的 Tutorials 里說(shuō)初始化塊會(huì)被復(fù)制到每個(gè)構(gòu)造方法里面其實(shí)是不嚴(yán)謹(jǐn)?shù)摹?/p>

具體到這個(gè)例子，需要考慮一個(gè)問(wèn)題，如果編譯器把初始化塊復(fù)制到每個(gè)構(gòu)造方法里面，那么對(duì)于在構(gòu)造方法里面調(diào)用了其他構(gòu)造方法的情況，這個(gè)初始化塊就會(huì)執(zhí)行兩次，就像例子里面的

public Employee(double salary) {this('Employee #' + nextId, salary); // 調(diào)用了另一個(gè)構(gòu)造方法}

如果編譯器把初始化塊里的代碼復(fù)制到了public Employee(double salary)和public Employee(String name, double salary)里面，這個(gè)初始化塊就會(huì)執(zhí)行兩次，為了避免這種情況，編譯器作了一個(gè)簡(jiǎn)單的處理，編譯器發(fā)現(xiàn)public Employee(double salary)調(diào)用了本類(lèi)的另一個(gè)構(gòu)造方法，就沒(méi)有把初始化塊的代碼拷貝到這個(gè)構(gòu)造方法里面。也就是說(shuō)在初始化第二個(gè)對(duì)象的時(shí)候，這個(gè)初始化塊是推遲到調(diào)用this('Employee #' + nextId, salary);后，在執(zhí)行Employee(String name, double salary)的時(shí)候才執(zhí)行的，由于推遲了初始化塊的執(zhí)行，在決定傳遞的參數(shù) nextId 的時(shí)候，仍然是未自增的值。如果把這個(gè)構(gòu)造方法修改為

public Employee(double salary) { // this('Employee #' + nextId, salary); this.name = 'Employee #' + nextId; this.salary = salary;}

輸出結(jié)果就會(huì)變?yōu)?/p>

id = 5473, name = Harry, salary = 40000.0id = 5474, name = Employee #5475, salary = 60000.0id = 5475, name = , salary = 0.0

而修改之前的情況，反編譯下 class 文件就能看出來(lái)編譯器最后的輸出結(jié)果，這里只貼三個(gè)構(gòu)造方法，可以很明顯的看出來(lái)，第二個(gè)構(gòu)造方法并沒(méi)有被復(fù)制初始化塊的內(nèi)容，直接調(diào)用了另一個(gè)構(gòu)造方法。

public Employee(java.lang.String, double); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object.'<init>':()V 4: aload_0 5: ldc #2 // String 7: putfield #3 // Field name:Ljava/lang/String; 10: aload_0 11: getstatic #4 // Field nextId:I 14: putfield #5 // Field id:I 17: getstatic #4 // Field nextId:I 20: iconst_1 21: iadd 22: putstatic #4 // Field nextId:I 25: aload_0 26: aload_1 27: putfield #3 // Field name:Ljava/lang/String; 30: aload_0 31: dload_2 32: putfield #6 // Field salary:D 35: return public Employee(double); Code: 0: aload_0 1: new #7 // class java/lang/StringBuilder 4: dup 5: invokespecial #8 // Method java/lang/StringBuilder.'<init>':()V 8: ldc #9 // String Employee # 10: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 13: getstatic #4 // Field nextId:I 16: invokevirtual #11 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder; 19: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 22: dload_1 23: invokespecial #13 // Method '<init>':(Ljava/lang/String;D)V 26: return public Employee(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object.'<init>':()V 4: aload_0 5: ldc #2 // String 7: putfield #3 // Field name:Ljava/lang/String; 10: aload_0 11: getstatic #4 // Field nextId:I 14: putfield #5 // Field id:I 17: getstatic #4 // Field nextId:I 20: iconst_1 21: iadd 22: putstatic #4 // Field nextId:I 25: return