Java:调用构造器的具体处理步骤

《Java 核心技术 卷1》关于调用构造器的具体处理步骤的叙述是:

调用构造器的具体处理步骤

所有数据域被初始化为默认值（0、false或者null）。

按照在类说明中出现的次数，一次执行所有域初始化语句和初始化块。

如果构造器第一行调用了第二个构造器，则执行第二个构造器主体。（this(“Employee #” + nextId, s)只能放在第一行，否则编译出错。）

执行这个构造器的主体。

怎么理解呢？接来下我们代码分析

public class ConstructorTest {
public static void main(String[] args) {
// TODO Auto-generated method stub
Employee[] staff = new Employee[3];
staff[0] = new Employee("Harry", 4000);
staff[1] = new Employee(6000);
staff[2] = new Employee();
for(Employee e:staff)
{
System.out.println("name="+e.getName()+" id="+e.getId()+" salary="+e.getSalary());
}
}
}

Employee类源代码

package com.ustc.constructor;
import java.util.Random;
public class Employee {
private static int nextId;
private int id;
private String name = "";
private Double salary=assignId();
//static initialization block
static {
Random generator = new Random();
nextId = generator.nextInt(10000);
}
//object initialization block
{
id = nextId;
nextId++;
}
public Employee(String n, double s) {
name = n;
salary = s;
}
public Employee(double s) {
this("Employee #" + nextId, s);
}
public Employee(){
salary=10000d;
System.out.println("name="+name+" id="+id+" salary="+salary);
}
public String getName() {
return name;
}
public double getSalary() {
return salary;
}
public int getId() {
return id;
}
}

staff[0]对象初始化

1.先执行静态域

static {
Random generator = new Random();
nextId = generator.nextInt(10000);
}

2.构造器参数拷贝,不执行构造器主体

Employee(String n, double s)//这时 n=Harry s=4000

3.实例域初始化

private String name = "";

4.初始化块

{
id = nextId;
nextId++;
}

5.执行构造器的主体

name = n;
salary = s;

staff[1]对象初始化

1.执行一个参数的构造器

public Employee(double s) {
this("Employee #" + nextId, s);
}

2.执行被调用的构造函数的参数拷贝，但是不执行构造器主体

Employee(String n, double s)//这时 n=Employee #3865 s=6000

3.实例域初始化

private String name = "";

4.初始化块

{
id = nextId;
nextId++;
}

5.执行被调用构造器的主体

name = n;
salary = s;

6.如果构造器有主体，则执行构造器主体

staff[2]对象初始化

1.执行无参构造器，构造器主体还没执行

public Employee()

2.实例域初始化

private String name = "";

3.初始化块

{
id = nextId;
nextId++;
}

4.执行构造器的主体

salary=10000d;
System.out.println("name="+name+" id="+id+" salary="+salary);