Java 初始化字段方式和顺序：
- 类加载时直接初始化静态字段;
- 类加载时调用静态方法初始化静态字段;
- 实例化对象时,在调用构造函数之前代码块中初始化字段;
- 实例化对象时,在调用构造函数之时初始化字段;
初始化字段顺序1->2->3->4
代码如下：
public class Employee {
//实例化对象时,在调用构造函数之前前初始化字段;
private int id; // 实例化对象时初始化为0
private String name; // 实例化对象时初始化为null
private boolean flag; // 实例化对象时初始化为false
private static int age = 22; // 加载类时初始化为22
// 在构造函数之前初始化
{
int num = 1111; // 实例化对象时初始化为1111
String name = "QA";// 实例化对象时初始化为QA
setId(num); // 实例化对象时调用
SetName(name); // 实例化对象时调用
System.out.println("call instance method");
}
//类加载时调用静态方法初始化静态字段;
static {
System.out.println("age is: " + age);
age = 30; // 加载类时初始化为33
print(); // 加载类时时调用
}
public void SetName(String name) {
this.name = name;
}
public int getId() {
return id;
}
public String getName() {
return name;
}
public static int getAge() {
return age;
}
public void setId(int num) {
id = num;
}
public static void print() {
System.out.println("call static method");
}
public Employee() {
// TODO Auto-generated constructor stub
}
public Employee(int id, String name) {
// TODO Auto-generated constructor stub
this.id = id;
this.name = name;
System.out.println("call constructor method");
}
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("---------------------------");
System.out.println("age is: " + Employee.getAge());
System.out.println("---------------------------");
Employee e = new Employee();
System.out.println("id is: " + e.getId());
System.out.println("name is: " + e.getName());
System.out.println("---------------------------");
Employee e2 = new Employee(2222, "Dev");
System.out.println("id is: " + e2.getId());
System.out.println("name is: " + e2.getName());
}
}
运行结果：
age is: 22
call static method
---------------------------
age is: 30
---------------------------
call instance method
id is: 1111
name is: QA
---------------------------
call instance method
call constructor method
id is: 2222
name is: Dev
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