Java实例域初始化方法及顺序

Java实例域初始化方式

1.构造器

public class Employee {
  private String name;
  private String gender;
  private int age;
  private double salary;

  public Employee() {
    this.salary = 1000.0;
  }

  public Employee(String name, String gender, int age, double salary) {
    this.name = name;
    this.gender = gender;
    this.age = age;
    this.salary = salary;
  }
}

2.域声明

public class Employee {
  private String name;
  private String gender;
  private int age;
  private double salary = 1000.0;

  public Employee() {
  }
  public Employee(String name, String gender, int age, double salary) {
    this.name = name;
    this.gender = gender;
    this.age = age;
    this.salary = salary;
  }
}

3.初始化块（initialization block）

public class Employee {
  private String name;
  private String gender;
  private int age;
  private double salary;
  private static String test;

  //静态初始化块：类第一次被加载时执行
  static {
    System.out.println("类加载");
    test = "hello world!"
  }
  
  //初始化块
  {
    name = "xiao";
    gender = "m";
    age = 20;
    salary = 1000.0;
    System.out.println("初始化");
  }

  @Override
  public String toString() {
    return "Employee{" +
        "name='" + name + '\'' +
        ", gender='" + gender + '\'' +
        ", age=" + age +
        ", salary=" + salary +
        '}';
  }
  public static void main(String[] args) {
    Employee employee = new Employee();
    System.out.println(employee.toString());
  }
}

初始化
Employee{name='xiao', gender='m', age=20, salary=1000.0}
Process finished with exit code 0

Java实例域初始化顺序

调用构造方法的具体处理步骤：

1. 如果类是第一次被使用，先执行静态初始化块

2. 所有数据域被初始化为默认值（0、false或null）

3. 按照在类声明中出现的次序，依次执行所有域初始化语句和初始化块。

4. 如果构造方法调用了其他构造方法，先执行其他方法。

5. 最后，执行构造方法。

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