Java- 构造方法的执行顺序

构造方法的执行过程

• 构造方法的执行过程遵循以下步骤：
  
  1. 调用本类或父类的构造方法，直至最高一层(Object)
  2. 按照声明顺序执行字段的初始化赋值
  3. 执行构造函数的各语句
• 简单的说：
  
  • 先父类构造，再本类成员赋值，最后执行构造方法中的语句。

class ConstructSequence {    public static void main(String[] args){         Person p = new Student("LiMing", 18, "PKU");    }}class Person{    String name="unnamed";  //step 2    int age=-1;    Person( String name, int age ){        super(); //step 1        //step 3        System.out.println( "start to create the constructor Person(),and now this.name="+this.name+",this.age="+this.age );        this.name=name; this.age=age;         System.out.println( "Person() constructed completely,and now this.name="+this.name+",this.age="+this.age );    }}class Student extends Person{    String school="unnamed"; //step2     Student( String name, int age, String school ){        super( name, age );  //step 1        //step 3        System.out.println( "start to create the constructor Student(),and now this.name="+this.name+",this.age="+this.age+",this.school="+this.school );        this.school = school;        System.out.println( "Student() constructed completely,and now this.name="+this.name+",this.age="+this.age+",this.school="+this.school );    }}-----------OUTPUT-----------  start to create the constructor Person(),and now this.name=unnamed,this.age=-1Person() constructed completely,and now this.name=LiMing,this.age=18start to create the constructor Student(),and now this.name=LiMing,this.age=18,this.school=unnamedStudent() constructed completely,and now this.name=LiMing,this.age=18,this.school=PKU

构造方法内部调用虚方法法

• 即调用的不是static，final，private方法。从语法上来说是合法的，但是有时会造成事实上的不合理
  
  • 父类调用该虚方法时，这个虚方法会跑到子类中去

class ConstructorInvokeVirtual2{    public static void main(String[] args) {        Person p = new Student("LiMing", 16, "PKU");    }}class Person{    String name = "unnamed";    int age = -1;    Person(String name, int age){        this.name = name;        this.age = age;        sayHello();    }    void sayHello(){        System.out.println("A Person, name: " + name + ", age: "+ age);    }}class Student extends Person{    String school = "unnamed";    Student(String name, int age, String school){        super(name, age);        this.school = school;    }    void sayHello(){        System.out.println("A Student, name:" + name + ", age: "+ age + ", school: " + school);    }}-----------OUTPUT-----------  A Student, name:LiMing, age: 16, school: null

• 在本例中，在构造函数中调用了一个动态绑定的方法sayHello()，这次，会使用该方法被覆盖的定义，即使用子类中的该方法，而这时对象尚未完全构建好，所以school还没有赋值。
• 因此，可能的话，在构建函数中避免调用任何方法，用尽可能简单的方法使对象进入就绪状态。
• 唯一能够安全调用的是具有final属性的方法。