java 初始化块

class Bike8{
    int speed;

    Bike8(){System.out.println("constructor is invoked");}

    {System.out.println("instance initializer block invoked");}

    public static void main(String args[]){
    Bike8 b1=new Bike8();
    Bike8 b2=new Bike8();
    }
}

In the above example, it seems that instance initializer block is firstly invoked but NO. Instance intializer block is invoked at the time of object creation. The java compiler copies the instance initializer block in the constructor after the first statement super(). So firstly, constructor is invoked. Let's understand it by the figure given below:

Rules for instance initializer block :

There are mainly three rules for the instance initializer block. They are as follows:

1. The instance initializer block is created when instance of the class is created.
2. The instance initializer block is invoked after the parent class constructor is invoked (i.e. after super() constructor call).
3. The instance initializer block comes in the order in which they appear.

class A {
A() {
System.out.println("parent class constructor invoked");
}
}

class Test extends A {
Test() {
super();
System.out.println("child class constructor invoked");
}

{
System.out.println("instance initializer block is invoked");
}

public static void main(String args[]) {
Test b = new Test();
}
}

输出：

parent class constructor invoked
instance initializer block is invoked
child class constructor invoked

class A {
A() {
System.out.println("parent class constructor invoked");
}
}

class Test extends A {
Test() {
super();
System.out.println("child class constructor invoked");
}

Test(int a) {
super();
System.out.println("child class constructor invoked " + a);
}

{
System.out.println("instance initializer block is invoked");
}

public static void main(String args[]) {
Test b1 = new Test();
Test b2 = new Test(10);
}
}

输出：

parent class constructor invoked
instance initializer block is invoked
child class constructor invoked
parent class constructor invoked
instance initializer block is invoked
child class constructor invoked 10