What are the details of using Java code blocks?

1. Basic introduction

The code block is also called the initialization block, which is a member of the class (part of the class). It is similar to a method. The logical statement is encapsulated in the method body and hugged with {} ;

But unlike methods, there is no method name, no return, no parameters, only the method body, and there is no need to explicitly call it through an object or class.

Basic syntax

(modifier)(optional){code};

Note:

1. Modifiers are optional. If you want to write, you can only write static

2. Code blocks can be divided into two categories. Those modified with static are called static code blocks, and those without static modification are called ordinary code blocks.

3.; Optional

Benefits

1. Equivalent to another form of constructor, which can perform initialization operations

2 .If there are repeated statements in multiple constructors, they can be extracted into code blocks.

Quick Start

package com.demo.codeblock_;
public class codeblock01 {
    public static void main(String[] args) {
        movie m01=new movie("环太平洋");
        movie m02=new movie("荒野大飞",66);
        movie m03=new movie("无暇赴死",55,"老K");
    }
}
class movie{
    private String name;
    private double price;
    private String director;
    {
        System.out.println("电影屏幕打开。。。");
        System.out.println("广告开始。。。");
        System.out.println("电影开始。。。");
    }
    //三个构造器重载
    public movie(String name) {
//        System.out.println("电影屏幕打开。。。");
//        System.out.println("广告开始。。。");
//        System.out.println("电影开始。。。");
        System.out.println("构造器movie(String name)被调用。。。");
        this.name = name;
    }
    public movie(String name, double price) {
        //        System.out.println("电影屏幕打开。。。");
//        System.out.println("广告开始。。。");
//        System.out.println("电影开始。。。");
        System.out.println("构造器movie(String name， double price)被调用。。。");
        this.name = name;
        this.price = price;
    }
    public movie(String name, double price, String director) {
        //        System.out.println("电影屏幕打开。。。");
//        System.out.println("广告开始。。。");
//        System.out.println("电影开始。。。");
        System.out.println("构造器movie(String name, double price, String director)被调用。。。");
        this.name = name;
        this.price = price;
        this.director = director;
    }
}

2. Code block details

Code block usage precautions and detailed discussion

1 ) static code block is also called static code block. Its function is to initialize the class, and it is executed as the class is loaded, and will only be executed once. If it is an ordinary code block, it will be executed every time an object is created.

2) When is the class loaded [Important!]

①When creating an object instance (new)

②When creating a subclass object instance, the parent class will also be loaded

③When using static members of the class (static properties, static methods)

Case demonstration: Class A extends the static block of class B

3) Ordinary code block, It is called implicitly when creating an object instance. Once created, it will be called once. If you only use static members of the class, the ordinary code block will not be executed.

package com.demo.codeblock_;
public class codeblock02 {
    public static void main(String[] args) {
        //类被加载的情况举例
        //1.创建对象时new
        //AA aa=new AA();
        //2.创建子类对象实例，父类也会被加载，而且，父类先被加载，子类后被加载
        AA aa01=new AA();
        //3.使用类的静态成员时
        System.out.println(cat.n);
        DD d1=new DD();
        DD d2=new DD();
    }
}
class DD{
    static {
        System.out.println("DD的静态代码被执行1次");
    }
}
class animal{
    static {
        System.out.println("animal的静态代码被执行");
    }
}
class cat extends animal{
    public static int n=888;
    //静态代码块
    static {
        System.out.println("cat的静态代码块被执行");
    }
}
class BB {
    static {
        System.out.println("BB的静态代码被执行");
    }
}
class AA extends BB{
    static {
        System.out.println("AA的静态代码被执行");
    }
}

Class calling sequence

When creating an object, in a class calling sequence: (Key points, difficulties)

①Call Static code blocks and static property initialization (Note: Static code blocks and static property initialization calls have the same priority. If there are multiple static code blocks and multiple static variable initializations, they will be called in the order they are defined)

②Call the initialization of ordinary code blocks and ordinary attributes (Note: The priority of ordinary code blocks and ordinary attribute initialization calls is the same. If there are multiple ordinary code blocks and multiple non-ordinary attribute initializations, they will be called in the order defined)

③Call the constructor method.

Example

package com.demo.codeblock_;
public class codeblock03 {
    public static void main(String[] args) {
        A a=new A();
    }
}
class A{
    public A(){
        System.out.println("A的无参构造被调用");
    }
    int n2=getn2();
    {//普通代码块
        System.out.println("A的普通代码块被调用");
    }
    int getn2(){
        System.out.println("getn2被调用");
        return 99;
    }
    private static int n=getn();
    static {
        System.out.println("A的静态代码被调用");
    }
    public static int getn(){
        System.out.println("getn被调用");
        return 100;
    }
}

Code block details 2

The front of the constructor actually implies super(and calls ordinary code blocks, Write a new class to demonstrate statically related code blocks and attribute initialization. When the class is loaded, it will be executed

, so it will be executed before the constructor and ordinary code blocks

class A {
public AO{
super0:
//调用普通代码块
_System.out.println("ok");
}
}

Example

package com.demo.codeblock_;
public class codeblock04 {
    public static void main(String[] args) {
        B b=new B();
    }
}
class AA{
    {
        System.out.println("AA的普通代码块");
    }
    public AA(){
        //1.super()
        //2.调用本类的普通代码块
        System.out.println("AA的构造器被调用");
    }
}
class B extends AA{
    {
        System.out.println("B的普通代码块");
    }
    public B(){
        //1.super()
        //2.调用本类的普通代码块
        System.out.println("B的构造器被调用");
    }
}

Code block details 2

Let’s take a look at when creating a subclass object (inheritance relationship), their static code blocks, static property initialization, ordinary Code block, ordinary attribute initialization, and constructor calling sequence are as follows:

1. The static code block of the parent class and the static properties (the priority is the same, hot-line in the order of definition

2. Child Static code blocks and static attributes of the class (same priority, executed in the order of definition)

3. Ordinary code blocks and ordinary attribute initialization of the parent class (same priority, executed in the order of definition)

4. Construction method of parent class

5. Ordinary code block and ordinary attribute initialization of subclass (same priority, executed in the order of definition)

6.Construction method of subclass

7. Static code blocks can only directly call static members (static properties and static methods), and ordinary code blocks can call any members.

Example

package com.demo.codeblock_;
public class codeblock05 {
    public static void main(String[] args) {
        //老师说明
        //(1) 进行类的加载
        //1.1 先加载 父类 A02 1.2 再加载 B02
        //(2) 创建对象
        //2.1 从子类的构造器开始
        //new B02();//对象
        new C02();
    }
}
class A02 { //父类
    private static int n1 = getVal01();
    static {
        System.out.println("A02的一个静态代码块..");//(2)
    }
    {
        System.out.println("A02的第一个普通代码块..");//(5)
    }
    public int n3 = getVal02();//普通属性的初始化
    public static int getVal01() {
        System.out.println("getVal01");//(1)
        return 10;
    }
    public int getVal02() {
        System.out.println("getVal02");//(6)
        return 10;
    }
    public A02() {//构造器
        //隐藏
        //super()
        //普通代码和普通属性的初始化......
        System.out.println("A02的构造器");//(7)
    }
}
class C02 {
    private int n1 = 100;
    private static  int n2 = 200;
    private void m1() {
    }
    private static void m2() {
    }
    static {
        //静态代码块，只能调用静态成员
        //System.out.println(n1);错误
        System.out.println(n2);//ok
        //m1();//错误
        m2();
    }
    {
        //普通代码块，可以使用任意成员
        System.out.println(n1);
        System.out.println(n2);//ok
        m1();
        m2();
    }
}
class B02 extends A02 { //
    private static int n3 = getVal03();
    static {
        System.out.println("B02的一个静态代码块..");//(4)
    }
    public int n5 = getVal04();
    {
        System.out.println("B02的第一个普通代码块..");//(9)
    }
    public static int getVal03() {
        System.out.println("getVal03");//(3)
        return 10;
    }
    public int getVal04() {
        System.out.println("getVal04");//(8)
        return 10;
    }
    //一定要慢慢的去品..
    public B02() {//构造器
        //隐藏了
        //super()
        //普通代码块和普通属性的初始化...
        System.out.println("B02的构造器");//(10)
        // TODO Auto-generated constructor stub
    }
}

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