024.static and final use traps continued

Lesson 24

1. about final Type member variable , Generally speaking, there are two ways to assign initial values ：
   1. In a statement final The initial value is assigned when the member variable of type .

package exercise;
public class exercise009 {
    
    final int a = 0;
}

2.  In a statement final Member variables of type are not assigned initial values , But in all construction methods of a class, it is assigned an initial value .

package exercise;
public class exercise009 {
    
    final int a;
    public exercise009(){
    
        a = 0;
    }
    public exercise009(int a){
    
        this.a = a;
    }
}

2. static Code block ： Static code block . The function of static code block is to complete some initialization . First execute the static code block , Then execute the constructor . Static code blocks are executed when the class is loaded , The construction method is executed when generating objects ; To call a class to generate an object , First, you need to load the class into Java Virtual machine （JVM）, Then from jvm Load this class to generate objects .

package exercise;
public class exercise009 {
    
    public static void main (String[] args){
    
        P p = new P();
        P p2 = new P();
    }
}
class P{
    
    static {
    
        System.out.println("static block");
    }
    public P(){
    
        System.out.println("P out");
    }
}

package exercise;
public class exercise009 {
    
    public static void main (String[] args){
    
        new S();
    }
}
class P{
    
    static {
    
        System.out.println("P static block");
    }
    public P(){
    
        System.out.println("P out");
    }
}
class Q extends P{
    
    static {
    
        System.out.println("Q static");
    }
    public Q(){
    
        System.out.println("Q out");
    }
}
class S extends Q{
    
    static {
    
        System.out.println("S static");
    }
    public S(){
    
        System.out.println("S out");
    }
}

3. The static code block of the class will be executed only once , It is executed when the class is loaded , Because each class will only be loaded once , All static code blocks will be executed only once ; The construction method is not , Every time an object is generated, the constructor of the class will be called , therefore new The constructor will be called once .
4. If there is an existing construction method in the inheritance system , There are also static code blocks , Then first execute the static code block of the topmost class , Execute all the way to the static code block of the lowest class . Be careful ： Static code blocks are executed only once .
5. Non static member variables cannot be accessed in static methods ; You can access static member variables in static methods . Static member variables can be accessed in non static methods .

package exercise;
public class exercise010 {
    
    public static void main (String[] args){
    
        W.change();
    }
}
class W {
    
    static int a = 10;
    public static void change(){
    
        a++;
    }
}

6. summary ： Static can only access static ; Non static access to everything
7. Cannot be used in static methods this keyword .