023.static and final use traps

Lesson 23

1. static Modification methods ：static The decorated method is called static method . For static methods , You can use the class name . Method name to access .
   normal

package exercise;
public class exercise006 {
    
    public static void main (String[] args){
    
        MyStatic2 test =new MyStatic2();
        test.out();
    }
}
class MyStatic2{
    
    public void out(){
    
        System.out.println("out");
    }
}

Revised

package exercise;
public class exercise006 {
    
    public static void main (String[] args){
    
        MyStatic2.out();
    }
}
class MyStatic2{
    
    public static void out(){
    
        System.out.println("out");
    }
}

contrast

package exercise;
public class exercise007 {
    
    public static void main (String[] args){
    
        N n = new N();
        n.output();
    }
}
class M{
    
    public static void output(){
    
        System.out.println("m");
    }
}
class N extends M{
    
    public static void output(){
    
        System.out.println("N");
    }
}

package exercise;
public class exercise007 {
    
    public static void main (String[] args){
    
        M m = new N();
        m.output();
    }
}
class M{
    
    public static void output(){
    
        System.out.println("m");
    }
}
class N extends M{
    
    public static void output(){
    
        System.out.println("N");
    }
}

2. Static methods can only inherit , Can not rewrite （Override）.
3. final keyword ：final You can modify properties 、 Method 、 class .
4. final decorator ： When a class is final When it is modified , Indicates that the class is a terminal class , That is, it cannot be inherited .

package exercise;
public class exercise008 {
    
    public static void main(String[] args){
    
        F f = new F();
    }
}
final class E{
    // Indicates that the class cannot be inherited 
}
class F extends E{
    
}

5. final Modification methods ： When a method is final When it is modified , Indicates that the method is a final state method , That is, it cannot be rewritten （Override） .
The wrong sample

package exercise;
public class exercise008 {
    
    public static void main(String[] args){
    
        H h =new H();
        h.out();
    }
}
class G{
    
    public final void out(){
    
        System.out.println("G");
    }
}
class H extends G{
     
    public void out(){
    
        System.out.println("H");
    }
}

6. final Modify properties ： When an attribute is final When it is modified , Indicates that the property cannot be overridden .
The wrong sample

package exercise;
public class exercise008 {
    
    public static void main(String[] args){
    
        People people = new People();
        people.age = 20;
    }
}
class People{
    
    final int age =10;
}

package exercise;
public class exercise008 {
    
    public static void main(String[] args){
    
        People people = new People();
        people.address.name = "shanghai";
    }
}
class People{
    
    final Address address = new Address();
}
class Address{
    
    String name = "dalian";
}

It can be executed

7. When final When decorating a native data type , The value representing the native data type cannot be changed （ For example, you can't 10 become 20）; If final When modifying a reference type , Indicates that the reference type cannot point to other objects ; But the content of the object that the reference refers to can change .