Analysis of static keyword, parent class and subclass loading and execution order in java

Many beginners may not be able to figure out the usage of the static method. Here I will explain my understanding:

• The invocation of the static method does not require the creation of class objects

• Since there is no need to create an object, the this keyword is not used in the static method

• Only static member variables and static methods can be called in the static method. Because ordinary methods need to be called by creating objects, this conflicts with static methods

• Static member variables and static methods can be called in ordinary methods, which can be directly passed in the form of class name.static method Call the

• static code block, which is executed when the class is loaded and only executed once

Now that the theory is over, let’s look at a practical example. ：

class Person{
    static{
        System.out.println("person static");
    }
    public Person(String str) {
        System.out.println("person "+str);
    }
}
public class Test {
	Person person = new Person("Test");
	
	public Test() {
		System.out.println("test constructor");
	}
	static{
        System.out.println("test static 1");
    }
	public static void main(String[] args) {
		new MyClass();
	}
	static{
		System.out.println("test static 2");
	}
}
 
 
class MyClass extends Test {
    Person person = new Person("MyClass");
    static{
        System.out.println("myclass static");
    }
     
    public MyClass() {
        System.out.println("myclass constructor");
    }
}

First guess his output, and then compare it to see what is wrong and deepen your understanding

test static 1
test static 2
myclass static
person static
person Test
test constructor
person MyClass
myclass constructor

1. First load the Test class, which contains two static codes block, then output test static 1, test static 2

2. in the main method, a MyClass is new. Load the MyClass class. If there is also a static code block in the MyClass class, the output will be myclass static

3. MyClass class inherits from the Test class. If the Test class has been loaded, it will not Then output the content in the static code block

4. After loading, start executing the Test class. When executing Person person = new Person("Test"), the Person class has not been loaded yet, and it is found during loading. The Person class also has a static code block, and the output is person static

5. . The constructor of Person is executed and the output is person Test

6. Continue to execute Test, enter the main method, new MyClass(), however MyClass inherits from the Test class, first execute the Test class constructor, and output test constructor

7. Continue to execute MyClass, Person person = new Person("MyClass"), execute the Person class construction method, and output person MyClass

8. Continue to execute MyClass Constructor method, outputmyclass constructor

9. Execution completed

Based on these, I summarized the following execution sequence:

Static code block——>Parent class construction method——>Subclass construction method

When the construction method and the new object exist at the same time, the construction method of the new object is executed first. (At this time, be careful not to generate nested loops, causing memory overflow)

Related articles:

A brief discussion on the loading order of parent classes and subclasses in Java

Java inheritance, whether the subclass inherits the constructor of the parent class

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