Java (static) variables and (static) code block execution sequence, java Variables

Java (static) variables and (static) code block execution sequence, java Variables
This article discusses the execution sequence of (static) variables and (static) code blocks in Java. First, three classes are created: 1. Foo class for printing variables.

public class Foo {    public Foo(String word) {        System.out.println(word);    }}

2. Parent class

public class Parent {    static Foo FOO = new Foo("Parent's static parameter");        Foo foo = new Foo("Parent's parameter");        static {        System.out.println("Parent's static code block");    }        {        System.out.println("Parent's code block");    }        public Parent() {        System.out.println("Parent.Parent()");    }}

3. Child class

public class Child extends Parent{    static Foo FOO = new Foo("Child's static parameter");        Foo foo = new Foo("Child's parameter");        static {        System.out.println("Child's static code block");    }        {        System.out.println("Child's code block");    }        public Child() {        System.out.println("Child.Child()");    }}

Next, run the instance:

public class Run {    public static void main(String[] args) {        new Child();    }}

Print result:

Parent's static parameterParent's static code blockChild's static parameterChild's static code blockParent's parameterParent's code blockParent.Parent()Child's parameterChild's code blockChild.Child()

Summary

Static variables and static code blocks in Java are executed during class loading. when instantiating an object, declare and instantiate the variable before executing the constructor. If the subclass inherits the parent class, the static variables and static code blocks of the parent class are executed first, and then the static variables and static code blocks of the subclass are executed. Similarly, the non-static code blocks and constructors of the parent class and subclass are executed.

Note: (static) variables and (static) code blocks also have execution sequence, consistent with the Code Writing sequence. (Static) variables can be used in (static) code blocks, but the used (static) variables must be declared before (static) code blocks.

Finally, the following steps are provided:

1. Parent static variables and static code blocks (first declared and executed );

2. Subclass static variables and static code blocks (first declared and executed );

3. Variables and code blocks of the parent class (first declared and executed );

4. constructor of the parent class;

5. Subclass variables and code blocks (first declared and executed );

6. constructor of the subclass.