For example, the execution order of code blocks in Java

Foreword
Today, when watching Android ContentProvider implementation, I suddenly thought of the execution order of static domains, static blocks, non -static blocks, non -static blocks, and constructive functions during the new process of the Java class. In fact, this is a very classic issue, which is very examined to grasp the basic knowledge of Java. I believe there are also such problems during the interview process, and take advantage of the time to review it on the weekend.

in conclusion
Here is the sorting conclusion to everyone, and then I write a program to verify our conclusions. During the Java class being NEW, the execution order is as follows:

• Realize your static attributes and static code blocks. (Decide from the order of the code to determine who executes first)
• Realize your own non -static attributes and non -static code blocks.
• Execute your own constructor.

In the process of implementing the inheritance class, the initialization execution order is as follows:

• Realize the public static attributes and static block code of the parent class.
• Realize your static attributes and static blocks.
• Realize the non -static attributes and non -static code blocks of the parent class.
• Execute the constructor of the parent class.
• Realize your own non -static attributes and non -static code blocks.
• Execute your own constructor.

Here you need to introduce static code blocks and non -static code blocks.
1. Static code block:

static {
}

2. Non -static code block

{{
}

The similarities and differences between static code blocks and non -static code blocks are as follows:

• The same point: When the JVM is loaded and executed before the constructor is executed, multiple can be defined in the class. Generally, some static variables are assigned in the code block.
• Different points: Static code blocks are executed before non -static code blocks (static code blocks> non -static code blocks). Static code blocks are only executed once at the first NEW, and then no longer execute. The non -static code block is executed once every time.

The best verification of the verification of the conclusion is to write the code to prove the results. First, let's take a look at the order of execution when the inheritance class initializes. The code is as follows:

 Public Class Initodertest {Public Static Stration Static_field = "Static Properties"; // Static block static {System.out.println (Static_field); System.out.println ("Static State Code Block ");} Public String Field =" Non -Static Properties "; // Non -static blocks {system.out.println (field); system.out.println (" non -static code block ");} public initodertest () {system.out.println );} Public Static void main (string [] args) {initodrtest test = new initodertest ();}}

Execution results:

• Static attribute
• Static code block
• Non -static attribute
• Non -static code block
• Non -parameter constructor

Next, let's verify that when the Java class is inherited, the execution order is in line with our conclusions. The test code is as follows:

 Class Parenttest {Public Static String Parent_Static_field = "Parent-Static Properties"; // Parent-Static block static {System.out.println (Parent_Static_field); System.out. Println ("Family-Static Code Block"); } PUBLIC Static String ParentField = "Family Class-Non-Static Attributes"; // Family-Non-Static Block {System.out.println (Parentfield); System.out.println ("Family-Non-Static Code Block"); } PUBLIC PAREATTTEST () {System.out.println ("Family -PuBlic Class Initodertest EXTENDS PAREATTTEST {Public Stration Static_field =" Static attribute "; // Static block static {system.out .println (static_field); System.out.println ("Static code block");} Public String Field = "Non -Static Properties"; // Non -Static Block {System.out.println (Field); System.out.pr. intln ("Non -Static Code Block");} Public Initodertest () {System.out.println ("Portomless Constructor"); Test ();} }

The execution results are as follows:

• Father-static attribute
• Father Category-Static code block
• Static attribute
• Static code block
• Father-non-static attribute
• Father-non-static code block
• Father -non -parameter structure function
• Non -static attribute
• Non -static code block
• Non -parameter constructor