It is said that more than half of Java programmers will make mistakes

After a period of more systematic self-study in Java, it is particularly interesting to see a program that claims more than half of Java programmers will make mistakes. And he went into it.

The program code is as follows:

1. Package Com.longpo;

• Class

person {

• Private

static person person = new person ();

• Public

static int count1;

• Public

static int count2 = 5;

• Private

Person () {

• count1++;
• count2++;
• }
• Public

Static Person getinstance ()

• {
• Return

Person

• }
• }
• Public

Class Testsingleton {

• Public

static void Main (string[] args) {

• Person Person=person.getinstance ();
• You can use direct person.count1

• System.out.println ("Count1:"

+PERSON.COUNT1);

• System.out.println ("Count2:"

+PERSON.COUNT2);

• }
• }

What is the result of the above code output?

It is easy to think that the output 1 and 6 (from the topic is certainly not so simple), then I guess the answer should be 1 and 5, but can not say why. I assign the code to run under Eclipse. Get

As I guessed, but I have no idea why. So began the Google knowledge of the road, after research, ClassLoader gradually reflected into my eyes. Let me say what I think is the reason that there are errors also hope to guide corrections

。

The class performs three steps before executing:

1. Class loading: Find and load the class's binary data, load the corresponding class file into memory

2. Connect

2.1. Verify: Ensure that the class being loaded is correct (primarily to prevent the disgusting class file from being loaded)

2.2. Prepare: Allocate memory for static variables of the class and initialize them to default values

2.3. Parsing: Converting a conforming reference in a class to a direct reference

3. Initialize: Assigns the correct initial value to the static variable of the class

found that steps 2.2 and 3 refer to the keyword static variables, focusing on these two steps, and 2.2 result will result in a static variable

Person,count1,count2 allocating memory and assigning values (default value)

Person=null;

Count1=count2=0

At step 3, the initialization will give the correct value to the static variable, then when will it be initialized? Then

Google Baidu, get:

all Java Virtual machine implementations must be "first actively used by Java programs in each class or interface"

"is initialized only when

Java's use of classes is divided into: active use, passive use

Active use of six kinds: (except for these 6 kinds, the other is passive use)

1. To create an instance of a class

2. Access static variables for a class or interface or assign a value to the static variable

3. To invoke a static method of a class

4. Reflection

5. Initializes subclasses of a class

6. Classes that are labeled to start classes when the Java Virtual machine is started

Understand the third step (initialization), then you can explain why the program output 1 and 5, I drew a diagram

Now you should understand the reason for it, in order to test whether it is really clear, you can change the code to

1. Package

Com.longpo;

• Class

person {

• Public

static int count1;

• Public

static int count2 = 5;

• Private

static person person = new person ();

• Private

Person () {

• count1++;
• count2++;
• }
• Public

Static Person getinstance ()

• {
• Return

Person

• }
• }
• Public

Class Testsingleton {

• Public

static void Main (string[] args) {

• Person Person=person.getinstance ();
• You can use direct person.count1

• System.out.println ("Count1:"

+PERSON.COUNT1);

• System.out.println ("Count2:"

+PERSON.COUNT2);

• }
• }  ?

Original from Techfox Technology Forum Java Community http://techfoxbbs.com/blog-1-3.html

It is said that more than half of Java programmers will make mistakes