Java only has value passing! Why?

Java technology stack 2020-11-08 16:12:36
java value passing


From the public number : Program control

A basic question for an interviewer to ask :Java Value passing or reference passing ?

Everyone must have their own views on this issue , So what the truth is , How can we answer the interviewer's question ? Today, let's analyze a wave of

Value passed ? reference ?

First , We need to know what Value passed , What do you mean reference , Know this to understand Java How to do it . To understand these two kinds of transmission, we need to understand the concept of formal parameter and actual parameter

Formal parameters : The parameters used when defining a function , Used to receive function input parameters , For example, let's write a function , The parameters in the function are formal parameters

(String str) { System..println(str)}

The actual parameter : When we call a function , The arguments in parentheses after the function name are called actual arguments , As shown in the following example

(String[] args) {
A a = A()a.test()}

You can find , When you call a function with parameters , The actual parameters are passed to the formal parameters ; So there are two situations in the process of transmission , namely Value passing and reference passing

Value passing is to give you the value of the parameter , When the function is called, a copy of the actual parameter is passed into the function , In this way, the modification of parameters inside the function will not affect the actual parameters ; And reference passing is different , It directly gives the actual address of the parameter to the calling function , The content of the address can be modified directly inside the function , Will affect the actual parameters

Let me give you an example , We have a database A, Only internal personnel are allowed to operate , Now there's a project to work with other companies , The data of this database needs to be handed over to the partner company , I can't put my database directly A Give them the address , Let them connect directly to our database A Well , If they delete the library and run away , Then I boss Don't you want to sacrifice me to heaven

So at this point , Copy our database table data to a new database B, Partners can look at this database B data , They can also operate at will , It will not affect our database

This operation is similar to value passing , If the cooperative company directly operates our database , It's like reference passing , Now we should know the difference between the two

Java Value passing or reference passing ?

We learned about the concepts of value passing and reference passing , that Java What kind of transmission is it in China ? Let's look at code analysis

(String[] args) {
Fans fans = Fans()t = fans.test(t)System..println(+ t) }
(t) {
t = System..println(+ t) }
// Output 
In testIn main

The above code , stay main In the definition of t The value of is 1, In function test Parameters have been modified in t The value of is 2, Is this result easy to analyze ?test The function does not change the incoming t Value , Then according to our introduction above, can we draw a conclusion :Java Value passing in

Some people may question , The parameters you pass in t Is the basic type , If you pass in a reference object type , That would change the content of the object ,OK, Let's look at a reference type code

= String = (Fans fans) {
fans.= fans.= System..println(+ fans.++fans.)}
(String[] args) {
Fans fa = Fans()fa.test(fa)System..println(+ fans.getA()++fans.getB())}
// Output 
In testABnewFans
In mainABnewFans

Ah , The output turns out to be the same , That is, the incoming fans Objects are functions test Revised , So is it a reference passing again ?

So I came to the conclusion that , The basic type is value passing , Reference type is reference passing ? Is this the case , Let's go through String In this reference type experiment

(String t) {
t = System..println(+ t)}
(String[] args) {
Fans fans = Fans()String tt = fans.test(tt)System..println(+ t)}
// Output 
In test: Official account 
In main:Java Technology stack 

ah , this , What a fat thing , The parameter value passed has not been modified , How did it become value passing again

The above three examples show different results , What is the conclusion ? Let's analyze

The first is the basic type , The basic type points to values , Passing is similar to an assignment operation , It will not affect the original value , It's like a=10,b=a,b=20 such , It doesn't make a Turn into 20;

The second reference object passes in a reference type fans The value of the address of , The original parameter passed in fa Pointing to the address 0x123456, So the function test Parameters of fans Point to 0x123456, Function internal reference fans Make changes , So it changed 0x123456 The value of the address , Causing external changes

The third reference object is String type , Again, the original parameters are passed in tt To the address 0x123456, function test Parameters t It's also a direction 0x123456 Value , Why is this different from the second result ? The point is t=" Official account "; This sentence was intended to try to make the content "Java Technology stack " Change to " Official account ", But helpless ,String The type is static final Type of , You should know that No , I don't know. I should go to make-up class to read String The source of the , So it became t=new String(" Official account "),t It points to another address , The content of this address is " Official account ", So the original quote tt Still pointing to the original address 0x123456, It hasn't changed

Some students may ask questions , Why the second one can change the content of this address , The third one can't ? Obviously ,String yes final Of , Do not modify the , The second one can modify the content of the address directly ; That's the question again , In this case , Can we call it value passing ?

Tell you , It's value passing , Because our second verification method is wrong , If you add... To the first line of the function fans = new Fans(); What else does it output , This becomes the third one String Similar to that , Changed the pointing position of function parameter , The output outside the function is different from the output inside the function , The inside of the function doesn't affect the outside ; If you pass... By reference , Even with this sentence , It should also be that the output is the same inside and outside the function , Besides, , This also goes against the concept of reference passing that changes the parameters passed in

reflection

Value passing and reference passing are not distinguished by what they are passing , What is passed is reference, not necessarily reference passing , Distinguish according to the result of definition ;

stay Java What we use Value passed ( Remember , Don't answer wrong in the next interview )

In other methods, changing the value of a reference type is done by reference , When passing a reference object , The object handle of the copied reference is passed , It was copied , It's in memory Copied a handle , These two handles point to the same object , So if you change the data of the space corresponding to this handle, it will affect the external variables

Although it's copied , But it points to the same address , When you point this handle to a reference to another object, it doesn't change the original value ( Example 3 String), Because you're using a copied handle

Finally nagging

A gift of rose , Fragrance in hand

If this article helps you , Explain that my next article will also help you , Keep an eye on it , mua

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