Let’s analyze whether Java is passed by value or by reference.

This article brings you relevant knowledge about java, which mainly introduces the relevant issues about whether Java is passed by value or passed by reference. Let's take a look at it together. I hope it will be helpful to everyone. helpful.

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Purpose of the article: Verify whether the Java language is pass by value or pass by reference and Java Implementation principle of parameter passing.

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Problem introduction:
Read the code segment first:

public static void main(String[] args){
    Person p=new Person("张三");
    f(p);
    System.out.println("实参:"+p);}public static void f(Person p){
    p.name="李四";
    System.out.println("形参:"+p);}

Running result:
Formal parameters :Person{name="李思"}
Actual parameters:Person{name="李思"}

We pass an object variable to the method, and then modify the object within the method Attributes, print actual parameters and formal parameters, on the surface, the values ​​of the actual parameters and formal parameters have been modified.Does Java also support reference transfer?

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The answer is: Java language only supports value transfer and does not support reference transfer.

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Before verifying the Java parameter transfer method, we First, you must understand the difference between function value passing and reference passing.

• Pass by value means copying a copy of the actual parameters and passing them to the function when calling the function, so that if Modifying parameters will not affect the actual parameters.
• Pass by reference refers to passing the address of the actual parameters directly to the function when calling the function. Then the modification of the parameters in the function will affect the actual parameters.

It can be seen that the difference between value passing and reference passing is whether the modification of the formal parameters will affect the actual parameters

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1. Basic Parameter passing of data type

We pass a basic data type to the method, and then modify the value of the formal parameter in the method, and find that the modification of the formal parameter does not affect the actual parameter.

public static void main(String[] args){
    int a=1;
    f(p);
    System.out.println("实参:"+a);}public static void f(int a){
    a=2;
    System.out.println("形参:"+a);}

2. Parameter passing of reference data types

In the example introduced by the question, it seems that the modification of the formal parameters affects the actual parameters, but as long as we create a new object and assign it to the formal parameters, then Print actual parameters and formal parameters, the two do not affect each other. It means that Java does not conform to reference transfer.

public static void main(String[] args){
    Person p=new Person("张三");
    f(p);
    System.out.println("实参:"+p);}public static void f(Person p){
    p=new Person("李四");
    System.out.println("形参:"+p);}

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3. Principle

We need to understand why this happens Phenomenon, you need to understand the basic principles behind Java:

The JVM is divided into many areas, the objects we create are placed on the heap, and basic data types and local variables are placed on the stack. When passing the basic data type, a copy of the data is created and passed to the method, so the actual parameters will not be affected by the modification of the formal parameters, as shown in the figure:

Since the object is placed In the heap area, so we can only use the reference of the object to operate this object.
When the object reference is passed to the method, a copy of the reference is actually created and points to the same object at the same time. When the object is manipulated through the formal parameter reference , it’s like the actual parameters have changed. In fact, the object content has changed, but the object variables have not changed, and the actual parameters themselves have not changed. Therefore, the example introduced in the question is not difficult to understand.

When we reassign the formal parameters, the actual parameters will not be affected in any way. At this time, the actual parameters and the formal parameters already point to two different objects.

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Therefore, Java only supports value transfer.

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