Java method parameter-value call, reference call problem

(blog content from the core Java Volume one)

1. XX Call: Method parameters are passed in the programming language:

Reference Call (call by reference) : Indicates that the method received a variable address provided by the caller.

value is called (Call by value) : Indicates that the method received a value supplied by the caller.

named Call (Call by name) : has become history.

2. Java uses value calls, and only values are called. This means that the method obtains a copy of the parameter value, not the parameter value itself, so the method cannot modify any of the parameter variables passed to it.

Look at the following code:

Public class Test {

Public Static void Main (string[] args) {

int percent = 10;

Triplevalue (percent);

System. out. println (percent);

}

Public Static void triplevalue (int x) {

x = x * 3;

}

}

Output: 10

As you can see, regardless of how this method is called, after execution, The value of Persent is still ten. Specific implementation process:

The ①x is initialized to a copy of the Persent (that is, ten). At this point x is x,persent is persent, but the value is the same.

②x is multiplied by 3 to be equal to. But persent is still ten.

③ This method ends, the parameter variable x is no longer used.

3. However, there are two types of method parameters: Basic data Type (numeric, Boolean), object reference.

A method cannot modify a parameter of a base data type. The object reference is different as a parameter. At this point, the method obtains a copy of the object reference. object reference and its copy, referencing an object at the same time.

Look at the following code:

Public class Test {

Public Static void Main (string[] args) {

Circle C = new circle ();

C.R = 1;

bigger (c);

System. out. println (C.R);

}

Public Static void bigger (Circle C2) {

C2.R = c2.r+3;

}

}

class circle{

int R;

}

Output: 4

Specific implementation process:

The ①C2 is initialized to a copy of the C value, which is a reference to an object.

The ②bigger method is applied to this object reference. Therefore, the R value of the Circle object referenced by C2 and C increases by 3.

after the ③ method finishes, the parameter variable c2 is no longer used. C continue referencing this R value has increased by 3 for the circle object.

4. many programming languages (especially C + + and Pascal) provide two ways to pass parameters: value invocation and reference invocation. Some programmers think that the Java programming language uses reference calls to objects, which is actually not true. As this misunderstanding has a certain universality, so here is a counter-example, to elaborate on this issue.

First, write a method that swaps two round objects:

Public Static void swap (Circle x,circle y) {

Circle temp = x;

x = y;

y = temp;

}

If the Java programming language uses reference calls to objects, this method should be able to achieve the effect of exchanging data:

Circle A = new circle (1);

Circle B = new circle (2);

Swap (A, B);

System. out. println (A.R);

System. out. println (B.R);

Output:

1

2

However, the method does not alter the object references stored in variables a and b . The parameters x and y of the swap method are initialized to a copy of two object references, and this method swaps the two copies,a and the b the object reference in does not change. Finally, in vain, the parameter variables x and y are discarded at the end of the method . This process illustratesthatJava does not use reference calls to objects, but rather to value passing.

5. Summary:

(1) A method cannot modify a parameter of a base data type (that is, numeric and Boolean).

(2) A method can manipulate the state of the object it refers to by using the object parameter.

(3) A method cannot have an object argument referencing a new object.

Note:C + + has value calls and reference calls. The reference parameter is marked with a & symbol. For example, it is easy to implement the Void swap (circle& a,circle& b) method and implement the purpose of modifying their reference parameters.

Java method parameter-value call, reference call problem