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Java in how to achieve function transfer mode value transfer, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain in detail for you, people with this need can come to learn, I hope you can gain something.

When a function is called in C++

Value transfer: generally copy a copy of the source data and then re-operate, the variable memory address is different.

Reference transfer: this point is more special to pass on the original variable, just the alias of the variable, internal modification of the function will directly lead to external changes.

Pointer passing: get the pointer to the object outside the function (that is, the address), and then the function will manipulate the value of the object under the address according to the address.

Similar conclusions in Java are as follows (explained in detail):

In Java, when a method is called, if the value passed in is a basic data type (including the String type), the change of the formal parameter does not affect the actual parameter, that is, the value is passed.

In Java, when a method is called, if the value passed in is a non-basic data type such as an object (except the String type), the address of the object is passed, and then the address of the object is copied to the formal parameter, and then the argument will be affected when manipulating the parameter, but keep in mind that the value is also passed but the object address is passed.

1. Pass by value (deep copy of the same value)

Import java.util.Scanner;public class learnJava {public static void main (String [] args) {int a = 12; fun (a); System.out.println ("A value of the object outside the method:" + a);} private static void fun (int a) {a + = 1 System.out.println ("A value of the object inside the function:" + a);}} = A value of the object inside the function: 13 A value of the object outside the method: 12package mytest;public class Client {public static void main (String [] args) {String a = "12"; fun (a) System.out.println ("A value of the object outside the method:" + a);} private static void fun (String a) {/ / a = new String ("34"); a = a + 34; System.out.println ("A value of the object inside the function:" + a) }} / * * String is an object, but it can be thought of as a basic type pass-through is a deep copy that directly creates an object that is like value but has nothing to do with the address. * A value of the internal object of the function: 1234 * the A value of the external object of the method: 12 * /

Note: the String type is also a reference type. Why doesn't the function call change the value of the external prototype?

This is mainly due to the fact that String saves values internally because String is saved through a final char [] array. Since the array value of type final cannot be changed, a reference to the String address is passed to the function's parameters when the function is called externally, and when the function modifies this reference, final char [] is unmodifiable. So when you assign a new value to a variable, you re-create a String instance (the address that the newly created String instance points to and the address pointed to by the external string variable is different, so you can't change the external variable) another point indicates that because the final String object is immutable, it can be shared, which means that String op= "abc" and String op1= "abc" point to the same reference object (the string is constant). Their values cannot be changed after creation-jdk Api)

Package mytest;import java.util.Scanner;public class learnJava {public static void main (String [] args) {AA = new A (5); fun (a); System.out.println ("A value of the external object of the method:" + a.a);} private static void fun (AA) {a.a + = 1 / / a = new A (); even so, the place where fun (a) was originally called will not change. Because it is the address passing under the value passing. It's not reference passing! System.out.println ("A value of the object inside the function:" + a.a);} static class A {public int a; public A (int a) {this.a = a } = = A value of the internal object of the function: 6 A value of the external object of the method: 6 value of the external object of the method: 6 this.id value * * Test parameter value transfer mechanism * @ author ljj * * / public class User4 {int id; / / id String name; / / account name String pwd; / / password public User4 (int id, String name) {this.id = id This.name = name;} public void testParameterTransfer01 (User4 u) {u. Name = "High Primary eight";} public void testParameterTransfer02 (User4 u) {u = new User4 Understand that the operation here will not affect the original layer of data} public static void main (String [] args) {User4 U1 = new User4; u1.testParameterTransfer01 (U1); System.out.println (u1.name); u1.testParameterTransfer02 (U1); System.out.println (u1.name);}}

Key notes on strings:

The string in java exists in the constant pool. The memory of java is divided into heap, stack, method area (including constant pool). The string in java exists in the constant pool. There are mainly class structures and static variables in the method area. The method area also contains a constant pool, which holds string constants.

The concept of String class

The String class is a class for string-related operations.

The class includes member variables and methods.

(1) the String class has a special member variable final char [], which holds the memory address of a string in the constant pool, which can also be understood as a pointer.

(2) the String class has some methods, such as indexOf (), charAt (). The String class has no method to modify the string.

Although the String class does not have a method to modify the string, the member variable that retains the string address can be modified, which means that the object of the String class can point to another string.

Analysis of two instantiation methods of String

String str= "abc" creation method

The process of creating an object

1 first look for the existence of a "abc" string object in the constant pool

2 if it does not exist, create "abc" in the constant pool and have str reference the object

3 if it exists, let str refer to the object directly

String str= new String ("abc") creation method

The process of creating an object

1 first create a specified object in the heap (not a constant pool) and point the str reference to the object.

2 check in the string constant pool to see if there is a string object with the content "abc"

3 if it exists, associate the string object from new with the object in the string constant pool (that is, let the pointer of that special member variable value point to it)

4 if it does not exist, create a string object with the content "abc" in the string constant pool and associate the objects in the heap with it. (it is possible that the "abc" in the constant pool has been recycled, so first create a string object with the content "abc".)

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