Difference: 1. Value transfer creates a copy, while reference transfer does not create a copy; 2. The original object cannot be changed in the function during value transfer, but the original object can be changed in the function during reference transfer. Passing by value means that a copy of the actual parameters is passed to the function when calling the function, so that if the parameters are modified in the function, the actual parameters will not be affected; passing by reference means that the actual parameters are copied when calling the function. The address is passed directly to the function, so modifications to the parameters in the function will affect the actual parameters.
The operating environment of this tutorial: windows7 system, java8 version, DELL G3 computer.
actual and formal parameters
We all know that parameters can be defined when defining a method in Java. For example, the main method in Java, public static void main(String[ ] args), the args here are parameters. Parameters are divided into formal parameters and actual parameters in programming languages.
Formal parameters: are the parameters used when defining the function name and function body. The purpose is to receive the parameters passed in when calling the function.
Actual parameters: When calling a parameterized function, there is a data transfer relationship between the calling function and the called function. When calling a function in the calling function, the parameters in parentheses after the function name are called "actual parameters"
A simple example:
public static void main( String[ ] args) {
ParamTest pt = new ParamTest();
pt.sout( "Hollis");//實(shí)際參數(shù)為Hollis
}
public void sout( String name) {/!形式參數(shù)為name
system.out.println(name);
}
The actual parameters are The content that is actually passed when calling a method with parameters, and the formal parameters are the parameters used to receive the content of the actual parameters.
Value passing and reference passing
As mentioned above, when we call a parameterized function, the actual parameters will be passed to the formal parameters. However, in programming languages, there are two cases of transfer in this transfer process, namely transfer by value and transfer by reference. Let's take a look at how passing by value and passing by reference are defined and distinguished in programming languages.
Value passing refers to copying a copy of the actual parameters to the function when calling the function, so that if the parameters are modified in the function, the actual parameters will not be affected. Passing 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.
With the above concepts, you can then write code and practice it. Let’s see whether it is value passing or reference passing in Java. So, the simplest piece of code came out:
public static void main( String[] args) {
ParamTest pt = new ParamTest();
int i = 10;
pt.pass(i);
System.out.println( "print in main , i is " +i);
}
public void pass(int j){
j = 20;
system.out.println( "print in pass , j is " + j);
}
In the above code, we modify the value of parameter j in the pass method, and then print the value of the parameter in the pass method and main method respectively. The output result is as follows:
print in pass , j is 20
print in main , i is 10
It can be seen that the modification of the value of i inside the pass method does not change the value of the actual parameter i. So, according to the above definition, someone came to the conclusion: Java's method passing is value passing. However, some people soon raised questions (haha, so don’t jump to conclusions easily.). Then, they will move out the following code:
public static void main(String[ ] args) {
ParamTest pt = new ParamTest();
User hollis = new User();
hollis.setName( "Hollis");
hollis.setGender("Male");
pt.pass(hollis);
system.out.println( "print in main , user is " + hollis);}public void pass(User user) {
user.setName( "hollischuang");
System.out.println( "print in pass , user is " + user);}
is also a pass method, and the value of the parameter is also modified within the pass method. The output result is as follows:
print in pass , user is User{name='hollischuang', gender='Male '}
print in main , user is User{name='hollischuang' , gender='Male '}
After the pass method is executed, the value of the actual parameter is changed. According to the definition of passing by reference above, the value of the actual parameter is changed. Isn’t this called passing by reference? . Therefore, based on the above two pieces of code, someone came to a new conclusion: in Java methods, when passing ordinary types, it is passed by value, and when passing object types, it is passed by reference. However, this statement is still wrong. If you don’t believe me, take a look at the following parameter transfer where the parameter type is an object:
public static void main( string[] args) {
ParamTest pt = new ParamTest();
string name = "Hollis";
pt.pass(name ) ;
System.out.println( "print in main , name is " + name);
}
public void pass(string name) {
name = "hollischuang";
system.out.println( "print in pass , name is " + name);
}
The output result of the above code is
print in pass , name is hollischuangprint in main , name is Hollis
What’s the explanation? An object is also passed, but the original parameter is The value has not been modified. Could it be that the transferred object has become a value transfer again?
Value transfer in Java
Above, we gave three examples to show the The results are different, which is why many beginners and even many advanced programmers are confused about Java's transfer types. In fact, what I want to tell you is that the above concept is not wrong, but there is a problem with the code example. Come on, let me outline the key points of the concept for you, and then give you a few truly appropriate examples.
Value passing refers to copying a copy of the actual parameters to the function when calling the function, so that if the parameters are modified in the function, the actual parameters will not be affected. Passing 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.
So, let me summarize for you the key points of the difference between value passing and reference passing.
##Pass by value
Pass by reference
Fundamental difference
Will create a copy
Does not create a copy
All
The original object cannot be changed in the function
The original object can be changed in the function
public static void main(String[ ] args){
ParamTest pt = new ParamTest();
User hollis = new User();
hollis.setName( "Hollis");
hollis.setGender("Male" );
pt.pass(hollis);
system.out.println("print in main , user is " + hollis);
public void pass(User user) {
user = new User();
user.setName( "hollischuang");
user.setGender( "Male");
system.out.println( "print in pass , user is " + user);
上面的代碼中，我們在pass方法中，改變了user對象，輸出結(jié)果如下:
print in pass , user is User{name='hollischuang ' , gender='Male '}
print in main , user is User{name='Hollis', gender= 'Male '}
Passing by sharing means that when a function is called, a copy of the address of the actual parameter is passed to the function (if the actual parameter is on the stack, the value is copied directly). When operating parameters inside a function, you need to copy the address to find the specific value before operating. If the value is on the stack, then because it is a direct copy of the value, operations on the parameters within the function will not affect external variables. If the original copy is the address of the original value in the heap, then you need to find the corresponding location in the heap based on the address before performing the operation. Because a copy of the address is passed, the operation on the value within the function is visible to the external variable.
To put it simply, transfer in Java is by value, and this value is actually a reference to the object. Passing by sharing is actually just a special case of passing by value. So we can say that passing in Java is passing by sharing, or that passing in Java is passing by value.
So operating parameters inside the function will not affect external variables. If the original copy is the address of the original value in the heap, then you need to find the corresponding location in the heap based on the address before performing the operation. Because a copy of the address is passed, the operation on the value within the function is visible to the external variable.
To put it simply, transfer in Java is by value, and this value is actually a reference to the object.
Passing by sharing is actually just a special case of passing by value. So we can say that passing in Java is passing by sharing, or that passing in Java is passing by value.
The above is the detailed content of What is the difference between pass by value and pass by reference in java. For more information, please follow other related articles on the PHP Chinese website!
Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
There are three common methods to traverse Map in Java: 1. Use entrySet to obtain keys and values at the same time, which is suitable for most scenarios; 2. Use keySet or values to traverse keys or values respectively; 3. Use Java8's forEach to simplify the code structure. entrySet returns a Set set containing all key-value pairs, and each loop gets the Map.Entry object, suitable for frequent access to keys and values; if only keys or values are required, you can call keySet() or values() respectively, or you can get the value through map.get(key) when traversing the keys; Java 8 can use forEach((key,value)->
In Java, Comparable is used to define default sorting rules internally, and Comparator is used to define multiple sorting logic externally. 1.Comparable is an interface implemented by the class itself. It defines the natural order by rewriting the compareTo() method. It is suitable for classes with fixed and most commonly used sorting methods, such as String or Integer. 2. Comparator is an externally defined functional interface, implemented through the compare() method, suitable for situations where multiple sorting methods are required for the same class, the class source code cannot be modified, or the sorting logic is often changed. The difference between the two is that Comparable can only define a sorting logic and needs to modify the class itself, while Compar
To deal with character encoding problems in Java, the key is to clearly specify the encoding used at each step. 1. Always specify encoding when reading and writing text, use InputStreamReader and OutputStreamWriter and pass in an explicit character set to avoid relying on system default encoding. 2. Make sure both ends are consistent when processing strings on the network boundary, set the correct Content-Type header and explicitly specify the encoding with the library. 3. Use String.getBytes() and newString(byte[]) with caution, and always manually specify StandardCharsets.UTF_8 to avoid data corruption caused by platform differences. In short, by
std::chrono is used in C to process time, including obtaining the current time, measuring execution time, operation time point and duration, and formatting analysis time. 1. Use std::chrono::system_clock::now() to obtain the current time, which can be converted into a readable string, but the system clock may not be monotonous; 2. Use std::chrono::steady_clock to measure the execution time to ensure monotony, and convert it into milliseconds, seconds and other units through duration_cast; 3. Time point (time_point) and duration (duration) can be interoperable, but attention should be paid to unit compatibility and clock epoch (epoch)
HashMap implements key-value pair storage through hash tables in Java, and its core lies in quickly positioning data locations. 1. First use the hashCode() method of the key to generate a hash value and convert it into an array index through bit operations; 2. Different objects may generate the same hash value, resulting in conflicts. At this time, the node is mounted in the form of a linked list. After JDK8, the linked list is too long (default length 8) and it will be converted to a red and black tree to improve efficiency; 3. When using a custom class as a key, the equals() and hashCode() methods must be rewritten; 4. HashMap dynamically expands capacity. When the number of elements exceeds the capacity and multiplies by the load factor (default 0.75), expand and rehash; 5. HashMap is not thread-safe, and Concu should be used in multithreaded
JavaScript data types are divided into primitive types and reference types. Primitive types include string, number, boolean, null, undefined, and symbol. The values are immutable and copies are copied when assigning values, so they do not affect each other; reference types such as objects, arrays and functions store memory addresses, and variables pointing to the same object will affect each other. Typeof and instanceof can be used to determine types, but pay attention to the historical issues of typeofnull. Understanding these two types of differences can help write more stable and reliable code.
ReentrantLock provides more flexible thread control in Java than synchronized. 1. It supports non-blocking acquisition locks (tryLock()), lock acquisition with timeout (tryLock(longtimeout, TimeUnitunit)) and interruptible wait locks; 2. Allows fair locks to avoid thread hunger; 3. Supports multiple condition variables to achieve a more refined wait/notification mechanism; 4. Need to manually release the lock, unlock() must be called in finally blocks to avoid resource leakage; 5. It is suitable for scenarios that require advanced synchronization control, such as custom synchronization tools or complex concurrent structures, but synchro is still recommended for simple mutual exclusion requirements.
Java
