The main method in Java is the entrance to a java application. No matter how complex, large or small a java application is, As long as it is a java program that can be run, there must be a main method.
The definition format of the main method is usually fixed as follows: (Recommended learning: java course)
public class Demo {
public static void main(String[] args) {
System.out.println("Hello Word");
}}1. Why Is public
ans1: Java specifies some accessible modifiers such as: private, protected, public. Each modifier has its corresponding permissions, and public has the highest permissions. To illustrate the problem, we assume that the main method is modified with private, then the Demo class in the main method is not visible to the outside world.
Then, the JVM cannot access the main method. Therefore, in order to ensure that the JVM can access the main method under any circumstances, use public modification.
ans2: Java has designed several access modifiers, including: private, protected, public. Any method or variable declared as public in Java can be accessed from outside the class. The JVM's access to the main method is obviously not within the class, so the main method needs to be defined as a public method.
2. Why is it static?
Static can make it more convenient for the JVM to call the main method without the need to call it through an object. What we know about the static keyword is that the method modified by the static keyword can be accessed directly through the class name without creating an instance. And statically modified methods and variables are stored in the method area of the virtual machine, not in the heap memory.
Then, the same is true for virtual machines. The main method is defined as static, so for the virtual machine, there is no need to create an instance after the program is started to call the main method.
3. Why is there no return value (void)
void means that the main method has no return value. The reason why there is no return value is because Java does not require the main method to report to the operating system. Return exit information. If the main method exits normally, the exit code of the Java application layer sequence is 0, indicating that the program was successfully run.
4. main
The name of main cannot be changed so that the JVM can identify the starting point of program running. The main method can be overloaded, but the overloaded main method will not be executed. The main method serves as the starting point for the initial thread of the program, and any other threads are started by this thread.
There are two kinds of threads inside the JVM, non-daemon threads and daemon threads. The main method belongs to the non-daemon thread. The daemon thread is usually used by the JVM itself. The Java program can also indicate that its thread is a daemon thread. When all non-daemon threads in the program terminate, the JVM exits. You can also use the Runtime class or System.exit() to exit.
5. String [] args
String[] args is the only place that can be changed in the main method! args is the abbreviation of arguments. It is just the default name of a variable. It is customary to write it, but it can also be changed. As long as it complies with the naming rules, you can write it as you like.
In today's world of using integrated development tools, String[] args is more like a decoration. Many beginners don't know its role. In fact, it is a parameter group passed in when the program is running.
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