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public static void main(String[] args) - Java main method
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Introduction
The Java main method is usually the first method you learn about when you start programming in Java because its the entry point for executing a Java program. The main method can contain code to execute or call other methods, and it can be placed in any class that’s part of a program. More complex programs usually have a class that contains only the main method. The class that contains the main method can have any name, although typically you can just call the class Main.
In the examples that follow, the class that contains the main method is called Test:
public class Test {
public static void main(String[] args){
System.out.println("Hello, World!");
}
}
In this article you’ll learn what each component of the main method means.
Java Main Method Syntax
The syntax of the main method is always:
public static void main(String[] args){
// some code
}
You can change only the name of the String array argument. For example, you can change args to myStringArgs. The String array argument can be written as String... args or String args[].
public
The access modifier of the main method needs to be public so that the JRE can access and execute this method. If a method isn’t public, then access is restricted. In the following example code, the main method is missing the public access modifier:
public class Test {
static void main(String[] args){
System.out.println("Hello, World!");
}
}
When you compile and run the program, the following error occurs because the main method isn’t public and the JRE can’t find it:
- javac Test.java
- java Test
OutputError: Main method not found in class Test, please define the `main` method as:
public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application
static
When the Java program starts, there is no object of the class present. The main method has to be static so that the JVM can load the class into memory and call the main method without creating an instance of the class first. In the following example code, the main method is missing the static modifier:
public class Test {
public void main(String[] args){
System.out.println("Hello, World!");
}
}
When you compile and run the program, the following error occurs because the main method isn’t static:
- javac Test.java
- java Test
OutputError: Main method is not static in class Test, please define the `main` method as:
public static void main(String[] args)
void
Every Java method must provide the return type. The Java main method return type is void because it doesn’t return anything. When the main method is finished executing, the Java program terminates, so there is no need for a returned object. In the following example code, the main method attempts to return something when the return type is void:
public class Test {
public static void main(String[] args){
return 0;
}
}
When you compile the program, the following error occurs because Java doesn’t expect a return value when the return type is void:
- javac Test.java
OutputTest.java:5: error: incompatible types: unexpected return value
return 0;
^
1 error
main
The Java main method is always named main. When a Java program starts, it always looks for the main method. The following example code shows a main method renamed to myMain:
public class Test {
public static void myMain(String[] args){
System.out.println("Hello, World!");
}
}
When you compile and run the program, the following error occurs because the JRE can’t find the main method in the class:
- javac Test.java
- java Test
OutputError: Main method not found in class Test, please define the `main` method as:
public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application
String[] args
Java main method accepts a single argument of type String array. Each string in the array is a command line argument. You can use command line arguments to affect the operation of the program, or to pass information to the program, at runtime. The following example code shows how to print the command line arguments that you enter when you run the program:
public class Test {
public static void main(String[] args){
for(String s : args){
System.out.println(s);
}
}
}
When you compile the program and then run it with a few command line arguments separated by spaces, the arguments get printed in the terminal:
- javac Test.java
- java Test 1 2 3 "Testing the main method"
Output1
2
3
Testing the main method
Conclusion
In this article you learned about each component of the Java main method. Continue your learning with more Java tutorials.
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