This question already has answers here:
How can you run a Java program without main method? [duplicate]
(4 answers)
Closed 8 years ago.
Is it possible to run a java program without main method ?
How can I execute this Java program in eclipse?
public class A {
static {
System.out.println("hello");
System.exit(0);
}
}
There are some ways to "run a Java program" without main method.
For example:
you could create a jUnit test. In that case Eclipse will offer you the option "Run as jUnit Test".
you can create a class which extends JApplet. In that case Eclipse will offer the option "Run as Java Applet".
Your question is copied from here, but the answer is :
Use a static initializer block to print the message. This way, as soon as your class is loaded the message will be printed. The trick then becomes using another program to load your class.
public class Hello {
static {
System.out.println("Hello, World!");
}
}
Of course, you can run the program as java Hello and you will see the message; however, the command will also fail with a message stating:
Exception in thread "main" java.lang.NoSuchMethodError: main
or
public class X { static {
System.out.println("Main not required to print this");
System.exit(0);
}}
Run from the cmdline with java X.
Related
This question already has answers here:
What does "Could not find or load main class" mean?
(61 answers)
Closed 4 years ago.
Hi i've compile this java project. This is my first project of hello world but when i try to run it on cmd i have this error. Can you help me?
This is my code:
public class CiaoMondo {
public static void main(String[] args)
{
// Prints "Hello, World" to the terminal window.
System.out.println("Hello, World");
}
}
This is the error:
A ClassNotFoundException can be caused for several reasons. The most common (in my experience) are because the java ClassName command is blocked by your firewall, or the file your class is in is improperly named.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Printing message on Console without using main() method
Can someone suggest how can a JAVA program run without writing a main method..
For eg:
System.out.println("Main not required to print this");
How can the above line be printed on console without using the public static void main(String arg[]) in the class.
Up to and including Java 6 it was possible to do this using the Static Initialization Block as was pointed out in the question Printing message on Console without using main() method. For instance using the following code:
public class Foo {
static {
System.out.println("Message");
System.exit(0);
}
}
The System.exit(0) lets the program exit before the JVM is looking for the main method, otherwise the following error will be thrown:
Exception in thread "main" java.lang.NoSuchMethodError: main
In Java 7, however, this does not work anymore, even though it compiles, the following error will appear when you try to execute it:
The program compiled successfully, but main class was not found.
Main class should contain method: public static void main (String[] args).
Here an alternative is to write your own launcher, this way you can define entry points as you want.
In the article JVM Launcher you will find the necessary information to get started:
This article explains how can we create a Java Virtual Machine
Launcher (like java.exe or javaw.exe). It explores how the Java
Virtual Machine launches a Java application. It gives you more ideas
on the JDK or JRE you are using. This launcher is very useful in
Cygwin (Linux emulator) with Java Native Interface. This article
assumes a basic understanding of JNI.
Up until JDK6, you could use a static initializer block to print the message. This way, as soon as your class is loaded the message will be printed. The trick then becomes using another program to load your class.
public class Hello {
static {
System.out.println("Hello, World!");
}
}
Of course, you can run the program as java Hello and you will see the message; however, the command will also fail with a message stating:
Exception in thread "main" java.lang.NoSuchMethodError: main
[Edit] as noted by others, you can avoid the NoSuchmethodError by simply calling System.exit(0) immediately after printing the message.
As of JDK6 onward, you no longer see the message from the static initializer block; details here.
public class X { static {
System.out.println("Main not required to print this");
System.exit(0);
}}
Run from the cmdline with java X.
Applets from what I remember do not need a main method, though I am not sure they are technically a program.
Closed. This question is not reproducible or was caused by typos. It is not currently accepting answers.
This question was caused by a typo or a problem that can no longer be reproduced. While similar questions may be on-topic here, this one was resolved in a way less likely to help future readers.
Closed 6 years ago.
Improve this question
class WithoutMain {
static
{
System.out.println("Without main class!!!");
System.exit(0);
}
}
when I am trying to to run above code in java version greater than 7 i am getting below error.
The program compiled successfully, but main class was not found. Main class should contain method: public static void main (String[] args).
can someone please guide me why Java does not support to run program without main after java7
AFAIK this change was specific to Java 7. In Java 8 you can do this. You can't do this in Java 7 as it looks for the method without loading the class first which fails. In any case, it has been changed back in Java 8.
public class Main {
static {
System.out.println("Without main class!!! with " + System.getProperty("java.version"));
System.exit(0);
}
}
prints
Without main class!!! with 1.8.0_66
Note: this will kill the whole program. If you want the program to keep running without a main you can do this
public class Main {
static {
// do something which starts threads
System.out.println("Without main class!!! with " + System.getProperty("java.version"));
if (true)
throw new ThreadDeath();
}
}
This will prevent the error message, but leave background threads running provided there is a non-daemon thread.
the static section
static
{
System.out.println("Without main class!!!");
System.exit(0);
}
will be executed every time the JVM load the class in memory, but if you want to start a java application, you will need a main method, because that it the start point of every java application, if you dont define it, then the JVM will have no idea where to start.
you can expand your code and do something like:
class WithoutMain {
static
{
System.out.println("Static section!!");
}
public static void main(String[] args){
System.out.println("Main class!!!");
}
}
and the output will be the static section first and then the code you define in the main method.
Static method is loaded with the class each time the JVM is started and the class is loaded into it, but there is nothing that calls it or outputs its contents inside of the JVM. As every language (that I know), you need a handler for your arguments and in case of Java, it is the main() method.
This question already has answers here:
Why does main method in Java always need arguments?
(8 answers)
Closed 9 years ago.
Is it necessary to add String[] args to main method in java, even if I'm sure that I won't pass any command line argument? If yes, why? And are there any other parameters that can be passed to main method???
Java Language Specification point ยง12.1.4 says: "Yes you have to."
It's not necessary to have arguments, but if you give the main method 0 arguments you won't be able to use it as an entry point. If you take away the arguments it will compile. For example, this code will compile and run:
package com.sandbox;
public class Sandbox {
public static void main(String[] args) {
//you can run this from the command line
main(); //you can call the faux main
}
public static void main() {
//you can't run this from the command line
System.out.println("called");
}
}
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Printing message on Console without using main() method
Can someone suggest how can a JAVA program run without writing a main method..
For eg:
System.out.println("Main not required to print this");
How can the above line be printed on console without using the public static void main(String arg[]) in the class.
Up to and including Java 6 it was possible to do this using the Static Initialization Block as was pointed out in the question Printing message on Console without using main() method. For instance using the following code:
public class Foo {
static {
System.out.println("Message");
System.exit(0);
}
}
The System.exit(0) lets the program exit before the JVM is looking for the main method, otherwise the following error will be thrown:
Exception in thread "main" java.lang.NoSuchMethodError: main
In Java 7, however, this does not work anymore, even though it compiles, the following error will appear when you try to execute it:
The program compiled successfully, but main class was not found.
Main class should contain method: public static void main (String[] args).
Here an alternative is to write your own launcher, this way you can define entry points as you want.
In the article JVM Launcher you will find the necessary information to get started:
This article explains how can we create a Java Virtual Machine
Launcher (like java.exe or javaw.exe). It explores how the Java
Virtual Machine launches a Java application. It gives you more ideas
on the JDK or JRE you are using. This launcher is very useful in
Cygwin (Linux emulator) with Java Native Interface. This article
assumes a basic understanding of JNI.
Up until JDK6, you could use a static initializer block to print the message. This way, as soon as your class is loaded the message will be printed. The trick then becomes using another program to load your class.
public class Hello {
static {
System.out.println("Hello, World!");
}
}
Of course, you can run the program as java Hello and you will see the message; however, the command will also fail with a message stating:
Exception in thread "main" java.lang.NoSuchMethodError: main
[Edit] as noted by others, you can avoid the NoSuchmethodError by simply calling System.exit(0) immediately after printing the message.
As of JDK6 onward, you no longer see the message from the static initializer block; details here.
public class X { static {
System.out.println("Main not required to print this");
System.exit(0);
}}
Run from the cmdline with java X.
Applets from what I remember do not need a main method, though I am not sure they are technically a program.