I have just started creating this program and I am trying to make a method outside of the main class. I researched the error and it says there are too many curly brackets outside the main function.
I was just wondering if it was actually possible to have a public static function outside of a main class?
class NewMain {
public static void main(String[] args) {
}
}
public static void plusOne(){
}
Think of a class as a collection of code. You can have as many classes as you want. But you can not put code outside of a class and there is no reason you should want to.
Is it possible to create object of class in which main method resides.I have been searching for this answer but I have been told that it depends on compiler some compiler will allow while others will not.Is that true?
Yes? The main method is just an entry point. The class is like any other, except it has an additional public static method. The main method is static and therefore not part of the instance of the object, but you shouldn't be using the main method anyway except for starting the program.
public class Scratchpad {
public static void main(String[] args) {
Scratchpad scratchpad = new Scratchpad();
scratchpad.someMethod();
}
public Scratchpad() {
}
private void someMethod() {
System.out.println("Non-static method prints");
}
}
Yes, you can create object for the class which has main method. There is no difference in this class and a class which don't have main method with respect to creating objects and using.
Can we call main() function inside any other function? I tried but did not come up with it.
If we can't call it then why?
Why main() is not like ordinary methods?
Yes why not try something like:
public class Main {
public static void main(String args[]) {
System.out.println("Hello World");
}
}
public class NewMain {
public static void main(String args[]) {
Main.main(args);
}
}
If you run:
java NewMain
the output is:
Hello World
Why main() is not like ordinary methods
Not like ordinary methods in what sense? It is like any other method. I'll try to explain why main looks like this, maybe it'll help you to understand what's going on.
It's void because when it finishes, it doesn't mean that the program finished. If it spawns a new thread it might be that these threads are still running.
It's public because it's called by the JVM, which is outside the scope of the project.
And of course it has to be static because when the JVM calls it, there's no object existing for the class being called.
Nothing special about it when you understand it, so yes.. it can be called like any other static method.
Sure you can, main() is like nay other method in that area.
public class A {
public static void main(String[] args) {
System.out.println("a's main()");
B.main(new String[0]);
}
}
public class B {
public static void main(String[] args) {
System.out.println("B's main()");
}
}
Running A's main() will produce:
a's main()
B's main()
All the actions that you do one other methods can also be done on the main method
The only difference between main method and other methods is that the main method servers as the starting point for running a class
The java command launches a Java application. It does this by starting a Java runtime environment, loading a specified class, and invoking that class's main method.
Except for this reason, there is nothing else.
I have two java classes.One in which my GUI is written and the other class in which i have implemented an interface(call it class 2).My project starts from the main method of GUI.I want to send a string to my GUI class from the class 2 for displaying it in the Text area but nothing is happening.
As my main gui class is
public class GraphicalInterface extends javax.swing.JFrame{
//I have created a function over here for displaying string in text area
public void show1(String name)
{
jTextArea1.setText(name);
}
//buttons code
public static void main(String args[]) {
//code
}
}
I have created an object of this class in my class 2 like below
GraphicalInterface b=new GraphicalInterface();
b.show1("pear");// it does not allow me to write this statement
Please help me out that how can i call main method class from another java class.Thanks.
You may be trying to call this code outside of a constructor or method (or initializer block) and in Java, this can't be done. Instead call this code inside of a method or constructor.
I guess that you have a design problem in your project. Let me expain. You say you have a GUI class "GraphicalInterface" which holds the main method which is the starting point of an application in Java. You say you need to call the main method of this class in another class,
"your class 2". If so why isn't the place belonging to the "main method" of your application in which you try to call this GUI's main method. Call GUI's main method x(), let the place that you call x() belong to the main method.
If you need to operate on the GUI's fields in another classes and also keep the main method still there, then I suggest you to apply Singleton Pattern to your GUI class. In that way you
will be able to refer the only instance of your public singleton class everywhere in your application.
public class GraphicalInterface extends javax.swing.JFrame
{
public String textAreaContent;
public getX()( return textAreaContent;)
public setX(String s)( this.textAreaContent = s;)
public void show1()
{
jTextArea1.setText(this.getTextAreaContent());
}
public static void main(String args[])
{
//code
}
}
From your other class:
GraphicalInterface b=new GraphicalInterface();
b.setX("text area content");
b.show1();
No, the best solution is not to do this, and if you feel you must, it is likely because your design is somehow broken. Instead write your code so that your classes are true OOPs classes that interact in an intelligent way (low coupling, high cohesion), and to only need one main method.
Also, you state:
GraphicalInterface b=new GraphicalInterface();
b.show1("pear");// it does not allow me to write this statement
What do you mean by "it does not allow me to write this statement"? Does the Java compiler give a compilation error? Does the JVM throw an exception? Does the JVM reach out of your monitor and slap you in the face? Please let us know all the details necessary to be able to help you.
You need to create a method in class2 and call it from your main method.
Sample code class1
public class Test1 {
public void show(String ab){
System.out.println(ab);
}
public static void main(String[] args) {
Test2.Test2();
}
}
Above code i create a class Test1.java like your class1 and create a method with one parameter and call it from class2 method.
Sample code class2
public class Test2 {
static void Test2(){
new Test1().show("Pass String to class1 show method");
}
}
here you can pass your string value.
I was wondering what the effect of creating extra main methods would do to your code.
For example,
public class TestClass {
public static void main (String[] args){
TestClass foo = new TestClass();
}
}
After the program initially starts up, foo will be created and it would have another public main method inside it. Will that cause any errors?
It will cause no errors. Just because you initialize an object, doesn't mean the main method gets executed. Java will only initially call the main method of the class passed to it, like
>java TestClass
However, doing something like:
public class TestClass
{
public static void main (String[] args)
{
TestClass foo = new TestClass();
foo.main(args);
}
}
Or
public class TestClass
{
public TestClass()
{
//This gets executed when you create an instance of TestClass
main(null);
}
public static void main (String[] args)
{
TestClass foo = new TestClass();
}
}
That would cause a StackOverflowError, because you are explicitly calling TestClass's main method, which will then call the main method again, and again, and again, and....
When in doubt, just test it out :-)
The main method is static, which means it belongs to the class rather than the object. So the object won't have another main method inside it at all.
You could call the main method on instances of the object, but if you do that it's literally just another way of calling TestClass.main() (and it's frowned upon by many, including me, to call a static method on an instance of an object anyway.)
If you're referring to multiple main methods in the same program, then this isn't a problem either. The main class is simply specified and its main method is executed to start the program (in the case of a jar file this is the main-class attribute in the manifest file.)
It won't have an additional main-method, as main is static. So it's once per class.
If you have multiple main-methods in your project, you will specify which one to launch when starting your application.
This is perfectly fine. Having multiple main methods doesn't cause any problems. When you first start a Java program, execution begins in some function called main in a class specified by the user or by the .jar file. Once the program has started running, all the other functions called main are essentially ignored or treated like other functions.
After searching for a Java Class with multiple main() methods or in plain words, overloaded main() methods, I came up with an example of my own. Please have a look
public class MultipleMain{
public static void main(String args[]){
main(1);
main('c');
main("MyString");
}
public static void main(int i){
System.out.println("Inside Overloaded main()");
}
public static void main(char i){
System.out.println("Inside Overloaded main()");
}
public static void main(String str){
System.out.println("Inside Overloaded main()");
}
}
I tested this Java Code on JDK 1.7 and works like a charm !
You need "public static void main(String args[])" to start with and then you can call overloaded main methods inside this main and it should work for sure.
Any comments and suggestion are highly appreciated. I am just a novice Java Developer willing to develop my Java skills.
Thanks,
PK
No, you can have any number of main-methods in a project. Since you specify which one you want to use when you launch the program it doesn't cause any conflicts.
You can have only one main method in one class, But you can call one main method to the another explicitly
class Expmain
{
public static void main(String[] ar)
{
System.out.println("main 1");
}
}
class Expmain1
{
public static void main(String[] ar)
{
System.out.println("main 2");
Expmain.main(ar);
}
}
when you run your Java class it will always look for the signature public static void main(String args[]) in the class. So suppose if you invoking by command line argument, it will look for the method Signature in the class and will not invoke other until if u explicitly inoke it by its class name.
class MainMethod1{
public static void main(String[] ags){
System.out.println("Hi main 1");
testig2 y = new testig2();
//in this case MainMethod1 is invoked/.......
// String[] a = new String[10];
// testig2.main(a);
}
}
class MainMethod2{
public static void main(String[] ags){
System.out.println("Hi main 2");
}
}
But when you try the same from eclipse it will ask for which class to compile. Means MainMethod1 or Mainmethod2. So if te class has the exact signature they can be used as individual entry point to start the application.
Coming to your question, If you remove the signature as u did above by changing the argument if main method. It will act as a normal method.
It is all about the execution engine of JVM. Remember, you write >java com.abc.MainClass on cmd prompt.
It explains everything. If main method is not found here it throws a run time Error:Main Method Not Found in class MainClass.
Now if main method is found here, it acts as the first point when Program Counters have to map and start executing the instructions. Referred classes are loaded then, referred methods may be called using the instances created inside. So, main is class specific though one class can have only one main method.
Please note, main method's signature never changes. You can have two overloaded main methods in same class, like
public static void main(String[] args) {}
public static void main() {} //overloaded in same class.
During Static binding, the original main is resolved and identified by execution engine.
Another interesting point to consider is a case where you have two different classes in of java file.
For example, you have Java file with two classes:
public class FirstClassMultiply {
public static void main (String args[]){
System.out.println("Using FirstClassMultiply");
FirstClassMultiply mult = new FirstClassMultiply();
System.out.println("Multiple is :" + mult.multiply(2, 4));
}
public static void main (int i){
System.out.println("Using FirstClassMultiply with integer argument");
FirstClassMultiply mult = new FirstClassMultiply();
System.out.println("Multiply is :" + mult.multiply(2, 5));
}
int multiply(int a, int b) {
return (a * b);
}
}
class SecondClass {
public static void main(String args[]) {
System.out.println("Using SecondClass");
FirstClassMultiply mult = new FirstClassMultiply();
System.out.println("Multiply is :" + mult.multiply(2, 3));
FirstClassMultiply.main(null);
FirstClassMultiply.main(1);
}
}
Compiling it with javac FirstClassMultiply.java will generate two .class files, first one is FirstClassMultiply.class and second one is SecondClass.class
And in order to run it you will need to do it for the generated .class files: java FirstClassMultiply and java SecondClass, not the original filename file.
Please note a couple of additional points:
You will be able to run SecondClass.class although it's class wasn't public in the original file!
FirstClassMultiply overloading of the main method
of is totally fine, but, the only entry point to your prog
will be the main method with String args[] argument.