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Can't find main(String[]) method in class: TapeDeck. The main method is in the other class which runs the program

I have two classes. When the I put class TapeDeckTestDrive first on the text editor, it runs fine. When I put the TestDrive class first, it gives the error that it can't find the main class. Why is this?
class TapeDeck {
boolean canRecord = false;
void playTape(){
System.out.println("tape playing");
}
void recordTape(){
System.out.println("tape recording");
}
}
class TapeDeckcTestDrive{
public static void main(String[] args){
TapeDeck t = new TapeDeck();
t.canRecord = true;
t.playTape();
if (t.canRecord == true) {
t.recordTape();
}
}
}
ERROR ON THIS FORMAT
VS
FOLLOWING WORKS FINE:
class TapeDeckcTestDrive{
public static void main(String[] args){
TapeDeck t = new TapeDeck();
t.canRecord = true;
t.playTape();
if (t.canRecord == true) {
t.recordTape();
}
}
}
class TapeDeck {
boolean canRecord = false;
void playTape(){
System.out.println("tape playing");
}
void recordTape(){
System.out.println("tape recording");
}
}

After you compile the code using the command:
javac fileName.java
Run the java .class file by only specifying fileName without the .java extension
java fileName
if you use fileName.java it won't run the specific .class file; it will try to interpret the .java file. if you want to interpret a .java file then parent class must contain the main(String[]) method.

First, You have to compile the File by using javac.
Then, You have to Run the file.
Classname where main is written.
javac filename.java
java classname

You Can Run the java program in two ways.
Directly run the java program by
java example_program.java
In this type compilation and Execution happens at runtime. That is
Byte codes is generated and executed immediately(works as a interpreter)
So,You must use the superclass(Containing the main method) at first followed by other
compound classes.
Note:
No .class file will generate. That means, it will generate byte code internally and will execute. Programmer's cannot view the class file.
In Second type, First, you should compile,
javac example_program.java
It will generate the example_program.class . Then, Execute the class file using,
java example_program
Here, the order of writing classes doesn't impact. you can write the classes in any order. it will work fine.

I split it into two files and added public to the classes/methods as well as the boolean. Now the code runs.

In some JDK's , JVM looks after the entry point function first due to which it need to be written first then the rest of the code. As main function is our entry point function it must be written first.

Steps 1.
--You have to compile the File by using javac. Then, You have to Run the file.
--Classname where main is written.
-- javac filename.java
-- java classname
It causing error due to:-
class TapeDeck {
boolean canRecord = false;
void playTape(){
System.out.println("tape playing");
}
void recordTape(){
System.out.println("tape recording");
}
}
class TapeDeckcTestDrive{
public static void main(String[] args){
TapeDeck t = new TapeDeck();
t.canRecord = true;
t.playTape();
if (t.canRecord == true) {
t.recordTape();
}
}
}
--Your tapedeck class doesn't main (String[]).

I got your problem.
First of all, check your classpath that you have set in Environment Variables
Follow the following steps:
***Step 1: *** Right Click on This PC --> Advanced system settings --> Environment Variables
***Step 2: *** Edit the variable classpath and add a new path or edit your old path that you have set. The path should be: C:\Program Files\Java_Home\jdk..\lib;.;
Note: The "." is must after a semicolon (;).
***Step 3: *** Close the CMD and open it again.
***Step 4: *** Now compile your using javac command: javac FileName.java
***Step 5: *** Run your code using java command: java ClassName
And there you go...

Try to put "public static void main" class first and then other methods in your code. It will definitely work.

running .class files with java.exe - Error: Could not find or load main class

code:
package pack1;
public class Demo01 {
public void run() {
System.out.println("--running Demo01--");
demoMethod1();
}
private void demoMethod1() {
int foo = 5;
int bar = 10;
int res = foo+bar;
System.out.println("res: "+res);
}
public static void main(String[] args){
Demo01 demo01 = new Demo01();
demo01.run();
// new change...
Demo02 demo02 = new Demo02();
demo02.run();
}
}
rest can be found here: https://code.google.com/p/ci-research-teamcity-test-project/source/browse/#svn%2Ftrunk%2Fsrc%2Fpack1
I'm trying to run the .class files with java.exe through the command line to no avail.
Yes I've looked for solutions, tried running the root folder with the -cp flag but I keep getting the same error. Works just fine in Eclipse.

Ok, we have several points to note at this point.
The class is in a package. Thus, it must be in a folder names exactly as the package name ("pack1" in your case).
Your folder structure must be like this:
"root folder" (X)
| pack1
| Demo01.class
| Demo02.class (as I just noticed that you are also referring to it in the code)
Then, in order to start it, you have to be in the parent folder (this has to be the current working directory; marked with X) of "pack1" and execute
java pack.Demo01
Note, you have to use the whole canonical class name for referencing it, without .class at the end.
If you don't want or cant change the current working directory to the "root folder" you can use -cp PATH as first parameters to java.exe.

How can I compile code that uses a .jar file class with BlueJ?

I am learning java with BlueJ, and recently I was given a .jar file called Imagen.jar. Apparently, what it does is return some pixel vectors depending on image file names given as parameters to it.
Anyway, I am supposed to make a program that will use a class called Imagen. Apparently, such class is within the mentioned .jar file.
Clearly, BlueJ won't compile if I'm using such class since I have not imported it or anything. But, I don't really know how to import such class in the first place.
I was given the following example code:
import java.io.PrintWriter;
public class Main {
public static void main(String arg[ ]){
if(arg.length > 1){
Imagen imagen = new Imagen(arg[0]);
int [][] m = imagen.getMatriz();
PrintWriter salida = null;
try {
salida = new PrintWriter(arg[1]);
}
catch(Exception e){
System.err.println(e);
}
for(int [] fila : m ){
for(int valor : fila){
System.out.print("\t"+valor);
salida.print("\t"+valor);
}
salida.println("");
System.out.println("");
}
if(salida!=null){
salida.close();
}
}
else {
System.out.println("Uso: java -classpath .;Imagen.jar Main nombreArchivo.gif");
}
}
}
Which does not compile using BlueJ. However, as you can see, at the end it says that to use it, you have to type in the terminal:
java -classpath .;Imagen.jar Main myImageFile.gif
And I do it. But it keeps throwing me the same message.
So I am stuck right now:
Why is the terminal line I was told to use not working?
How can I import the class that is contained within a .jar file?

You need to do the following once.
Select the menu option Tools -> Preferences.
In the resulting dialog, click on the Libraries tab.
Click the Add button.
Navigate to the folder containing jar file. Select jar file.
Restart BlueJ.
Answer extracted from this place

you need to import Imagen class as it is being used in the main method.

Create a class instance

I am just starting to use Java. I am using NetBeans and inside my .pkg1 file I have two .java files. I am doing the Coursera course on Algorithms by the way, so my code references that:
CourseraAlgorithmsWeek1.java
package coursera.algorithms.week.pkg1;
public class CourseraAlgorithmsWeek1 {
public static void main(String[] args) {
QuickFindUF mystuff(10); // DOES NOT WORK!
}
}
QuickFindUF.java
public class QuickFindUF {
private int[] id;
public QuickFindUF(int N){
id = new int[N];
for(int i =0; i< N; i++){
id[i] = i;
}
}
}
My problem is that the first line in my main function does not recognize the QuickFindUF object creation. I read that I need to compile the second file into a .class file, and then into a .jar file. How can I do this with netbeans?
I also read a bit about the Classpath. Can I only add .jar files to the classpath?

change
QuickFindUF mystuff(10);
to
QuickFindUF mystuff = new QuickFindUF(10);

Move QuickFindUF.java to the same package of your main class by adding
package coursera.algorithms.week.pkg1;
before class definition.

You can add a directory to the classpath as well. You could do something like
export CLASSPATH = "."
and that would include the directory you are currently in. This should allow you to compile and execute code in that directory.
If the classpath is set correctly, you can either move the QuickFindUF class to the coursera.algorithms.week.pkg1 package by adding this declaration at the top of the page:
package coursera.algorithms.week.pkg1;
or, you can import the class by using the declaration:
import coursera.algorithms.week.pkg1.CourseraAlgorithmsWeek1;

1. Use Composition.
QuickFindUF q = new QuickFindUF();
q.mystuff(10);

Use java in matlab

I encountered a problem when I tried to use java from matlab. I read through the tutorials from MathWork.com several times, also I re-installed the JDK1.6, in order to be compatible with matlab. However, after my work, it still doesn't work...
Here is the contents in classpath.txt:
C:\Program Files\MATLAB\R2010a\java\jarext\xstream.jar
C:\Program Files\MATLAB\R2010a\toolbox\javabuilder\jar\win64 \javabuilder.jar
DYNAMIC JAVA PATH
C:\Users\Gao\Desktop\connected_components_labeling
Clearly, the directory is included in the file.
The connected_component_labeling is just a folder on my disk. The classes that I want to use in the connected_components_labeling are: Disjoint_Set.class and Node.class are in the connected_components_labeling folder.
I tried:
x = Disjoint_Set();
also
x = connected_components_labeling.Disjoint_Set();
None of them work. The only feedback I got from matlab is:
??? Undefined variable "connected_components_labeling" or class
"connected_components_labeling.Disjoint_Set".
I'm pretty frustrated. Could anyone help me out? I'd appreciate it. Thanks a ton!

Make sure that you are compiling the java files using a JRE/JDK that MATLAB is compatible with. As far as I can tell, MATLAB does not work properly with Java 7, so stick with Java 6 for the moment...
There are a couple of environment variables that affect MATLAB. In my case I have:
JAVA_HOME = C:\Program Files\Java\jdk1.6.0_32
MATLAB_JAVA = C:\Program Files\Java\jre6
PATH = ...;C:\Program Files\Java\jdk1.6.0_32\bin
Here is a simple test I just did:
C:\work\Student.java
public class Student {
private String name;
public Student(String str) {
name = str;
}
public void setName(String str) {
name = str;
}
public String getName() {
return name;
}
public static void main(String args[]) {
Student s = new Student("amro");
s.setName("unknown");
System.out.println("Hello " + s.getName());
}
}
I compile: javac Student.java (the output is placed in the same directory c:\work\Student.class). Now I test it from MATLAB:
javaaddpath('C:\work')
javaMethod('main','Student','')
s = Student('me')
char( s.getName() )
I get:
Hello unknown
s =
Student#8d6877
ans =
me