Thread: Help with infix to postfix problem.

The assignment is to change an input that is infix into postfix and then output it. I am supposed to implement a stack, which I have, in order to accomplish this. The final solution will have a file as an input, but for practice I've decided to try it by writing in an input first. I think I have the Stack class down, at least for now, but its the main method I'm currently having trouble with. When I try to run my program, this is the error I get from the compiler:

Exception in thread "main" java.lang.NoClassDefFoundError: javaprogram2/Stack
	at javaprogram2.JavaProgram2.main(JavaProgram2.java:24)
Caused by: java.lang.ClassNotFoundException: javaprogram2.Stack
	at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:356)

And this is my current code:

public class JavaProgram2 {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args)throws Exception {
        String s;
        BufferedReader br= new BufferedReader(new InputStreamReader(System.in));
        Stack one = new Stack();
        System.out.println("Enter input string.");
        s = br.readLine();
        System.out.println("Input string: " + s);
        System.out.println("Output string: ");
        one.postFix(s);
    }
 
}

public class Stack {
    char stackChar[]= new char[20];
    int top;
 
 
    public void push(char ch){
        top++;
        stackChar[top]=ch;
    }
 
    public char pop(){
        char ch;
        ch=stackChar[top];
        top--;
        return ch;
    }
 
    public int pre(char ch){
        switch(ch){
            case'-': return 1;
            case'+': return 1;
            case'*': return 2;
            case'/': return 2;
        }
        return 0;
    }
 
    public boolean operator(char ch){
        if(ch=='-' || ch=='+' || ch=='*' || ch=='/') {
            return true;
        }
        else {
            return false;
        }
    }
 
    public boolean isAlpha(char ch){
        if(ch>='a' && ch<='z'|| ch>='0' && ch=='9') {
            return true;
        }
        else {
            return false;
        }
    }
 
    public void postFix(String str){
        char output[] = new char[str.length()];
        char ch;
        int t = 0;
        int i;
        int j;
 
        for(i=0; i<str.length(); i++){
            ch = str.charAt(i);
            if(ch=='('){
                push(ch);
            }
            else if(isAlpha(ch)){
                output[t++]=ch;
            }
            else if(operator(ch)){
                if(stackChar[top]== 0 || (pre(ch)>pre(stackChar[top]))||
                        stackChar[top]=='('){
                    push(ch);
                }
            }
            else if(pre(ch)<=pre(stackChar[top])){
                output[t++]=pop();
                push(ch);
            }
            else if(ch=='('){
                while((ch=pop())!='('){
                    output[t++]=ch;
                }
            }
        }
        while(top!=0){
            output[t++]=pop();
        }
        for(j=0; j<str.length(); j++){
            System.out.print(output[j]);
        }
    }
 
}

I asked my teacher for help, and she recommended that I grab a skeleton online and try to fill it in with the methods for the stack, which I did. Any help with understanding whats wrong would be greatly appreciated.