Thread: Important

I don't know what wrong with my code. can anyone help me.

Error:
Exception in thread "main" java.lang.NullPointerException
at InToPost$LinkList.deleteFront(InToPost.java:57)
at InToPost.pop(InToPost.java:26)
at InToPost.evaluate(InToPost.java:166)
at InToPost.main(InToPost.java:210)

My Code :

import javax.swing.JOptionPane;
 
public class InToPost {
	private LinkList list;
 
	private String inString;
 
	private String outString = "";
 
	public InToPost myStack;
 
	public InToPost(String in) {
		inString = in;
		myStack = new InToPost();
	}
 
	public InToPost() {
		list = new LinkList();
	}
 
	public void push(char j) {
		list.insertFront(j);
	}
 
	public char pop() {
		return list.deleteFront();
	}
 
	public boolean isEmpty() {
		return (list.isEmpty());
	}
 
	public void displayStack() {
		System.out.print("Stack: ");
		list.displayList();
	}
 
class LinkList {
	private Link head;
 
	public LinkList() {
		head = null;
	}
 
	public boolean isEmpty() {
		return (head == null);
	}
 
	public void insertFront(char d) {
		Link newLink = new Link(d);
		newLink.next = head;
		head = newLink;
	}
 
	public char deleteFront() {
		Link temp = head;
		head = head.next;
		return temp.data;
	}
 
	public void displayList() {
		Link current = head;
		while (current != null) {
			current.displayLink();
			current = current.next;
		}
		System.out.println("");
	}
 
	class Link {
		public char data; // data item
 
		public Link next; // next link in list
 
		public Link(char d) {
			data = d;
		}
 
		public void displayLink() {
			System.out.print(data + " ");
		}
	}
}
public String doTrans() {
	for (int j = 0; j < inString.length(); j++) {
		char ch = inString.charAt(j);
		switch (ch) {
			case '+': 
			case '-':
				gotOper(ch, 1); 
				break; //   (precedence 1)
			case '*': // it's * or /
			case '/':
				gotOper(ch, 2); // go pop operators
				break; //   (precedence 2)
			case '(': // it's a left paren
				myStack.push(ch); // push it
				break;
			case ')': // it's a right paren
				gotParen(ch); // go pop operators
				break;
			default: // must be an operand
				outString = outString + ch; // write it to output
				System.out.println(outString);
				break;
		}
	}
 
	while (!myStack.isEmpty()) {
		outString = outString + myStack.pop();
	}
	System.out.println(outString);
	return outString; // return postfix
}
public void evaluate() {
	char a;
	char b;
	int c;
	int d;
	int total;
	for (int j = 0; j < outString.length(); j++) {
		char result = 0;
		char ch = outString.charAt(j);
		switch (ch) {
			case '+':
				a = myStack.pop();
				c = Character.getNumericValue(a);
				b = myStack.pop();
				d = Character.getNumericValue(b);
				total = c + d;
				result =(char)total;
				myStack.push(result);
				break;
			case '-':
				a = myStack.pop();
				c = Character.getNumericValue(a);
				b = myStack.pop();
				d = Character.getNumericValue(b);
				total = c - d;
				result =(char)total;
				myStack.push(result);
				break;
			case '*':
				a = myStack.pop();
				c = Character.getNumericValue(a);
				b = myStack.pop();
				d = Character.getNumericValue(b);
				total = c * d;
				result =(char)total;
				myStack.push(result);
				break;
			case '/':
				a = myStack.pop();
				c = Character.getNumericValue(a);
				b = myStack.pop();
				d = Character.getNumericValue(b);
				total = c / d;
				result =(char)total;
				myStack.push(result);
				break;
			default:
				myStack.push(ch);
				break;
		}
	}
	System.out.println(myStack.pop());
}
 
public void gotOper(char opThis, int prec1) {
	while (!myStack.isEmpty()) {
		char opTop = myStack.pop();
		if (opTop == '(') {
			myStack.push(opTop);
			break;
		}// it's an operator
		else {// precedence of new op
			int prec2;
			if (opTop == '+' || opTop == '-')
				prec2 = 1;
			else
			prec2 = 2;
			if (prec2 < prec1) // if prec of new operator is less
			{ //    than prec of old
				myStack.push(opTop); // save newly-popped operator
				break;
			} else
			// prec of new operator is not less than prec of old
			outString = outString + opTop; 
		}
	}
	myStack.push(opThis);
}
public void gotParen(char ch){ 
	while (!myStack.isEmpty()) {
		char chx = myStack.pop();
		if (chx == '(') 
			break; 
		else
		outString = outString + chx; 
	}
}
public static void main(String[] args) {
	String inString = JOptionPane.showInputDialog("Enter your expression: ");
	System.out.println(inString);
	String outString;
	InToPost theTrans = new InToPost(inString);
	outString = theTrans.doTrans();
	System.out.println("Postfix is " +outString+ '\n');
	InToPost calculate = new InToPost(outString);
	calculate.evaluate();
	}
}

Exception in thread "main" java.lang.NullPointerException
at InToPost$LinkList.deleteFront(InToPost.java:57)

Look at line 57 in your code. What variable has a null value there? Back track in the code to see why it does NOT have a valid value.
If you can't see what variable, print out the values of all the variables on that line to see which is null

Please edit your code and wrap it in code tags. See: BB Code List - Java Programming Forums or use Go Advanced and use the # icon

i know where is the problem.
but i cant to solve it logically in my mind.
its so much important for me to solve it can anyone help me directly to re: edit my code.