I can\'t Figure out why my program not working. When I run it shows Usage: java

Question

I can't Figure out why my program not working. When I run it shows Usage: java EvaluateExpression "expression"

import java.util.Stack;

public class EvaluateExpression {
  

public static void main(String[] args) {
  
// Check number of arguments passed
if (args.length != 1) {
System.out.println(
"Usage: java EvaluateExpression "expression"");
System.exit(1);
}

try {
System.out.println(evaluateExpression(args[0]));
}
catch (Exception ex) {
System.out.println("Wrong expression: " + args[0]);
}
}

/** Evaluate an expression */
public static int evaluateExpression(String expression) {
// Create operandStack to store operands
Stack<Integer> operandStack = new Stack<Integer>();
  
// Create operatorStack to store operators
Stack<Character> operatorStack = new Stack<Character>();
  
// Insert blanks around (, ), +, -, /, and *
expression = insertBlanks(expression);

// Extract operands and operators
String[] tokens = expression.split(" ");

// Phase 1: Scan tokens
for (String token: tokens) {
if (token.length() == 0) // Blank space
continue; // Back to the while loop to extract the next token
else if (token.charAt(0) == '+' || token.charAt(0) == '-') {
// Process all +, -, *, / in the top of the operator stack
while (!operatorStack.isEmpty() &&
(operatorStack.peek() == '+' ||
operatorStack.peek() == '-' ||
operatorStack.peek() == '*' ||
operatorStack.peek() == '/'||
operatorStack.peek()=='^'||operatorStack.peek()=='%')){
processAnOperator(operandStack, operatorStack);
  
}

// Push the + or - operator into the operator stack
operatorStack.push(token.charAt(0));
  
}
else if (token.charAt(0) =='^') {
  
operatorStack.push(token.charAt(0));
  
}
else if (token.charAt(0) == '*' || token.charAt(0) == '/'||token.charAt(0)=='%') {
// Process all *, /,% in the top of the operator stack
while (!operatorStack.isEmpty() &&
(operatorStack.peek() == '*' ||
operatorStack.peek() == '/'||
operatorStack.peek()=='%'||operatorStack.peek()=='^')) {
processAnOperator(operandStack, operatorStack);
  
}

// Push the * or / operator into the operator stack
operatorStack.push(token.charAt(0));
}
else if (token.trim().charAt(0) == '(') {
operatorStack.push('('); // Push '(' to stack
}
else if (token.trim().charAt(0) == ')') {
// Process all the operators in the stack until seeing '('
while (operatorStack.peek() != '(') {
processAnOperator(operandStack, operatorStack);
}
  
operatorStack.pop(); // Pop the '(' symbol from the stack
}
else { // An operand scanned
// Push an operand to the stack
operandStack.push(new Integer(token));
}
}

// Phase 2: process all the remaining operators in the stack
while (!operatorStack.isEmpty()) {
processAnOperator(operandStack, operatorStack);
}

// Return the result
return operandStack.pop();
  
}

/** Process one operator: Take an operator from operatorStack and
* apply it on the operands in the operandStack */
public static void processAnOperator(
  
Stack<Integer> operandStack, Stack<Character> operatorStack) {
char op = operatorStack.pop();
int op1 = operandStack.pop();
int op2 = operandStack.pop();
if (op == '+')
operandStack.push(op2 + op1);
else if (op == '-')
operandStack.push(op2 - op1);
else if (op == '*')
operandStack.push(op2 * op1);
else if (op == '/')
operandStack.push(op2 / op1);
else if(op=='%')
operandStack.push(op2%op1);
else if(op=='^'){
operandStack.push((int)Math.pow((double)op2,(double)op1));}
}
  
public static String insertBlanks(String s) {
String result = "";
  
for (int i = 0; i < s.length(); i++) {
if (s.charAt(i) == '(' || s.charAt(i) == ')' ||
s.charAt(i) == '+' || s.charAt(i) == '-' ||
s.charAt(i) == '*' || s.charAt(i) == '/'||
s.charAt(i)=='%' || s.charAt(i)=='^')
result += " " + s.charAt(i) + " ";
else
result += s.charAt(i);
}
  
return result;
  
}
}

Explanation / Answer

The string array args[] contains the parameters that we pass as command-line-arguments with the program name. For example, if we run, <program_file_name> <argument1> <argument2>, args[] will contain ["<argument1>", "<arguement2"].

In your case, when you're running the program, you're not passing any arguments, which means that the array args[] is empty i.e. the length of array args[] is 0.

In the program, if the length of args[] is not 1, then the program exits with the message "Usage: java EvaluateExpression expression" in the print statement and that is exactly what is happening here.

If you want to check for no arguments, then change if(args.length != 1) to if(args.length != 0) and you'll not face this problem.

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