Question

JAVA programming

The task of parsing a file for correctness is an essential part of any programmer toolkit. The check for a
balanced set of brackets in a file a Stack can be used and the following algorithm:
• The source file is read character by character
• Each time an opening '{' is read it is pushed onto the stack
• Each time a closing '}' is read the stack is popped
• If the stack is empty when a closing '}' is read then there must be a missing '{'
• If the stack is not empty at the end of the file then there must be a missing closing '}'

Create the following class.
1. BracketChecker - this class contains:
i. a single private instance variable of type Stack<Character> and initialise it to new
Stack<Character>()
ii. a single method check that takes a single formal parameter of type String and returns a
boolean, i.e.

public boolean check(String text) { return false; }
2. Modify the main method so that it:
i. Prints the message "Starting Bracket Checker application"
ii. Defines a variable called checker of type BracketChecker, initialised to a new
BracketChecker object.
iii. Defines a variable called in of type Scanner, initialised to new Scanner(System.in). This
will allow input from the keyboard.
iv. Calls in.nextLine() and passing the result to the check method of checker
v. Prints the result of the method call.
1. if it is true then "Syntax Correct"
2. if it is false then "Syntax Error"
Run the project. The following output should be produced (example input, {{}} is shown in bold below):
Starting Bracket Checker application
{{}}
Syntax Error
NOTE: this program will always print Syntax Error as the stub method check always returns false

The Tasks
1. Fully Implement the check method in BracketChecker so that it examines each character (hint:
use .charAt(int index)) in the String passed as a parameter
i. if it is an opening '{' push it onto the stack
ii. if it is a closing '}'
▪ and the stack is empty, then return false
▪ pop the stack, .pop()
iii. any other character is ignored (not pushed onto the stack)
iv. when the end of the string is reached
▪ if the stack is empty, then return true
▪ if the stack is not empty, then return false
2. Add a while loop to main method in Practical03.java so that multiple lines can entered for
checking
3. Add a print statement at the end stating Exiting Checker
4. Run the application. The following output should be produced (example input is indicated in bold):
Starting Bracket Checker application
{{}}
Syntax Correct
{{}
Syntax Error
{"Hello{}World"}
Syntax Correct
Exiting Checker

Answer 1

CODE:

import java.util.*;
class BracketChecker
{
    private Stack<Character> stack = new Stack<Character>();
    public boolean check(String text)
    {
        for(int i=0;i<text.length();i++)
        {
            if(text.charAt(i)=='}')
            {
                if(stack.empty())
                    return false;
                else
                    stack.pop();
            }
            if(text.charAt(i)=='{')
                stack.push('{');
        }
        if(stack.empty())
            return true;
        else
            return false;
    }
    public static void main(String args[])
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("Starting Bracket Checker Application");
        while(true)
        {
            String ip = sc.nextLine();
            if(ip.equals("")) // If the user does not enter any string or empty string then break the loop
            {
                System.out.println("Exiting Checker");
                break;
            }
            BracketChecker ob = new BracketChecker();
            if(ob.check(ip))
                System.out.println("Syntax Correct");
            else
                System.out.println("Syntax Error");
        }
    }
    
}

OUTPUT: