[for statement] I can't assign a value correctly

[Matthew Park]

Hello. I'm new to Java and English.
This program is for calculating a student's GPA with given grade (e.g., A, B, C, D, and F) entered by user.
But according to my code, I wonder why student's GPA is 0.0 regardless of any inputs.
I tried many times and found there's a problem when assigning a varible 'sum'.
I think it's value is always zero.
The problem is, I can't find the way to modify it correctly.
Could somebody help me find what is the problem and how can I mend it?

Here's my code. Variable 'sum' appears in the for statement.

import javax.swing.*;

class MultipleStudentGPA {

	public static void main ( String[] args ) {

		String name, grade, studentNumStr, countStr;

		int countSubject, sum, studentNum;

		double avg;

		studentNumStr = JOptionPane.showInputDialog(null, "=GPA Calculator=\nHow many students you want to compute GPA?");
		studentNum = Integer.parseInt(studentNumStr);

		for (int i = 0; i < studentNum; i++) {

		name = JOptionPane.showInputDialog(null, "=GPA Calculator=\nEnter a name of the student.");

		if (name == "0")
			System.exit(0);
		
		countStr = JOptionPane.showInputDialog(null, "=GPA Calculator=\nHow many subject did he/she take? (max.12)");
		countSubject = Integer.parseInt(countStr);

		for(i = sum = 0; i < countSubject; i++) {
			grade = JOptionPane.showInputDialog(null, "=GPA Calculator=\nEnter a grade.");
			if (grade == "A")
				sum += 4;
			else if (grade == "B")
				sum += 3;
			else if (grade == "C")
				sum += 2;
			else if (grade == "D")
				sum += 1;
		}

		avg = sum / countSubject;

		JOptionPane.showMessageDialog(null, "=GPA Calculator=\nName : " + name + "\nGPA : " + avg);

		}
	}
}

[K. Tsang]

There's 2 problems I see. First you are using == to check a string. You should uses equals() or equalsIgnoreCase(). If you change this, your sum will no longer be 0 at least.

The 2nd problem is the avg, when you input "A" then "B" you should get (4+3)/2=3.5 but actually got 3.0? And yes you defined it as double. Can you figure out how to get the proper result?

[Matthew Park]

K. Tsang wrote:There's 2 problems I see. First you are using == to check a string. You should uses equals() or equalsIgnoreCase(). If you change this, your sum will no longer be 0 at least.

The 2nd problem is the avg, when you input "A" then "B" you should get (4+3)/2=3.5 but actually got 3.0? And yes you defined it as double. Can you figure out how to get the proper result?

Wonderful! I solved 2 problems you mentioned above.

About typecasting, I absoulutely forgot! Thank you for reminding me.