change how your interact with Parser

Calculator.java

@@ -1,5 +1,6 @@
 public class Calculator {
     public static void main(String[] args) {
+        GFrame frame = new GFrame("Calculator");
         String expr = "(84 / 4 * 3 - 9) * 2 + 1 / 5"; // 108.2
         Parser p;
         if (args.length == 0) {
@@ -7,8 +8,7 @@ public class Calculator {
         } else {
             p = new Parser(args[0]);
         }
-        String postfix = p.toPostFix();
-        // System.out.println("pfix => \t " + postfix);
-        System.out.println(p.evalExpr(postfix));
+        System.out.println("postfix => \t " + p.getPostfix());
+        System.out.println(p.eval());
     }
 }

GFrame.java

@@ -0,0 +1,54 @@
+import javax.swing.*;
+import java.awt.Color;
+import java.awt.Dimension;
+import java.awt.event.*;
+
+public class GFrame extends JFrame {
+    Parser p;
+
+    GFrame(String title) {
+        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
+        this.setTitle(title);
+        this.getContentPane().setBackground(Color.decode("#4E586e"));
+        JButton b = new JButton();
+        b.setBounds(130, 100, 100, 40);
+        b.setBackground(Color.decode("#F78361"));
+        b.setText("Evaluate");
+
+        JTextField tf = new JTextField();
+        tf.setBounds(130, 50, 220, 40);
+
+        JButton bClear = new JButton();
+        bClear.setBounds(250, 100, 100, 40);
+        bClear.setBackground(Color.decode("#F78361"));
+        bClear.setText("Clear");
+
+        this.add(b);
+        this.add(bClear);
+        this.add(tf);
+        this.setSize(400, 500);
+        this.setLayout(null);
+        this.setVisible(true);
+
+        JLabel label = new JLabel("Enter Expression: ");
+        label.setBounds(130, 20, 150, 40);
+        this.add(label);
+
+        b.addActionListener(new ActionListener() {
+            public void actionPerformed(ActionEvent e) {
+                if (tf.getText().length() != 0) {
+                    p = new Parser(tf.getText());
+                    tf.setText(Double.toString(p.eval()));
+                } else {
+                    tf.setText("No input");
+                }
+            }
+        });
+
+        bClear.addActionListener(new ActionListener() {
+            public void actionPerformed(ActionEvent e) {
+                tf.setText("");
+            }
+        });
+    }
+}

Parser.java

@@ -8,13 +8,15 @@ import java.util.StringJoiner;
 
 public class Parser {
     private String expr;
+    private String postfix;
     private Stack<Character> operatorStack;
 
     public Parser(String infixExpr) {
         expr = infixExpr.trim().replaceAll("\\s", ""); // remove whitespaces
+        postfix = toPostFix();
     }
 
-    public static boolean isOperand(char c) {
+    private boolean isOperand(char c) {
         switch (c) {
             case '0':
             case '1':
@@ -32,11 +34,11 @@ public class Parser {
         }
     }
 
-    public static boolean isOperand(String c) {
+    private boolean isOperand(String c) {
         return isOperand(c.charAt(0));
     }
 
-    public int getPresedence(char c) {
+    private int getPresedence(char c) {
         switch (c) {
             case '^':
                 return 3;
@@ -53,7 +55,7 @@ public class Parser {
         }
     }
 
-    public boolean hasLeftAssociativity(char c) {
+    private boolean hasLeftAssociativity(char c) {
         if (c == '^') {
             return false;
         } else {
@@ -61,7 +63,7 @@ public class Parser {
         }
     }
 
-    public String toPostFix() {
+    private String toPostFix() {
         operatorStack = new Stack<>();
         StringJoiner output = new StringJoiner(" ");
         StringBuilder operand = new StringBuilder();
@@ -136,7 +138,7 @@ public class Parser {
         }
     }
 
-    public double evalExpr(String postfix) {
+    public double eval() {
         Stack<Double> stack = new Stack<Double>();
         for (String c : postfix.split(" ")) {
             if (isOperand(c)) {
@@ -149,4 +151,8 @@ public class Parser {
         }
         return stack.pop();
     }
+
+    public String getPostfix() {
+            return postfix;
+    }
 }