Expression evaluation

app/src/androidTest/java/com/exuberant/calci/ExampleInstrumentedTest.java

@@ -8,7 +8,9 @@
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+
+//import androidx.test.runner.AndroidJUnit4;
 
 /**
  * Instrumented test, which will execute on an Android device.

app/src/main/java/com/exuberant/calci/HomeActivity.java

@@ -7,11 +7,12 @@
 import android.widget.TextView;
 import android.widget.Toast;
 
+import com.exuberant.calci.utility.Expression;
 import com.google.android.material.button.MaterialButton;
 
 public class HomeActivity extends AppCompatActivity implements View.OnClickListener {
 
-    private String currentNumber = "", totalCalculation = "";
+    private String currentEquation = "", totalCalculation = "";
     private TextView totalCalculationTextView, currentAnswerTextView;
 
     @Override
@@ -46,67 +47,35 @@ public void onClick(View view) {
         MaterialButton button = (MaterialButton) view;
         int id = button.getId();
         switch (id){
-            //Handling all operators
-            case R.id.btn_addition :
-            case R.id.btn_division :
-            case R.id.btn_multiplication :
-            case R.id.btn_subtraction :
-                handleOperatorClick(button.getText().toString());
-                break;
-
             //Handling clear button
             case R.id.btn_clear:
-                currentNumber = "";
+                currentEquation = "";
                 totalCalculation = "";
                 break;
 
             //Handle calculation
             case R.id.btn_equals:
-                totalCalculation += currentNumber;
+                //totalCalculation += currentNumber;
                 calculateAnswer();
                 break;
 
-            //Handle other numerical button clicks
+            //Handle other button clicks
             default:
-                currentNumber += button.getText().toString();
+                currentEquation += button.getText().toString();
         }
         updateDisplay();
     }
 
     private void updateDisplay(){
         totalCalculationTextView.setText(totalCalculation);
-        currentAnswerTextView.setText(currentNumber);
-    }
-
-    private void handleOperatorClick(String operator){
-        if (!(currentNumber.equals("") || currentNumber.length() == 0)) {
-            totalCalculation += currentNumber + operator;
-            currentNumber = "";
-        } else {
-            totalCalculation = totalCalculation.substring(0, totalCalculation.length() - 1);
-            totalCalculation += operator;
-        }
-    }
-
-    private double add(double a, double b){
-        return a + b;
-    }
-
-    private double sub(double a, double b){
-        return  a - b;
-    }
-
-    private double mul(double a, double b){
-        return  a * b;
-    }
-
-    private double div(double a, double b){
-        return a / b;
+        currentAnswerTextView.setText(currentEquation);
     }
 
     private void calculateAnswer(){
         //Use totalCalculation string to get final answer and display it
-        double answer = 0.0;
+        double answer = Expression.evaluateExpression(currentEquation);
+        totalCalculation = currentEquation;
+        currentEquation = String.valueOf(answer);
         updateDisplay();
     }
 }

app/src/main/java/com/exuberant/calci/utility/Expression.java

@@ -0,0 +1,111 @@
+package com.exuberant.calci.utility;
+
+import java.util.Stack;
+
+public class Expression {
+
+    public static double evaluateExpression(String expression) {
+        expression = infixToPostfix(expression);
+        String[] inputs = expression.split(" ");
+
+        Stack<Double> stack = new Stack<>();
+
+        for(int i=0; i<inputs.length; i++) {
+            if( isNumeric(inputs[i]) )
+                stack.push( Double.parseDouble(inputs[i]) );
+            else {
+                double num2 = stack.pop();
+                double num1 = stack.pop();
+
+                double interAns;
+                switch( inputs[i] ) {
+                    case "+":
+                        interAns = num1 + num2;
+                        break;
+                    case "-":
+                        interAns = num1 - num2;
+                        break;
+                    case "x":
+                        interAns = num1 * num2;
+                        break;
+                    case "/":
+                        interAns = num1 / num2;
+                        break;
+                    default:
+                        interAns = 0;
+                }
+
+                stack.push(interAns);
+            }
+        }
+
+        return stack.pop();
+    }
+
+    private static String infixToPostfix(String expression) {
+        StringBuilder result = new StringBuilder();
+        Stack<Character> stack = new Stack<>();
+
+        for(int i=0; i<expression.length(); i++) {
+            char c = expression.charAt(i);
+
+            if(c == ' ') continue;
+
+            if( Character.isDigit(c) || c == '.' ) {
+                result.append(c);
+            }
+
+            else if(c == '(')
+                stack.push(c);
+
+            else if(c == ')') {
+                result.append(' ');
+                while(!stack.isEmpty() && stack.peek() != '(')
+                    result.append(stack.pop());
+
+                if( !stack.isEmpty() && stack.peek() != '(' )
+                    throw new IllegalStateException();
+                else stack.pop();
+            }
+
+            else {
+                while(!stack.isEmpty() && priority(c) <= priority(stack.peek())) {
+                    if(stack.peek() == '(') throw new IllegalStateException();
+                    result.append(' ');
+                    result.append(stack.pop());
+                }
+                result.append(' ');
+                stack.push(c);
+            }
+        }
+
+        while(!stack.isEmpty()) {
+            if(stack.peek() == '(') throw new IllegalStateException();
+            result.append(' ');
+            result.append(stack.pop());
+        }
+
+        return result.toString().replaceAll("\\s+", " ");
+    }
+
+    private static boolean isNumeric(String input) {
+        try {
+            Double.parseDouble(input);
+            return true;
+        } catch(NumberFormatException ex ) {
+            return false;
+        }
+    }
+
+    private static int priority(char c) {
+        switch(c) {
+            case '+':
+            case '-':
+                return 1;
+            case 'x':
+            case '/':
+                return 2;
+        }
+        return -1;
+    }
+}

app/src/main/res/layout/content_number_board.xml

@@ -85,7 +85,7 @@
         app:layout_constraintTop_toTopOf="@+id/btn_eight" />
 
     <com.google.android.material.button.MaterialButton
-        android:text="X"
+        android:text="x"
         android:textColor="@android:color/white"
         android:backgroundTint="@color/colorOperatorKey"
         android:id="@+id/btn_multiplication"