Tried to add functionality to operator buttons, but with the exceptio…

…n of sqrt, it doesn't work.

app/src/main/java/com/alexlowe/calculator/MainActivity.java

@@ -2,18 +2,17 @@
 
 import android.support.v7.app.ActionBarActivity;
 import android.os.Bundle;
-import android.view.Menu;
-import android.view.MenuItem;
 import android.view.View;
-import android.widget.Button;
 import android.widget.TextView;
 
 
 public class MainActivity extends ActionBarActivity {
 
     String screenString = "";
     TextView screen;
-
+    Double number1 = null;
+    Double number2 = null;
+    String operator;
 
 
     @Override
@@ -76,15 +75,133 @@ public void press0(View view) {
     }
 
     public void pressPoint(View view) {
-        screenString += ".";
+        if (!screenString.contains(".")) {
+            screenString += ".";
+            displayScreen(screenString);
+        }
+    }
+
+    public void pressBack(View view) {
+        if (screenString.length() > 0) {
+            screenString = screenString.substring(0, screenString.length() - 1);
+        }
         displayScreen(screenString);
     }
 
     public void pressAC(View view) {
+        number1 = null;
+        number2 = null;
+        operator = "";
         screenString = "";
         displayScreen(screenString);
     }
 
+    public void pressSqrt(View view) {
+        if (!screenString.equals("")) {
+            number1 = Math.sqrt(Double.parseDouble(screenString));
+            screenString = String.valueOf(number1);
+            operator = "SR";
+            displayScreen(screenString);
+        }
+    }
+
+    public void pressPercent(View view) {
+        operation("%");
+    }
+
+    public void pressPlus(View view) {
+        operation("+");
+        /* --Old Code
+        if(screenString != "") {
+            if(number1 != null && number2 == null){
+                number2 = Double.parseDouble(screenString);
+                operator = "+";
+                Double result = calculate(number1, number2, operator);
+                screenString = String.valueOf(result);
+                displayScreen(screenString);
+            }else{
+                number1 = Double.parseDouble(screenString);
+                number2 = null;
+                operator = "+";
+                screenString = "";
+                displayScreen(screenString);
+            }
+        }*/
+    }
+
+    public void pressMinus(View view) {
+        operation("-");
+    }
+
+    public void pressMult(View view) {
+        operation("*");
+    }
+
+
+    public void pressDiv(View view) {
+        operation("/");
+    }
+
+    public void pressEqual(View view) {
+        //pick up here, trying to get plus operation to work, switch statment for operator, check for value in holder and screen
+        //moved operation to calculate method instead of in each operation button's method
+        if (!operator.equals("") && !screenString.equals("")) {
+            if (operator.equals("SR")) {
+                pressSqrt(view);
+            } else {
+                number2 = Double.parseDouble(screenString);
+                number1 = calculate(number1, number2, operator);
+                screenString = String.valueOf(number1);
+            }
+        }
+    }
+
+    private Double calculate(Double num1, Double num2, String operator) {
+        Double result = 0.0;
+        switch (operator) {
+            case "+":
+                result = num1 + num2;
+                break;
+            case "-":
+                result = num1 - num2;
+                break;
+            case "*":
+                result = num1 * num2;
+                break;
+            case "/":
+                result = num1 / num2;
+                break;
+            case "%":
+                result = (num1 / 100) * num2;
+                break;
+        }
+        return result;
+    }
+
+    private void operation(String op) {
+        if (!screenString.equals("")) {
+            if (number1 != null && number2 == null) {
+
+                number2 = Double.parseDouble(screenString);
+                operator = op;
+
+                screenString = "";
+                displayScreen(screenString);
+
+                number1 = calculate(number1, number2, operator);
+                number2 = null;
+            } else {
+
+                number1 = Double.parseDouble(screenString);
+                number2 = null;
+                operator = op;
+
+                screenString = "";
+                displayScreen(screenString);
+            }
+        }
+    }
+
     private void displayScreen(String screenString) {
         screen.setText(screenString);
     }

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

@@ -12,6 +12,7 @@
         android:text="0"
         android:layout_width="match_parent"
         android:layout_height="150dp"
+        android:maxLength="12"
         android:id="@+id/screen"/>
 
     <GridLayout
@@ -38,6 +39,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="%"
+        android:onClick="pressPercent"
         android:id="@+id/buttonPercent"
         android:singleLine="true"
         />
@@ -47,6 +49,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="/"
+        android:onClick="pressDiv"
         android:id="@+id/buttonDivide"
         android:singleLine="true"
        />
@@ -57,6 +60,7 @@
         android:layout_height="wrap_content"
         android:text="&lt;"
         android:id="@+id/buttonBackspace"
+        android:onClick="pressBack"
         android:singleLine="true"
         />
 
@@ -95,6 +99,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="x"
+        android:onClick="pressMult"
         android:id="@+id/buttonMult"
         android:singleLine="true"
         />
@@ -133,6 +138,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="-"
+        android:onClick="pressMinus"
         android:id="@+id/buttonSub"
         android:singleLine="true"
        />
@@ -172,6 +178,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="+"
+        android:onClick="pressPlus"
         android:id="@+id/buttonPlus"
         android:singleLine="true"
         />
@@ -202,6 +209,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="sr"
+        android:onClick="pressSqrt"
         android:id="@+id/buttonSquareRoot"
         android:singleLine="true"
        />
@@ -211,6 +219,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="="
+        android:onClick="pressEqual"
         android:id="@+id/buttonEqual"
         android:singleLine="true"
         />