A Simple Android Application For Simple Calculator


Open main.xml and draw the screen:

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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/linearLayout1"
        android:layout_marginLeft="10pt"
        android:layout_marginRight="10pt"
        android:layout_marginTop="3pt">
        <EditText
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:layout_marginRight="5pt"
            android:id="@+id/etNum1"
            android:layout_width="match_parent"
            android:inputType="numberDecimal">
        </EditText>
        <EditText
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:layout_marginLeft="5pt"
            android:id="@+id/etNum2"
            android:layout_width="match_parent"
            android:inputType="numberDecimal">
        </EditText>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/linearLayout2"
        android:layout_marginTop="3pt"
        android:layout_marginLeft="5pt"
        android:layout_marginRight="5pt">
        <Button
            android:layout_height="wrap_content"
            android:layout_width="match_parent"
            android:layout_weight="1"
            android:text="+"
            android:textSize="8pt"
            android:id="@+id/btnAdd">
        </Button>
        <Button
            android:layout_height="wrap_content"
            android:layout_width="match_parent"
            android:layout_weight="1"
            android:text="-"
            android:textSize="8pt"
            android:id="@+id/btnSub">
        </Button>
        <Button
            android:layout_height="wrap_content"
            android:layout_width="match_parent"
            android:layout_weight="1"
            android:text="*"
            android:textSize="8pt"
            android:id="@+id/btnMult">
        </Button>
        <Button
            android:layout_height="wrap_content"
            android:layout_width="match_parent"
            android:layout_weight="1"
            android:text="/"
            android:textSize="8pt"
            android:id="@+id/btnDiv">
        </Button>
    </LinearLayout>
    <TextView
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:layout_marginLeft="5pt"
        android:layout_marginRight="5pt"
        android:textSize="12pt"
        android:layout_marginTop="3pt"
        android:id="@+id/tvResult"
        android:gravity="center_horizontal">
    </TextView>
</LinearLayout>


Open MainActivity.java and write the code:

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public class MainActivity extends Activity implements OnClickListener {
 
  EditText etNum1;
  EditText etNum2;
 
  Button btnAdd;
  Button btnSub;
  Button btnMult;
  Button btnDiv;
 
  TextView tvResult;
 
  String oper = "";
 
  /** Called when the activity is first created. */
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
 
    // find the elements
    etNum1 = (EditText) findViewById(R.id.etNum1);
    etNum2 = (EditText) findViewById(R.id.etNum2);
 
    btnAdd = (Button) findViewById(R.id.btnAdd);
    btnSub = (Button) findViewById(R.id.btnSub);
    btnMult = (Button) findViewById(R.id.btnMult);
    btnDiv = (Button) findViewById(R.id.btnDiv);
 
    tvResult = (TextView) findViewById(R.id.tvResult);
 
    // set a listener
    btnAdd.setOnClickListener(this);
    btnSub.setOnClickListener(this);
    btnMult.setOnClickListener(this);
    btnDiv.setOnClickListener(this);
 
  }
 
  @Override
  public void onClick(View v) {
    // TODO Auto-generated method stub
    float num1 = 0;
    float num2 = 0;
    float result = 0;
 
    // check if the fields are empty
    if (TextUtils.isEmpty(etNum1.getText().toString())
        || TextUtils.isEmpty(etNum2.getText().toString())) {
      return;
    }
 
    // read EditText and fill variables with numbers
    num1 = Float.parseFloat(etNum1.getText().toString());
    num2 = Float.parseFloat(etNum2.getText().toString());
 
    // defines the button that has been clicked and performs the corresponding operation
    // write operation into oper, we will use it later for output
    switch (v.getId()) {
    case R.id.btnAdd:
      oper = "+";
      result = num1 + num2;
      break;
    case R.id.btnSub:
      oper = "-";
      result = num1 - num2;
      break;
    case R.id.btnMult:
      oper = "*";
      result = num1 * num2;
      break;
    case R.id.btnDiv:
      oper = "/";
      result = num1 / num2;
      break;
    default:
      break;
    }
 
    // form the output line
    tvResult.setText(num1 + " " + oper + " " + num2 + " = " + result);
  }
}

Let’s extend the functionality by making menu with clearing fields and exit items. Items will be named Reset and Quit.
Add two constants - these will be IDs of the menu items.
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public class MainActivity extends Activity implements OnClickListener {
 
final int MENU_RESET_ID = 1;
 final int MENU_QUIT_ID = 2;
 
 EditText etNum1;
(add only the highlighted code)

And write code for menu creation and processing:
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// menu creation
@Override
public boolean onCreateOptionsMenu(Menu menu) {
  // TODO Auto-generated method stub
  menu.add(0, MENU_RESET_ID, 0, "Reset");
  menu.add(0, MENU_QUIT_ID, 0, "Quit");
  return super.onCreateOptionsMenu(menu);
}
 
// process menu item clicks
@Override
public boolean onOptionsItemSelected(MenuItem item) {
  // TODO Auto-generated method stub
  switch (item.getItemId()) {
  case MENU_RESET_ID:
    // clear the fields
    etNum1.setText("");
    etNum2.setText("");
    tvResult.setText("");
    break;
  case MENU_QUIT_ID:
    // exit the application
    finish();
    break;
  }
  return super.onOptionsItemSelected(item);
}

Save everything and run. Two menu items appeared:
Reset - clears all the fields
Quit - closes the application

























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