Calculator

Draw the interface as

Add respective onClick events to each button.
And in Main Activity..
    Declare following variables:

    int vFirstNum=0,vSecondNum=0,vResult=0;
    int vTemp=0;
    String vOperation="";

    and then for each digit button click:

 public void btn0_Click(View view){
    EditText editText=(EditText)findViewById(R.id.editText1);
    vTemp=Integer.parseInt(editText.getText().toString());
        if(vTemp==0)
    editText.setText("0");
    else if(vTemp<=999999)
    editText.setText(editText.getText()+"0");
    }

For operation button click:

 public void btnPlus_Click(View view){
    vOperation="Add";
        EditText editText=(EditText)findViewById(R.id.editText1);
    vFirstNum=Integer.parseInt(editText.getText().toString());
        editText.setText("0");
    } 

For Clear Button:

public void btnC_Click(View view){
    EditText editText=(EditText)findViewById(R.id.editText1);
    editText.setText("0");
    } 

and for the "=" button:

 public void btnResult_Click(View view){
    EditText editText=(EditText)findViewById(R.id.editText1);
    vSecondNum=Integer.parseInt(editText.getText().toString());
        if(vOperation=="Add")
    vResult=vFirstNum+vSecondNum;
        else if(vOperation=="Multiply")
    vResult=vFirstNum*vSecondNum;
        else if(vOperation=="Divide")
    vResult=vFirstNum/vSecondNum;
        else if(vOperation=="Minus")
    vResult=vFirstNum-vSecondNum;
        String vMessage=""+vResult;
    editText.setText(vMessage);
    vFirstNum=0;
    vSecondNum=0;
    vResult=0;
    vOperation="";
    } 

yeah, that's it and ur handy calculator is ready.