Android Simple Calculator

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:text=""
        android:textAppearance="?android:attr/textAppearanceLarge" />

    <Button
        android:id="@+id/Button04"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/Button03"
        android:layout_below="@+id/Button03"
        android:layout_marginTop="18dp"
         android:onClick="onClickBtn"
        android:text="*" />

    <Button
        android:id="@+id/Button06"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button04"
        android:layout_alignBottom="@+id/Button04"
        android:layout_toLeftOf="@+id/Button01"
         android:onClick="onClickBtn"
        android:text="4" />

    <Button
        android:id="@+id/Button12"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/Button11"
        android:layout_below="@+id/Button11"
        android:layout_marginTop="16dp"
         android:onClick="onClickBtn"
        android:text="+" />

    <Button
        android:id="@+id/Button01"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/button1"
        android:layout_alignBottom="@+id/button1"
        android:layout_marginLeft="18dp"
        android:layout_toRightOf="@+id/button1"
         android:onClick="onClickBtn"
        android:text="8" />

    <Button
        android:id="@+id/Button02"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button01"
        android:layout_alignBottom="@+id/Button01"
        android:layout_marginLeft="14dp"
        android:layout_toRightOf="@+id/Button01"
         android:onClick="onClickBtn"
        android:text="9" />

    <Button
        android:id="@+id/Button03"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button02"
        android:layout_alignBottom="@+id/Button02"
        android:layout_marginLeft="17dp"
        android:layout_toRightOf="@+id/Button02"
         android:onClick="onClickBtn"
        android:text="/" />

    <Button
        android:id="@+id/Button05"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button04"
        android:layout_alignBottom="@+id/Button04"
        android:layout_toLeftOf="@+id/Button03"
         android:onClick="onClickBtn"
        android:text="6" />

    <Button
        android:id="@+id/Button14"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button12"
        android:layout_alignBottom="@+id/Button12"
        android:layout_toLeftOf="@+id/Button03"
         android:onClick="onClickBtn"
        android:text="=" />

    <Button
        android:id="@+id/Button15"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button14"
        android:layout_alignBottom="@+id/Button14"
        android:layout_toLeftOf="@+id/Button02"
         android:onClick="onClickBtn"
        android:text="0" />

    <Button
        android:id="@+id/Button13"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button15"
        android:layout_alignBottom="@+id/Button15"
        android:layout_toLeftOf="@+id/Button01"
         android:onClick="onClickBtn"
        android:text="&lt;-" />

    <Button
        android:id="@+id/Button07"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/Button05"
        android:layout_alignBottom="@+id/Button05"
        android:layout_toLeftOf="@+id/Button02"
         android:onClick="onClickBtn"
        android:text="5" />

    <Button
        android:id="@+id/Button11"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/Button04"
        android:layout_below="@+id/Button04"
        android:layout_marginTop="20dp"
         android:onClick="onClickBtn"
        android:text="-" />

    <Button
        android:id="@+id/Button10"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/Button12"
        android:layout_alignLeft="@+id/Button14"
         android:onClick="onClickBtn"
        android:text="3" />

    <Button
        android:id="@+id/Button08"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/Button12"
        android:layout_toLeftOf="@+id/Button02"
         android:onClick="onClickBtn"
        android:text="2" />

    <Button
        android:id="@+id/Button09"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/Button12"
        android:layout_toLeftOf="@+id/Button01"
         android:onClick="onClickBtn"
        android:text="1" />

    <Button
        android:id="@+id/button1"
        style="?android:attr/buttonStyleSmall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_below="@+id/textView1"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="50dp"
         android:onClick="onClickBtn"
        android:text="7" />

</RelativeLayout>

MainActivity.java

package com.example.calc;


import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {
	String symbol = "";
	float no1 = 0;
	float no2 = 0;
	Button btn;
	TextView display;
	
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        display = (TextView) findViewById(R.id.textView1);
        display.setText("Welcome");
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
    public void setDisplay(float ans) {
    	no1 = 0;
		no2 = no1;
		symbol = "";
		display.setText("Answer: "+ans);
    }
    
    public void onClickBtn(View v) {
    	if (no1 == 0 && no2 == 0 && symbol == ""){
    		display.setText("");
    	}
    	btn = (Button) findViewById(v.getId());
    	try {
    		int tmp = Integer.parseInt(btn.getText().toString());
    		if (symbol == "") {
    			if (no1 != 0) {
    				no1 = Integer.parseInt(Integer.toString((int) no1) + btn.getText().toString());
    			}
    		
    		}
    		else {
    			if (no2 != 0) {
    				no2 = Integer.parseInt(Integer.toString((int) no2) + btn.getText().toString());
    			}
    		}
    		if (no1 == 0){
    			no1 = tmp;
    		}
    		else {
    			no2 = tmp;
    		}
    		display.setText(display.getText().toString() + " " + tmp);
    	}
    	catch (Exception e) {
    		String s = btn.getText().toString();
    		if (s.equals("=")) {
    			if (no1 == 0 && no2 == 0){
    				setDisplay(0);
    			}
    			else {
    				if (symbol.equals("+")) {
    					setDisplay(no1 + no2);
    				}
    				else if (symbol.equals("-")) {
    					setDisplay(no1 - no2);
    				}
    				else if (symbol.equals("*")) {
    					setDisplay(no1 * no2);
    				}
    				else if (symbol.equals("/")) {
    					setDisplay(no1/no2);
    				}
    				else {
    					no1 = 0;
						no2 = no1;
						symbol = "";
						display.setText("Error");
    				}
    			
    			}
    		}
    		else if(s.equals("<-")) {
    			if (no2 != 0)
    				no2 = 0;
    			else if (no1 != 0)
    				no1 = 0;
    			else
    				symbol = "";
    		}
    		else {
    			symbol = s;
    			display.setText(display.getText().toString() + " " + symbol);
    		}
    	}
    }
    public void onClick(View v) {
    	
    }
    
}