安卓编写简单的计算器

import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.app.Activity;

public class MainActivity extends Activity implements OnClickListener{

private EditText edit;
private Button night;
private Button eight;
private Button seven;
private Button six;
private Button fine;
private Button four;
private Button three;
private Button two;
private Button one;
private Button zero;
private Button divide;
private Button add;
private Button ride;
private Button reduce;
private Button piont;
private Button equals;
private Button delete;

private String tag="";
private boolean flag=true;
private int i;
private double num1;
private double num2;
private double result;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

edit = (EditText) findViewById(R.id.edit);
edit.setOnClickListener(this);

night = (Button) findViewById(R.id.night);
night.setOnClickListener(this);

eight = (Button) findViewById(R.id.eight);
eight.setOnClickListener(this);

seven=(Button) findViewById(R.id.seven);
seven.setOnClickListener(this);

six=(Button) findViewById(R.id.six);
six.setOnClickListener(this);

fine=(Button) findViewById(R.id.fine);
fine.setOnClickListener(this);

four=(Button) findViewById(R.id.four);
four.setOnClickListener(this);

three=(Button) findViewById(R.id.three);
three.setOnClickListener(this);

two=(Button) findViewById(R.id.two);
two.setOnClickListener(this);

one=(Button) findViewById(R.id.one);
one.setOnClickListener(this);

zero=(Button) findViewById(R.id.zero);
zero.setOnClickListener(this);

divide=(Button) findViewById(R.id.divide);
divide.setOnClickListener(this);

ride=(Button) findViewById(R.id.ride);
ride.setOnClickListener(this);

reduce=(Button) findViewById(R.id.reduce);
reduce.setOnClickListener(this);

add=(Button) findViewById(R.id.add);
add.setOnClickListener(this);

piont=(Button) findViewById(R.id.point);
piont.setOnClickListener(this);

equals=(Button) findViewById(R.id.equals);
equals.setOnClickListener(this);

delete=(Button) findViewById(R.id.delete);
delete.setOnClickListener(this);
}

@Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.delete:
edit.setText("");
tag="";
break;
case R.id.point:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+".";
edit.setText(tag);
break;
case R.id.zero:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"0";
edit.setText(tag);
break;
case R.id.one:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"1";
edit.setText(tag);
break;
case R.id.two:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"2";
edit.setText(tag);
break;
case R.id.three:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"3";
edit.setText(tag);
break;
case R.id.four:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"4";
edit.setText(tag);
break;
case R.id.fine:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"5";
edit.setText(tag);
break;
case R.id.six:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"6";
edit.setText(tag);
break;
case R.id.seven:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"7";
edit.setText(tag);
break;
case R.id.eight:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"8";
edit.setText(tag);
break;
case R.id.night:
if(flag){
edit.setText("");
flag=false;
}
tag=tag+"9";
edit.setText(tag);
break;
case R.id.add:
tag=tag+"+";
edit.setText(tag);
i=1;
break;
case R.id.divide:
tag=tag+"/";
edit.setText(tag);
i=2;
break;
case R.id.reduce:
tag=tag+"-";
edit.setText(tag);
i=3;
break;
case R.id.ride:
tag=tag+"*";
edit.setText(tag);
i=4;
break;

case R.id.equals:
edit.setText("");
switch (i) {
case 1:
num1=Double.parseDouble(tag.substring(0, tag.indexOf("+")));
num2=Double.parseDouble(tag.substring(tag.indexOf("+")+1, tag.length()));
result=num1+num2;
break;
case 2:
num1=Double.parseDouble(tag.substring(0, tag.indexOf("/")));
num2=Double.parseDouble(tag.substring(tag.indexOf("/")+1, tag.length()));
if(num2==0){
edit.setText("除数不能为0");
}else{
result=num1/num2;
}
break;
case 3:
num1=Double.parseDouble(tag.substring(0, tag.indexOf("-")));
num2=Double.parseDouble(tag.substring(tag.indexOf("-")+1, tag.length()));
result=num1-num2;
break;
case 4:
num1=Double.parseDouble(tag.substring(0, tag.indexOf("*")));
num2=Double.parseDouble(tag.substring(tag.indexOf("*")+1, tag.length()));
result=num1*num2;
break;

}
edit.setText(String.valueOf(result));
flag=true;
tag="";
break;
}

}

}