為什么我只能計算一次？

package com.example.calculator;

import android.os.Bundle;

import android.app.Activity;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.EditText;

public class MainActivity extends Activity implements OnClickListener{

Button btn_0;

Button btn_1;

Button btn_2;

Button btn_3;

Button btn_4;

Button btn_5;

Button btn_6;

Button btn_7;

Button btn_8;

Button btn_9;

Button btn_point;

Button btn_clear;

Button btn_del;

Button btn_plus;

Button btn_minus;

Button btn_multiply;

Button btn_divide;

Button btn_equal;

EditText et_input;//顯示輸入

boolean clear_flag;//清空標識

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

btn_0 = (Button) findViewById(R.id.btn_0);

btn_1 = (Button) findViewById(R.id.btn_1);

btn_2 = (Button) findViewById(R.id.btn_2);

btn_3 = (Button) findViewById(R.id.btn_3);

btn_4 = (Button) findViewById(R.id.btn_4);

btn_5 = (Button) findViewById(R.id.btn_5);

btn_6 = (Button) findViewById(R.id.btn_6);

btn_7 = (Button) findViewById(R.id.btn_7);

btn_8 = (Button) findViewById(R.id.btn_8);

btn_9 = (Button) findViewById(R.id.btn_9);

btn_point = (Button) findViewById(R.id.btn_point);

btn_clear = (Button) findViewById(R.id.btn_clear);

btn_del = (Button) findViewById(R.id.btn_del);

btn_plus = (Button) findViewById(R.id.btn_plus);

btn_minus = (Button) findViewById(R.id.btn_minus);

btn_multiply = (Button) findViewById(R.id.btn_multiply);

btn_divide = (Button) findViewById(R.id.btn_divide);

btn_equal = (Button) findViewById(R.id.btn_equal);

//實例化按鈕

et_input = (EditText) findViewById(R.id.et_input);

//實例化顯示屏

btn_0.setOnClickListener(this);

btn_1.setOnClickListener(this);

btn_2.setOnClickListener(this);

btn_3.setOnClickListener(this);

btn_4.setOnClickListener(this);

btn_5.setOnClickListener(this);

btn_6.setOnClickListener(this);

btn_7.setOnClickListener(this);

btn_8.setOnClickListener(this);

btn_9.setOnClickListener(this);

btn_point.setOnClickListener(this);

btn_clear.setOnClickListener(this);

btn_del.setOnClickListener(this);

btn_plus.setOnClickListener(this);

btn_minus.setOnClickListener(this);

btn_multiply.setOnClickListener(this);

btn_divide.setOnClickListener(this);

btn_equal.setOnClickListener(this);

//設置按鈕的點擊事件

}

@Override

public void onClick(View v){

// TODO Auto-generated method stub

String str = et_input.getText().toString();

switch(v.getId()){

case R.id.btn_0:

case R.id.btn_1:

case R.id.btn_2:

case R.id.btn_3:

case R.id.btn_4:

case R.id.btn_5:

case R.id.btn_6:

case R.id.btn_7:

case R.id.btn_8:

case R.id.btn_9:

case R.id.btn_point:

if(clear_flag){

clear_flag = false;

str = " ";

et_input.setText(" ");

}

et_input.setText(str+((Button)v).getText());

break;

case R.id.btn_plus:

case R.id.btn_minus:

case R.id.btn_multiply:

case R.id.btn_divide:

if(clear_flag){

clear_flag = false;

str = "";

et_input.setText(" ");

}

et_input.setText(str+" "+((Button)v).getText()+" ");

break;

case R.id.btn_clear:

clear_flag = false;

str = " ";

et_input.setText(" ?");

break;

case R.id.btn_del:

if(clear_flag){

clear_flag = false;

et_input.setText(" ");

}

else if(str!=null&&!str.equals(" ")){

et_input.setText(str.substring(0, str.length()-1));

}

break;

case R.id.btn_equal:

if(clear_flag){break;}

? ? ? ? ? ? ?clear_flag=true;

getResult();

break;

default:

break;

}

}

private void getResult(){

String exp = et_input.getText().toString();

if(exp==null||exp.equals(" ")){

return;

}

if(!exp.contains(" ")){

return;

}

if(clear_flag){

clear_flag = false;

return;

}

clear_flag = true;

double result = 0;

String s1 = exp.substring(0, exp.indexOf(" "));

String op = exp.substring(exp.indexOf(" ")+1,exp.indexOf(" ")+2);

String s2 = exp.substring(exp.indexOf(" ")+3);

if(!s1.equals(" ")&&!s2.equals(" ")){

double d1 = Double.parseDouble(s1);

double d2 = Double.parseDouble(s2);

if(op.equals("＋")){

result = d1 + d2;

}

else if(op.equals("－")){

result = d1 - d2;

}

else if(op.equals("×")){

result = d1 * d2;

}

else if(op.equals("÷")){

if(d2==0){

result = 0;

}

else{

result = d1 / d2;

}

}

if(!s1.contains(".")&&!s2.contains(".")&&op.contains("÷")){

int r = (int) result;

et_input.setText(r+" ");

}

else{

et_input.setText(result+" ");

}

}

else if(!s1.equals(" ")&&s2.equals(" ")){

et_input.setText(exp);

}

else if(s1.equals(" ")&&!s2.equals(" ")){

double d2 = Double.parseDouble(s2);

if(op.equals("＋")){

result = 0 + d2;

}

else if(op.equals("－")){

result = 0 - d2;

}

else if(op.equals("×")){

result = 0;

}

else if(op.equals("÷")){

result = 0;

}

if(!s2.contains(".")){

int r = (int) result;

et_input.setText(r+" ");

}

else{

et_input.setText(result+" ");

}

}

else if(s1.equals(" ")&&s2.equals(" ")){

et_input.setText(" ");

}

}

}