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                        /Android攻城狮的第一门课（入门篇）

为什么我点等于号程序会闪退

package com.example.demo.caculatordemo;

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

public class MainActivity extends Activity implements View.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;
boolean clear_flag;

@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_mutiply);
btn_divide = (Button)findViewById(R.id.btn_divide);
btn_equal = (Button)findViewById(R.id.btn_equal);
et_input = (EditText)findViewById(R.id.et_input);

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);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();

//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}

return super.onOptionsItemSelected(item);
}

@Override
public void onClick(View v) {
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_mutiply:
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_del:
if(clear_flag){
clear_flag = false;
str="";
et_input.setText("");
}
else if(str!=null && !str.equals("")){
et_input.setText(str.substring(0,str.length()-1));
}
break;
case R.id.btn_clear:
clear_flag = false;
str="";
et_input.setText("");
break;
case R.id.btn_equal:
getResult();
break;
default:
}
}
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(" "));//运算符前面的字符
String op = exp.substring(exp.indexOf(" ")+1,exp.indexOf(" ")+2);//运算符
String s2 = exp.substring(0,exp.indexOf(" ")+3);//运算符后面的字符
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.equals("÷")){
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 {
et_input.setText("");
}
}
}