求大神帮忙看下代码，加减乘都不对

package com.example.xierenjie.myapplication;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class CalculatorActivity 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_reduce;
Button btn_equal;
Button btn_add;
Button btn_point;
Button btn_clear;
Button btn_del;
Button btn_divide;
Button btn_mul;
EditText et_input;
boolean clear_flag;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_calculator);
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_reduce = (Button) findViewById(R.id.btn_reduce);
btn_equal = (Button) findViewById(R.id.btn_equal);
btn_add = (Button) findViewById(R.id.btn_add);
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_divide = (Button) findViewById(R.id.btn_divide);
btn_mul = (Button) findViewById(R.id.btn_mul);
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_del.setOnClickListener(this);
btn_divide.setOnClickListener(this);
btn_reduce.setOnClickListener(this);
//btn_add.setOnClickListener(this);
btn_equal.setOnClickListener(this);
btn_mul.setOnClickListener(this);
btn_point.setOnClickListener(this);
btn_clear.setOnClickListener(this);
}
/*ctrl+o对父类方法进行重写*/
@Override
public void onClick(View v) {
String str = et_input.getText().toString();/*取出显示的内容*/
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_reduce:
case R.id.btn_add:
case R.id.btn_divide:
case R.id.btn_mul:
if (clear_flag) {
clear_flag = false;
str = "";
et_input.setText("");
}
et_input.setText(str + " " + ((Button) v).getText() + " ");
break;
case R.id.btn_clear:
et_input.setText("");
break;
case R.id.btn_del:
if (clear_flag) {
clear_flag = false;
str = "";
et_input.setText("");
}
if (str != null && !str.equals("")) {
et_input.setText(str.substring(0, str.length() - 1));
}
break;
case R.id.btn_equal:
getResult();
break;
}
}
private void getResult() {
String exp = et_input.getText().toString();
if (exp.equals(" ") && exp == null) {
return;
}
if (!exp.contains(" ")) {
return;
}
if (clear_flag) {
clear_flag = false;
return;
}
clear_flag = true;
String str1 = exp.substring(0, exp.indexOf(" "));/*第一个数字*/
String str2 = exp.substring(exp.indexOf(" ") + 3);/*第二个数字*/
String str3 = exp.substring(exp.indexOf(" ") + 1, exp.indexOf(" ") + 2);/*运算符*/
double result = 0;
if (!str1.equals("") && !str2.equals("")) {
double s1 = Double.parseDouble(str1);
double s2 = Double.parseDouble(str2);
if (str3.equals(" + ")) {
result = s1 + s2;
} else if (str3.equals(" － ")) {
result = s1 - s2;
} else if (str3.equals(" × ")) {
result = s1 * s2;
} else if (str3.equals(" ÷ ")) {
if (s2 == 0)
result = 0;
} else {
result = s1 / s2;
}
if (!str1.contains(".") && !str2.contains(".") && !str3.equals("÷")) {
int r = (int) result;
et_input.setText(r + "");
} else {
et_input.setText(result + "");
}
} else if (str1.equals("") && !str2.equals("")) {
double s2 = Double.parseDouble(str2);
if (str3.equals("＋")) {
result = 0 + s2;
} else if (str3.equals("－")) {
result = 0 - s2;
} else if (str3.equals("×")) {
result = 0;
} else if (str3.equals("÷")) {
result = 0;
}
if (!str2.contains(".")) {
int r = (int) result;
et_input.setText(r + "");
} else {
et_input.setText(result + "");
}
} else if (!str1.equals("") && str2.equals("")) {
et_input.setText(exp);
} else {
et_input.setText("");
}
}
}