不知道为啥，一按按钮，软件就闪退。求援助。
这个是报错日志
E FATAL EXCEPTION: main
Process: com.example.exercise2_4, PID: 10924
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.example.exercise2_4.MainActivity.refreshText(MainActivity.java:105)
at com.example.exercise2_4.MainActivity.onClick(MainActivity.java:78)
at android.view.View.performClick(View.java:7441)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1131)
at android.view.View.performClickInternal(View.java:7418)
at android.view.View.access$3700(View.java:835)
at android.view.View$PerformClick.run(View.java:28676)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:201)
at android.os.Looper.loop(Looper.java:288)
at android.app.ActivityThread.main(ActivityThread.java:7839)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
这个是java源代码 报错处我已经加粗
package com.example.exercise2_4;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private TextView txt;
private String firstNum = "";
private String operator = "";
private String secondNum = "";
private String result = "";
private String showText = "";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView txt =findViewById(R.id.tv01);
findViewById(R.id.btn_cancel).setOnClickListener(this);
findViewById(R.id.one).setOnClickListener(this);
findViewById(R.id.two).setOnClickListener(this);
findViewById(R.id.three).setOnClickListener(this);
findViewById(R.id.four).setOnClickListener(this);
findViewById(R.id.five).setOnClickListener(this);
findViewById(R.id.six).setOnClickListener(this);
findViewById(R.id.seven).setOnClickListener(this);
findViewById(R.id.eight).setOnClickListener(this);
findViewById(R.id.nine).setOnClickListener(this);
findViewById(R.id.zero).setOnClickListener(this);
findViewById(R.id.add).setOnClickListener(this);
findViewById(R.id.minus).setOnClickListener(this);
findViewById(R.id.multiply).setOnClickListener(this);
findViewById(R.id.chuyu).setOnClickListener(this);
findViewById(R.id.denyu).setOnClickListener(this);
findViewById(R.id.point).setOnClickListener(this);
}
@Override
public void onClick(View v) {
String inPutText;
inPutText = ((TextView)v).getText().toString();
switch(v.getId()){
case R.id.btn_cancel:
clear();
break;
case R.id.add:
case R.id.minus:
case R.id.multiply:
case R.id.chuyu:
operator = inPutText;
refreshText(showText + operator);
break;
case R.id.denyu:
double calculate_result =calculateFour();
refreashOperate(String.valueOf(calculate_result));
refreshText(showText + "=" + result);
break;
default:
if(result.length() > 0 && operator.equals("")){
clear();
}
if(operator.equals("")){
firstNum = firstNum + inPutText;
}else{
secondNum = secondNum +inPutText;
}
if(showText.equals("0") && !inPutText.equals(".")) {
refreshText(inPutText);
}else{
** refreshText(showText + inPutText);**
}
}
}
private double calculateFour() {
switch(operator){
case "+":
return Double.parseDouble(firstNum) + Double.parseDouble(secondNum);
case "-":
return Double.parseDouble(firstNum) - Double.parseDouble(secondNum);
case "*":
return Double.parseDouble(firstNum) * Double.parseDouble(secondNum);
case "/":
return Double.parseDouble(firstNum) / Double.parseDouble(secondNum);
}
return 0;
}
private void refreshText(String text) {
showText = text;
**txt.setText(showText);**
}
private void refreashOperate(String new_result){
result = new_result;
firstNum = result;
secondNum = "";
operator = "";
}
private void clear() {
refreshText("");
refreashOperate("");
}
}
下面是我的布局文件
<?xml version="1.0" encoding="utf-8"?>
<GridLayout
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"
tools:context=".MainActivity"
android:rowCount="6"
android:columnCount="4"
android:padding="5dp">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_columnSpan="4"
android:gravity="center"
android:text="计算器"
android:textSize="20sp"
android:background="#EEEEEE">
</TextView>
<TextView
android:id="@+id/tv01"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_columnSpan="4"
android:lines="2"
android:gravity="right|bottom"
android:text="0"
android:textSize="50dp">
</TextView>
<Button
android:id="@+id/btn_cancel"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_columnSpan="4"
android:text="清除"
android:textSize="26sp"
android:layout_margin="8dp">
</Button>
<Button
android:id="@+id/one"
android:text="1"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/two"
android:text="2"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/three"
android:text="3"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/add"
android:text="+"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/four"
android:text="4"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/five"
android:text="5"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/six"
android:text="6"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/minus"
android:text="-"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/seven"
android:text="7"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/eight"
android:text="8"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/nine"
android:text="9"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/multiply"
android:text="*"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/point"
android:text="."
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/zero"
android:text="0"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/denyu"
android:text="="
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
<Button
android:id="@+id/chuyu"
android:text="/"
android:textSize="26sp"
android:layout_margin="5dp">
</Button>
</GridLayout>
