我是Android编程的新手，我在编写基本应用程序时遇到了问题。该应用程序应该计算课程考试所需的标记，以便在课程中获得某个标记（这不是一个作业问题，只是我正在研究的东西）。具体来说，我在查找在textView小部件中显示计算结果的方法时遇到了麻烦。我在下面发布了我的代码。另外，我的应用程序也会在启动时立即崩溃，所以如果你发现任何可疑的东西，那真的会有所帮助。谢谢！
更新：感谢hai hack，我的应用程序现在正常打开。但是，单击按钮后仍然会崩溃。我已更新我的代码和logcat以显示所做的更改
MainActivity：
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Button;
import static com.managergmail.time.finite.finitemanager02.R.id.calculateButton;
import static com.managergmail.time.finite.finitemanager02.R.id.textViewExamMarkNeeded;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button buttonCalculate = (Button) findViewById(R.id.buttonCalculate);
buttonCalculate.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
EditText currentGradeInput = (EditText) findViewById(R.id.currentGradeInput);
float currentGradeValue = Float.valueOf(currentGradeInput.getText().toString());
//obtaining value of currentGrade inputted by the user and converting to float
EditText desiredGradeInput = (EditText) findViewById(R.id.desiredGradeInput);
float desiredGradeValue = Float.valueOf(desiredGradeInput.getText().toString());
//obtaining value of currentGrade inputted by the user and converting to float
EditText examWeightInput = (EditText) findViewById(R.id.examWeightInput);
float examWeightValue = Float.valueOf(examWeightInput.getText().toString());
//obtaining value of currentGrade inputted by the user and converting to float
float currentGradeWeight = 100-examWeightValue;
//calculating current grade weight
final float examMarkNeededValue;
examMarkNeededValue= ((100*desiredGradeValue)-currentGradeValue*currentGradeWeight)/examWeightValue;
textViewExamMarkNeeded.setText((Float.toString(examMarkNeededValue)));
}
});
}
}
xml代码：
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
<TextView
android:id="@+id/textView"
android:layout_width="180dp"
android:layout_height="45dp"
android:text="@string/input_the_weighting_of_your_exam"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHorizontal_bias="0.078"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.356" />
<TextView
android:id="@+id/textView2"
android:layout_width="180dp"
android:layout_height="45dp"
android:text="@string/input_your_desired_grade"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHorizontal_bias="0.078"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.19" />
<TextView
android:id="@+id/textViewExamMarkNeeded"
android:layout_width="265dp"
android:layout_height="64dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.662"
app:layout_constraintHorizontal_bias="0.378" />
<TextView
android:layout_width="180dp"
android:layout_height="45dp"
android:text="@string/input_your_current_grade"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHorizontal_bias="0.073"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.034"
android:id="@+id/textView3" />
<EditText
android:id="@+id/currentGradeInput"
android:layout_width="0dp"
android:layout_height="0dp"
android:ems="10"
android:inputType="numberDecimal"
tools:layout_constraintTop_creator="1"
tools:layout_constraintRight_creator="1"
tools:layout_constraintBottom_creator="1"
android:layout_marginStart="17dp"
app:layout_constraintBottom_toBottomOf="@+id/textView3"
android:layout_marginEnd="33dp"
app:layout_constraintRight_toRightOf="parent"
tools:layout_constraintLeft_creator="1"
app:layout_constraintTop_toTopOf="@+id/textView3"
app:layout_constraintLeft_toRightOf="@+id/textView3" />
<EditText
android:id="@+id/desiredGradeInput"
android:layout_width="133dp"
android:layout_height="31dp"
android:ems="10"
android:inputType="numberDecimal"
tools:layout_constraintTop_creator="1"
tools:layout_constraintRight_creator="1"
tools:layout_constraintBottom_creator="1"
app:layout_constraintBottom_toTopOf="@+id/examWeightInput"
android:layout_marginEnd="33dp"
app:layout_constraintRight_toRightOf="parent"
android:layout_marginTop="44dp"
app:layout_constraintTop_toBottomOf="@+id/currentGradeInput"
android:layout_marginBottom="42dp" />
<EditText
android:id="@+id/examWeightInput"
android:layout_width="0dp"
android:layout_height="33dp"
android:ems="10"
android:inputType="numberDecimal"
tools:layout_constraintTop_creator="1"
tools:layout_constraintRight_creator="1"
tools:layout_constraintBottom_creator="1"
app:layout_constraintBottom_toTopOf="@+id/calculateButton"
android:layout_marginStart="2dp"
android:layout_marginEnd="2dp"
app:layout_constraintRight_toRightOf="@+id/desiredGradeInput"
android:layout_marginTop="12dp"
tools:layout_constraintLeft_creator="1"
android:layout_marginBottom="19dp"
app:layout_constraintLeft_toLeftOf="@+id/desiredGradeInput"
app:layout_constraintTop_toTopOf="@+id/textView" />
<Button
android:id="@+id/buttonCalculate"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="17dp"
android:layout_marginEnd="7dp"
android:onClick="onButtonClick"
android:text="@string/calculate"
app:layout_constraintBottom_toTopOf="@+id/textViewExamMarkNeeded"
app:layout_constraintRight_toRightOf="@+id/textViewExamMarkNeeded"
tools:layout_constraintBottom_creator="1"
tools:layout_constraintRight_creator="1" />
logcat的
12-17 21:14:05.840 18312-18312/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.managergmail.time.finite.finitemanager02, PID: 18312
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.managergmail.time.finite.finitemanager02/com.managergmail.time.finite.finitemanager02.MainActivity}: java.lang.NumberFormatException: Invalid float: ""
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2581)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2647)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1502)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:207)
at android.app.ActivityThread.main(ActivityThread.java:5763)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:888)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:749)
Caused by: java.lang.NumberFormatException: Invalid float: ""
at java.lang.StringToReal.invalidReal(StringToReal.java:63)
at java.lang.StringToReal.parseFloat(StringToReal.java:308)
at java.lang.Float.parseFloat(Float.java:306)
at java.lang.Float.valueOf(Float.java:343)
at com.managergmail.time.finite.finitemanager02.MainActivity.onCreate(MainActivity.java:20)
at android.app.Activity.performCreate(Activity.java:6280)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1116)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2534)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2647)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1502)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:207)
at android.app.ActivityThread.main(ActivityThread.java:5763)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:888)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:749)
答案 0 :(得分：1)
尝试将此添加到您的editext
final EditText currentGradeInput = (EditText)findViewById(R.id.currentGradeInput);
currentGradeInput.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {
}
@Override
public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
currentGradeValue = Float.valueOf(currentGradeInput.getText().toString());
}
@Override
public void afterTextChanged(Editable editable) {
}
});
上面的代码将完美地工作，而不会抛出任何数字格式异常。
答案 1 :(得分：0)
似乎examMarkNeededValue计算为null。所以我建议你使用：
String.valueOf(examMarkNeededValue)
而不是
Float.toString(examMarkNeededValue)
因为在Float.toString的情况下，如果实例为null，则会引发NullPointerException，但在这种情况下，String.valueOf将返回字符串"null" 。
计算为null，因为您没有在按钮上使用事件侦听器。
答案 2 :(得分：0)
从一开始，你的EditTexts没有任何价值，所以在onCreate（）方法中使用tableView.WeakDelegate= this;
tableView.WeakDataSource= this;
[Export("tableView:heightForRowAtIndexPath:")]
public nfloat GetHeightForRow(UITableView tableView, NSIndexPath indexPath)
{
return 50;
}
[Export("numberOfSectionsInTableView:")]
public nint NumberOfSections(UITableView tableView)
{
return 1;
}
[Export("tableView:cellForRowAtIndexPath:")]
public UITableViewCell GetCell(UITableView tableView, NSIndexPath indexPath)
{
}
意味着你要转换一个空字符串＆＃34;＆＃34;浮点数。这使得您的应用无法崩溃。
在这种情况下，您的应用中应该有一个“提交”按钮来获取输入值并进行转换。在onCreate（）中，添加：
Float.valueOf(currentGradeInput.getText().toString())