我如何让Android Studio在textView中显示计算的值
我是Android编程的新手，我遇到了编程基本应用程序的问题。该应用程序应该计算课程考试所需的标记，以获得课程中的某个标记（这不是一个家庭作业问题，而只是我正在研究的内容）。具体来说，我无法找到一种方法来在textView小部件中显示计算结果。我已经在下面发布了我的代码。另一方面，我的应用程序在启动时也会立即崩溃，所以如果你发现任何可疑的东西，那真的会有帮助。谢谢！我如何让Android Studio在textView中显示计算的值
更新：感谢hai hack，我的应用程序现在可以正常打开。但是，点击按钮后它仍然崩溃。我有更新我的代码和logcat的显示所做的更改
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)
回答：
尝试添加以下内容到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) {
}
});
上面的代码将很好地工作而不引发任何数字格式例外。
回答：
看来， 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情况下，如果实例null，然后NullPointerException被抛出，但在这种情况下String.valueOf会返回一个字符串"null"。 计算结果为空，因为您没有在按钮上使用事件侦听器。
回答：
从一开始，你的EditTexts就没有价值，所以在onCreate（）方法中使用Float.valueOf(currentGradeInput.getText().toString())意味着你将一个空字符串“”转换为浮点数。这不可能让你的应用崩溃。
在这种情况下，您应该在应用中有一个提交按钮来获取输入值并将其转换。里面的onCreate（），添加：
Button btnSubmit = findViewById(R.id.btn_button); btnSubmit.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
float desiredGradeValue = Float.valueOf(desiredGradeInput.getText().toString());
float examWeightValue = Float.valueOf(examWeightInput.getText().toString());
textViewExamMarkNeeded.setText((Float.toString(examMarkNeededValue)));
}
});
以上是 我如何让Android Studio在textView中显示计算的值 的全部内容， 来源链接： utcz.com/qa/260355.html
