How do I get Android Studio to display the value of a calculation in a textView

Question

I'm new to android programming and I'm having issues programming a basic app. The app is supposed to calculate the mark needed on a course exam in order to obtain a certain mark in a course (this is not a homework problem and is just something that I'm working on). Specifically, I'm having trouble finding a way to display the result of the calculation in a textView widget. I have posted my code below. On a side note, my app also crashes immediately on startup, so if you find anything suspicious, that would really help out. Thanks!

Update: Thanks to hai hack, my app opens up properly now. However, it still crashes after I click the button. I have update my code and logcat to show the changes made

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 code:

<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

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) 

Answer 1

Try adding this to your 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) {

       }
   });

Above code will work perfectly without throwing any number format exception.

Answer 2

From the beginning, your EditTexts had no value, so using Float.valueOf(currentGradeInput.getText().toString()) in onCreate() method meant that you were converting a empty string "" to float number. That's impossible making your app to crash.

In this case, you should have a Submit button in your app to get the input values and convert them. Inside onCreate(), add:

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)));
                    }
                }); 

Answer 3

It seems that examMarkNeededValue is computed to null. So I would suggest you use:

String.valueOf(examMarkNeededValue)

instead of

Float.toString(examMarkNeededValue)

Because in case of Float.toString, if the instance is null, then NullPointerException is thrown, but in that case String.valueOf would return a string "null". The computation is null because you haven't used event listener on the button.