由于java.lang.NumberFormatException：Android单元转换器不断崩溃：无效的double值：“”

Question

我正在用android做一个双向单位转换器。我想我已经很清楚了，但是当我想编写一个单独的convert方法而不是在进行其余的单位转换之前编写多个addTextChangedListener（）时。我一直使用现在已注释掉的方法来获取NumberFormatExceptions。有没有更好的方法可以做到这一点，或者我错过了一个简单的解决方法？

此问题被标记为重复，但是我尝试了将我重定向到的修复程序，并遇到了同样的问题。请有人将我指向另一个方向。

activity_main.xml

   <?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/temperatureTextView"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="32dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:gravity="center"
        android:text="@string/temperature"
        android:textColor="@android:color/black"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <LinearLayout
        android:id="@+id/tempLinearLayout"
        android:layout_width="395dp"
        android:layout_height="50dp"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:orientation="horizontal"
        android:weightSum="3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/temperatureTextView">

        <EditText
            android:id="@+id/celsiusEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/celsius"
            android:inputType="numberDecimal" />

        <ImageView
            android:id="@+id/imageView"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:scaleType="fitXY"
            app:srcCompat="@drawable/double_arrow" />

        <EditText
            android:id="@+id/fahrenheitEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/fahrenheit"
            android:inputType="numberDecimal" />
    </LinearLayout>

    <TextView
        android:id="@+id/distanceTextView"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="32dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:gravity="center"
        android:text="@string/distance"
        android:textColor="@android:color/black"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tempLinearLayout" />

    <LinearLayout
        android:id="@+id/distanceLinearLayout"
        android:layout_width="395dp"
        android:layout_height="50dp"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:orientation="horizontal"
        android:weightSum="3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/distanceTextView">

        <EditText
            android:id="@+id/kmEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/kilometers"
            android:inputType="numberDecimal" />

        <ImageView
            android:id="@+id/imageView2"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:scaleType="fitXY"
            app:srcCompat="@drawable/double_arrow" />

        <EditText
            android:id="@+id/mileEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/miles"
            android:inputType="numberDecimal" />
    </LinearLayout>

    <TextView
        android:id="@+id/weightTextView"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="32dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:gravity="center"
        android:text="@string/weight"
        android:textColor="@android:color/black"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/distanceLinearLayout" />

    <LinearLayout
        android:id="@+id/weightLinearLayout"
        android:layout_width="395dp"
        android:layout_height="50dp"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:orientation="horizontal"
        android:weightSum="3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/weightTextView">

        <EditText
            android:id="@+id/kgEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/kilograms"
            android:inputType="numberDecimal" />

        <ImageView
            android:id="@+id/imageView3"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:scaleType="fitXY"
            app:srcCompat="@drawable/double_arrow" />

        <EditText
            android:id="@+id/lbEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/pounds"
            android:inputType="numberDecimal" />
    </LinearLayout>

    <TextView
        android:id="@+id/volumeTextView"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="32dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:gravity="center"
        android:text="@string/volume"
        android:textColor="@android:color/black"
        android:textSize="18sp"
        android:textStyle="bold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/weightLinearLayout" />

    <LinearLayout
        android:id="@+id/volumeLinearLayout"
        android:layout_width="395dp"
        android:layout_height="50dp"
        android:layout_marginStart="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        android:orientation="horizontal"
        android:weightSum="3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/volumeTextView">

        <EditText
            android:id="@+id/literEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/litres"
            android:inputType="numberDecimal" />

        <ImageView
            android:id="@+id/imageView4"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:scaleType="fitXY"
            app:srcCompat="@drawable/double_arrow" />

        <EditText
            android:id="@+id/gallonEditText"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:background="@drawable/border"
            android:ems="10"
            android:gravity="center"
            android:hint="@string/gallons"
            android:inputType="numberDecimal" />
    </LinearLayout>

</android.support.constraint.ConstraintLayout>

MainActivity.java

package com.comp3710.unitconverter;

import android.annotation.SuppressLint;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.MotionEvent;
import android.view.View;
import android.widget.EditText;

import java.text.DecimalFormat;

public class MainActivity extends AppCompatActivity {
    DecimalFormat fmt = new DecimalFormat("#,##0.0#");
    private EditText celsius, fahrenheit, kilometers, miles, kilograms, pounds, litres, gallons;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        celsius = (EditText) findViewById(R.id.celsiusEditText);
        fahrenheit = (EditText) findViewById(R.id.fahrenheitEditText);
        kilometers = (EditText) findViewById(R.id.kmEditText);
        miles = (EditText) findViewById(R.id.mileEditText);
        kilograms = (EditText) findViewById(R.id.kgEditText);
        pounds = (EditText) findViewById(R.id.lbEditText);
        litres = (EditText) findViewById(R.id.literEditText);
        gallons = (EditText) findViewById(R.id.gallonEditText);

        clearText(celsius);
        celsius.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                if(celsius.hasFocus()) {
                    String c = celsius.getText().toString();
                    if (c.isEmpty()) {
                        fahrenheit.setText(null);
                    } else {
                        String f = toFahrenheit() + '\u2109';
                        fahrenheit.setText(f);
                    }
                }
            }
        });

        //convert(celsius, fahrenheit, toFahrenheit(), "\u2109");

        clearText(fahrenheit);
        fahrenheit.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                if(fahrenheit.hasFocus()) {
                    String f = fahrenheit.getText().toString();
                    if (f.isEmpty()) {
                        celsius.setText(null);
                    } else {
                        String c = toCelsius() + '\u2103';
                        celsius.setText(c);
                    }
                }
            }
        });
        //convert(fahrenheit, celsius, toCelsius(), "\u2103");

    }

    /*private void convert(final EditText source, final EditText target, final String conversion, final String unit) {
        source.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                if(source.hasFocus()) {
                    String src = source.getText().toString();
                    if (src.isEmpty()) {
                        target.setText(0);
                    } else {
                        String t = conversion + unit;
                        target.setText(t);
                    }
                }
            }
        });
    }*/

    @SuppressLint("ClickableViewAccessibility")
    private void clearText(final EditText editText) {
        editText.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                editText.getText().clear();
                return false;
            }
        });
    }

    private String toCelsius() {
        return fmt.format((Double.parseDouble(fahrenheit.getText().toString()) - 32.0) * (5.0 / 9.0));

    }

    private String toFahrenheit() {
        return fmt.format((Double.parseDouble(celsius.getText().toString()) * (9.0 / 5.0)) + 32.0);

    }
}