java - 无法检索另一个类中的字符串值

Question

我试图从 MainActivity.java 中提取另一个类 converter.java 类中某些特定字符串值的值，但无法显示。没有显示日志。甚至，如果我想从另一个类获取数据 spinnerSelects.java 在某些地方也不再显示。你能帮我么？

MainActivity.java 代码在这里:

package com.gazzali.spinitmeow;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener, View.OnClickListener{

    Spinner spinnerMainChoice;
    Spinner spinnerInputChoice;
    Spinner spinnerOutputChoice;

    EditText getInputValueID;
    Double inputValue;

    TextView outputValueTextViewFromConverter;

    Button buttonConvert;

    String selectedMainChoice;



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

        /* ------------ Main code Starts Here ----------------*/

        /* Main conversion Type choice with Spinner (Drop Down menu)*/
        spinnerMainChoice = findViewById(R.id.spinnerIDMainChoice);
        // [IMPORTANT] Set Spinner Click Listener
        spinnerMainChoice.setOnItemSelectedListener(this);
        // Create an ArrayAdapter using the string array and a default spinner layout
        ArrayAdapter<CharSequence> adapterMainChoice = ArrayAdapter.createFromResource(this,
                R.array.MainChoices_array, android.R.layout.simple_spinner_item);
        // Specify the layout to use when the list of choices appears
        adapterMainChoice.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        // Apply the adapter to the spinner
        spinnerMainChoice.setAdapter(adapterMainChoice);

        /* Input Conversion type choice with Spinner */
        spinnerInputChoice = findViewById(R.id.spinnerIDInputChoice);

        /* Output Conversion type choice with Spinner */

        spinnerOutputChoice = findViewById(R.id.spinnerIDOutputChoice);

        /* for input and output fields */
        getInputValueID = findViewById(R.id.editTextIDInputValue);
        /* ---- Setting Button Properties -----*/
        buttonConvert = findViewById(R.id.buttonIDConvert);
        buttonConvert.setOnClickListener(this);

        /* --- Setting Output TextView field ----*/
        outputValueTextViewFromConverter = findViewById(R.id.textViewIDoutputValueToConverter);

    }

    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
        // An item was selected. retrieve the selected item
        selectedMainChoice = parent.getSelectedItem().toString();
        Log.i("Selected", selectedMainChoice);
        /* Toast.makeText(MainActivity.this, String.valueOf(inputValue), Toast.LENGTH_SHORT).show();*/

            /* Implement object of spinnerSelects class*/
            spinnerSelects spinnerSelectsInMain = new spinnerSelects(this, spinnerInputChoice, spinnerOutputChoice);
            /* the main EVIL '(context) this' in  the 2nd parameter, 5 hours wasted, but I learnt many more */
            spinnerSelectsInMain.setInputOutputSpinners(selectedMainChoice);

            /* calling test for converter class */
        /*testOnConverter();*/

    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) {
        // Another interface callback
    }

    public void testOnConverter(){
        converter converterInMain = new converter(selectedMainChoice);
    }

    @Override
    public void onClick(View view)
    {
        String inputValueString = getInputValueID.getText().toString();
        inputValue = Double.parseDouble(inputValueString);

        /*Toast.makeText(this, String.valueOf(inputValue), Toast.LENGTH_SHORT).show();*/
        converter converterInMain = new converter(selectedMainChoice);
        double convertedValue = converterInMain.convert(inputValue);
        outputValueTextViewFromConverter.setText(String.valueOf(convertedValue));
    }
}

converter.java 代码如下:

package com.gazzali.spinitmeow;

import android.content.Context;
import android.util.Log;
import android.widget.Spinner;
import android.widget.Switch;
import android.widget.Toast;

public class converter {


    public String MainChoice, inputChoice, outputChoice;


    public converter() {
    }

    public converter(String selectedMainChoiceFromMain) {
        this.MainChoice = selectedMainChoiceFromMain;

        /* No Log for Main Choice is Being Shown Here */

        Log.i("Main Choice is", MainChoice);
    }

    public converter(String inputChoiceFromSpinnerSelects, String outputChoiceFromSpinnerSelects){
        this.inputChoice = inputChoiceFromSpinnerSelects;
        this.outputChoice = outputChoiceFromSpinnerSelects;

        /* This Logs Shows But ONLY here */
        /* IF I try to show the logs anywhere else in the class, */
        /* Either null exception or No Log output */

        Log.i("Sub Input Choices are:", inputChoice);
        Log.i("Sub Output Choices are:", outputChoice);
    }


    public double convert(double inputValueForEditTextFieldFromMain)
    {
        double inputValueInConverter = inputValueForEditTextFieldFromMain;
        double outputValueToConverterToMain= 0.00;

        /* Here I can't see the Log */
        /* Apps Crashes When I press Convert Button*/
         Log.i("Sub Input Choices are:", inputChoice);

        converterLength converterLengthInConverter = new converterLength();

        outputValueToConverterToMain = inputValueInConverter * 22;


        /*switch (MainChoice)
        {
            case "Length":
                outputValueToConverterToMain = converterLengthInConverter.convertLength(inputChoice, outputChoice, inputValueInConverter);
            break;
        }*/
        return outputValueToConverterToMain;
    }
}

当我按下转换按钮时，应用程序不断崩溃。

定位错误显示:

2019-07-21 16:48:33.406 10979-10979/com.gazzali.spinitmeow E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.gazzali.spinitmeow, PID: 10979
java.lang.NullPointerException: println needs a message
    at android.util.Log.println_native(Native Method)
    at android.util.Log.i(Log.java:166)
    at com.gazzali.spinitmeow.converter.convert(converter.java:46)
    at com.gazzali.spinitmeow.MainActivity.onClick(MainActivity.java:104)

更新这是我的 spinnerSelects 类，它将 inputChoice 和 outputChoice 参数传递给 converter 类，但我仍然遇到了那个奇怪的错误。尽管我通过 spinnerSelects 传递的参数设置了 converter 类的构造函数值，但它从未设置它的值，因为我看不到 inputChoice 值在 converter 类内

package com.gazzali.spinitmeow;

import android.content.Context;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Spinner;

public class spinnerSelects implements AdapterView.OnItemSelectedListener{

    public String inputChoice, outputChoice;
    public Spinner spinnerInputChoice, spinnerOutputChoice;
    public ArrayAdapter<CharSequence> adapterInputChoice, adapterOutputChoice;

    private Context contextInSpinnerSelects;

    public Context getContextInSpinnerSelects() {
        return contextInSpinnerSelects;
    }

    public spinnerSelects() {
        /* Empty Constructor */
    }

    public spinnerSelects(Context contextFromMain, Spinner spinnerInputChoiceFromMain, Spinner spinnerOutputChoiceFromMain) {
        this.spinnerInputChoice = spinnerInputChoiceFromMain;
        this.spinnerOutputChoice = spinnerOutputChoiceFromMain;
        this.contextInSpinnerSelects = contextFromMain;
    }

    /**
     *
     * @param selectedMainChoice String retrieves Main Conversion spinner's type
     */
    public void setInputOutputSpinners(String selectedMainChoice) {
        switch (selectedMainChoice)
        {
            case "Length": {
                adapterInputChoice = ArrayAdapter.createFromResource(contextInSpinnerSelects,
                        R.array.LengthChoices_array, android.R.layout.simple_spinner_item);

                adapterOutputChoice = ArrayAdapter.createFromResource(contextInSpinnerSelects,
                        R.array.LengthChoices_array, android.R.layout.simple_spinner_item);

                setInputOutputListenerAndDropDownAndAdapter();
            }
            break;

            case "Temperature": {
                adapterInputChoice = ArrayAdapter.createFromResource(contextInSpinnerSelects,
                        R.array.TemperatureChoices_array, android.R.layout.simple_spinner_item);

                adapterOutputChoice = ArrayAdapter.createFromResource(contextInSpinnerSelects,
                        R.array.TemperatureChoices_array, android.R.layout.simple_spinner_item);

                setInputOutputListenerAndDropDownAndAdapter();
            }
            break;

            case "Weight": {
                adapterInputChoice = ArrayAdapter.createFromResource(contextInSpinnerSelects,
                        R.array.WeightChoices_array, android.R.layout.simple_spinner_item);

                adapterOutputChoice = ArrayAdapter.createFromResource(contextInSpinnerSelects,
                        R.array.WeightChoices_array, android.R.layout.simple_spinner_item);

                setInputOutputListenerAndDropDownAndAdapter();

            }
            break;
        }
    }
    private void setInputOutputListenerAndDropDownAndAdapter() {

        spinnerInputChoice.setOnItemSelectedListener(this);
        spinnerOutputChoice.setOnItemSelectedListener(this);

        adapterInputChoice.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinnerInputChoice.setAdapter(adapterInputChoice);
        adapterOutputChoice.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinnerOutputChoice.setAdapter(adapterOutputChoice);

    }

    public Spinner getSpinnerInputChoice() {
        return spinnerInputChoice;
    }

    public Spinner getSpinnerOutputChoice() {
        return spinnerOutputChoice;
    }


    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

        inputChoice = spinnerInputChoice.getSelectedItem().toString();
        outputChoice = spinnerOutputChoice.getSelectedItem().toString();

        converter converterInSpinnerSelects = new converter(inputChoice, outputChoice);
        /*converterInSpinnerSelects.setInputOutputChoice(inputChoice, outputChoice);*/
    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) {

    }
}

Answer 1

您的问题可能是 inputChoice从未设置。

您正在创建 converter 的新实例- 具有以下代码的类:

converter converterInMain = new converter(selectedMainChoice);

这将仅在您的 converter 中运行此代码-实例:

public converter(String selectedMainChoiceFromMain) {
    this.MainChoice = selectedMainChoiceFromMain;
    /* No Log for Main Choice is Being Shown Here */
    Log.i("Main Choice is", MainChoice);
}

因此第二个构造函数永远不会执行 public converter(String inputChoiceFromSpinnerSelects, String outputChoiceFromSpinnerSelects){这意味着你从未设置 inputChoice也不outputChoice在你的converter -实例，这意味着他们留下null

Log.i("Sub Input Choices are:", inputChoice);因此得到 null如inputChoice

但我认为这不是问题所在。它应该仍然可以工作，但只是输出 null进入你的控制台。我记得 Log.i() 的工作原理如下: Log.i(TAG, MESSAGE)所以你的代码应该是这样的:

Log.i("CONVERTER", "Sub Input Choices are: " + inputChoice);

更新现在，您正在创建 convertor 的新实例在(AdapterView<?> parent, View view, int position, long id) {

converter converterInSpinnerSelects = new converter(inputChoice, outputChoice);

这将不会更新您在onClick(View view)中创建的转换器实例的值。 -功能。

解决此问题的一种方法可能是简单地将字符串置于静态:

public static String MainChoice, inputChoice, outputChoice;

但是这样就不需要创建 converter 的新实例了每次...所以只需这样做:

@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
    converter.inputChoice = spinnerInputChoice.getSelectedItem().toString();
    converter.outputChoice = spinnerOutputChoice.getSelectedItem().toString();
}