微调器问题：如何调用微调器中的列表项，并分别对每个数学项使用数学？

Question

I have a snippet of code I've been working on with four conversion ratios depending on what unit conversion option is picked within the spinner. How would I call each individual ratio depending on what the user selects.

For Example , If option 1 is selected in the spinner then the conversion to use would be Miles to Kilometers or resultOutput = userInput * 1.6093 Since the conversion would be a userInput of miles then kilometers results in the miles * 1.6093 . Now how would I inject that and multiple other if statements into this source code? Would I create a new method or just do it right inside public void onItemSelected ?

Would I type: if(conversionType = list.get(0)){ resultOutput = userInput * 1.6093 ?

I just need a general direction on where to go with this as I have been stuck on it for a few days. Thanks for any help available!

MainActivity.java :

package com.example.corey.unitconversionappbycoreywhitlow;

import java.util.ArrayList;
import java.util.List;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity implements OnItemSelectedListener {

    Spinner spinnerConversions;
    TextView conversionFrom, conversionTo, outputResult;
    EditText userInput;
    List<String> list = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        conversionFrom = findViewById(R.id.fromValue);
        conversionTo = findViewById(R.id.toValue);
        outputResult = findViewById(R.id.resultOutput);
        userInput = findViewById(R.id.numInput);
    }

    public void onItemSelected(AdapterView<?> parent, View view, int pos, long id){
        spinnerConversions = findViewById(R.id.conversionSpinner);
        spinnerConversions.setOnItemSelectedListener(this);

        list.add("Miles to Kilometers");
        list.add("Kilometers to Miles");
        list.add("Inches to Centimeters");
        list.add("Centimeters to Inches");
        ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, list);

        adapter.setDropDownViewResource
             (android.R.layout.simple_spinner_dropdown_item);    

        spinnerConversions.setAdapter(adapter);
        String conversionType = list.get(1);
        String[] units = conversionType.split("to");
        String conFrom = units[0].trim();
        String conTo = units[1].trim();

        conversionFrom.setText(conFrom);
        conversionTo.setText(conTo);
    }

    public void onNothingSelected(AdapterView<?> parent){}
}

Answer 1

I made changes to your code:

public class MainActivity extends AppCompatActivity implements OnItemSelectedListener {

    Spinner spinnerConversions;
    TextView conversionFrom, conversionTo, outputResult;
    EditText userInput;
    List<String> list = new ArrayList<>();

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

        conversionFrom = findViewById(R.id.fromValue);
        conversionTo = findViewById(R.id.toValue);
        outputResult = findViewById(R.id.resultOutput);
        userInput = findViewById(R.id.numInput);

        spinnerConversions = findViewById(R.id.conversionSpinner);

        list.add("Miles to Kilometers");
        list.add("Kilometers to Miles");
        list.add("Inches to Centimeters");
        list.add("Centimeters to Inches");
        ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, list);

        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

        spinnerConversions.setAdapter(adapter);

        String conversionType = list.get(0);
        String[] units = conversionType.split("to");
        String conFrom = units[0].trim();
        String conTo = units[1].trim();

        conversionFrom.setText(conFrom);
        conversionTo.setText(conTo);

        spinnerConversions.setOnItemSelectedListener(this);
    }

    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
                Double d = 0.0;
                try {
                    d = Double.parseDouble(userInput.getText().toString().trim());
                } catch (NumberFormatException e) {
                    e.printStackTrace();
                }
        switch (position){
            case 0:
                outputResult.setText(d * 1.6093 + "");

                break;
            case 1:
                //Do something

                break;
            case 2:
                //Do something

                break;
            case 3:
                //Do something

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

}

You had a lot of initialization code in onItemSelected but it should be placed in your activity's onCreate()
The switch statement allows you to choose what to do with the user's choice. You will make there the conversions you need.
In onCreate() you have all the initialization code: variables and views declarations, setting the adapter and the listener for the spinner etc.