无法使微调器在Android Studio中工作

Question

I am having great difficulty understanding how to code a spinner. I've looked at the many items on this site and others but don't understand why it has to be SO complicated, with some solutions even involving creating a class. All I want to be able to do is, when the user selects one of the items from the spinner drop-down, which one has been clicked needs to be identified then the code triggers a new activity or a method or some other action. The web site I-Programmer seemed to offer a relatively simple solution and I have followed the coding instructions to the letter but it doesn't work. I have written to them but they haven't replied yet. Their code now follows, but I don't understand how it can work, as there doesn't seem to be a link between the AdapterView block and the actual spinner stuff before it. It doesn't compile as is. When I add a few semi-colons and braces, it DOES compile but still doesn't work. Can anyone sort it out for me please?

package com.example.owner.spinners;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Spinner;

public class MainActivity extends AppCompatActivity
{

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

    String[] country = {"Canada", "Mexico", "USA"};

    ArrayAdapter<String> stringArrayAdapter=
            new ArrayAdapter<String>(this,
                    android.R.layout.simple_spinner_dropdown_item,
                    country);

    Spinner spinner=
            (Spinner)findViewById(R.id.spinner);
    spinner.setAdapter(stringArrayAdapter);
}

AdapterView.OnItemSelectedListener onSpinner=
        new AdapterView.OnItemSelectedListener(){  // Error 1 here
            AdapterView.OnItemSelectedListener onSpinner =
                    new AdapterView.OnItemSelectedListener() {
                        @Override
                        public void onItemSelected(
                                AdapterView<?> parent,
                                View view,
                                int position,
                                long id) {  // Does my 'if' code go here?
                        }

                        @Override
                        public void onNothingSelected(
                                AdapterView<?>  parent) {  // and here?
                        }
                    } // Error 2 here. ; expected
        } // Error 3 here. ; expected

@Override
public boolean onCreateOptionsMenu(Menu menu)
{  
    ...
}

Error 1 reads, Class 'Anonymous class derived from OnItemSelectedListener' must either be declared abstract or implement abstract method 'onNothingSelected (AdapterView)' in 'OnItemSelectedListener' I'm afraid I don't follow all that. I have really tried. Thanks in advance.

Answer 1

If you don't want to use an extra class, you can use an anonymous inner class .

Be sure to register the Listener to your spinner.

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

    final String[] country = {"Canada", "Mexico", "USA"};

    ArrayAdapter<String> stringArrayAdapter=  new ArrayAdapter<>(this, android.R.layout.simple_spinner_dropdown_item, country);

    Spinner spinner = (Spinner)findViewById(R.id.spinner);
    spinner.setAdapter(stringArrayAdapter);

    spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() // register the listener
    {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id)
        {
            // User selected item
            Toast.makeText(getApplicationContext(), country[position] + " selected!", Toast.LENGTH_SHORT).show();
        }

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

        }
    });
}

Answer 2

First, why you have a compile error:

• Error 1 - because you tried create a variable in this place, but you can't do that. In this place you can use a existing variable/field or create a simple anonymous class.

• Error 2 - compiler expected ; because you are ended of the part of code, so you have to end this by ; .

• Error 3 - it the same situation like in Error 2.

So, I a little improved your code and added some comments. I hope now the code is more understandable for you.

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

    // create a adapter
    String[] country = {"Canada", "Mexico", "USA"};
    ArrayAdapter<String> stringArrayAdapter = new ArrayAdapter(this, android.R.layout.simple_spinner_dropdown_item, country);

    // create a spinner
    Spinner spinner = (Spinner) findViewById(R.id.spinner);
    // add adapter to spinner
    spinner.setAdapter(stringArrayAdapter);
    // create listener and add to spinner
    spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
            // put code which recognize a selected element
        }

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

        }
    });
}