Android-单击另一个类中的按钮后，如何重新启动MainActivity？

Question

so I'm fairly new to Android. Lemme just explain first: I'm trying to make a small portable music app, just for my phone and to test my skills/review what I've learned. I have the mainactivity setup to pick a song with 1 of 4 buttons, and it'll start another activity with buttons to pause, resume, and go back to the song selection screen (MainActivity). I'm trying to get the back button to both release the player and finish the activity, and I've tried many different things but nothing seems to work; the app closes due to some exception (a common one being NullPointerException).

So here's MainActivity:

package me.lemmy.portablemusic.app;

import android.media.MediaPlayer;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.content.Intent;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;

public class MainActivity extends ActionBarActivity {

Button wreckingBall, happy, lig, ligMulti;
Intent songPickedActivity = new Intent("me.lemmy.portablemusic.app.SONGPICKED");
public static MediaPlayer player;

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


    //set buttons
    wreckingBall = (Button) findViewById(R.id.song_wreck);
    happy = (Button) findViewById(R.id.song_happy);
    lig = (Button) findViewById(R.id.song_lig);
    ligMulti = (Button) findViewById(R.id.song_lig_multi);

    //listeners
    wreckingBall.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(songPickedActivity);
            player = MediaPlayer.create(MainActivity.this, R.raw.wrecking_ball);
            player.start();
        }
    });

    happy.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(songPickedActivity);
            player = MediaPlayer.create(MainActivity.this, R.raw.happy);
            player.start();
        }
    });

    lig.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(songPickedActivity);
            player = MediaPlayer.create(MainActivity.this, R.raw.let_it_go);
            player.start();
        }
    });

    ligMulti.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(songPickedActivity);
            player = MediaPlayer.create(MainActivity.this, R.raw.let_it_go_multi);
            player.start();
        }
    });
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

public static MediaPlayer getPlayer(){
    return player;
}

}

and here's my SongPicked activity:

package me.lemmy.portablemusic.app;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.media.MediaPlayer;
import android.widget.TextView;

/**
 * Created by Lemuel on 6/20/14.
 */
public class SongPicked extends Activity {

TextView text;
Button pause, resume, back;
MediaPlayer player;

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

    text = (TextView) findViewById(R.id.tvPlaying);
    pause = (Button) findViewById(R.id.buttonPause);
    resume = (Button) findViewById(R.id.buttonResume);
    back = (Button) findViewById(R.id.buttonBack);

    player = MainActivity.getPlayer();

    pause.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            player.pause();
        }
    });

    resume.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            player.start();
        }
    });

    back.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view){
            player.release();
        }
    });

}

@Override
protected void onPause(){
    super.onPause();
    finish();
}
}

Any help is appreciated, thanks! PS I know I can't use copyrighted music in my apps, this is just a test.

Answer 1

I suggest to first make sure the activity is in the AndroidManifest file. Second is to change your intent from this:

Intent songPickedActivity = new Intent("me.lemmy.portablemusic.app.SONGPICKED");

into:

Intent songPickedActivity = new Intent(this, SongPicked.class);

Do it as well on the songPickedActivity when going back to MainActivity. You can use putExtra to send data to next intent. For more information click this link .