在android studio中使用意图将数据从主活动传递到另一个活动

Question

I have a problem when passing data from the main activity to another activity using Intents .
I made MainActivity , BaiHat class and SecondActivity for getting data from the MainActivity .

Open logcat, there is an error like this:

(java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference at com.myappnvt.truyendulieu.MainActivity$1.onItemClick(MainActivity.java:30)) MainActivity.java:30 is line " String ten_bai_hat = listView.getItemAtPosition(position).toString();"

Here is the code:

 package com.myappnvt.truyendulieu;

import android.os.Parcel;
import android.os.Parcelable;

import java.lang.reflect.Array;

public class BaiHat implements Parcelable {
    private String tenBaiHat;
    private int tenFile;

    public BaiHat(String tenBaiHat, int tenFile) {
        this.tenBaiHat = tenBaiHat;
        this.tenFile = tenFile;
    }

    public BaiHat(Parcel in) {
        tenBaiHat = in.readString();
        tenFile = in.readInt();
    }

    public static final Creator<BaiHat> CREATOR = new Creator<BaiHat>() {
        @Override
        public BaiHat createFromParcel(Parcel in) {
            return new BaiHat(in);
        }

        @Override
        public BaiHat[] newArray(int size) {
            return new BaiHat[size];
        }
    };

    public String getTenBaiHat() {
        return tenBaiHat;
    }

    public void setTenBaiHat(String tenBaiHat) {
        this.tenBaiHat = tenBaiHat;
    }

    public int getTenFile() {
        return tenFile;
    }

    public void setTenFile(int tenFile) {
        this.tenFile = tenFile;
    }

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(tenBaiHat);
        dest.writeInt(tenFile);

    }
    public  static final Parcelable.Creator<BaiHat> creator = new Parcelable.Creator<BaiHat>(){
        public BaiHat createFromParcel(Parcel in){
            return new BaiHat(in);
        }
        public BaiHat[] newArray (int size){
            return  new BaiHat[size];
        }
    };
}

    package com.myappnvt.truyendulieu;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.os.Parcelable;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {
    BaiHatAdapter adapter;
    ListView listView;
    ArrayList<BaiHat> baiHatArrayList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Addlist();
        adapter = new BaiHatAdapter(MainActivity.this, R.layout.dong_bai_hat, baiHatArrayList);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String ten_bai_hat = listView.getItemAtPosition(position).toString();
                startActivity(new Intent(getApplicationContext(),SecondActivity.class)
                        .putExtra("BAIHAT", baiHatArrayList)
                .putExtra("TENBAIHAT", ten_bai_hat)
                .putExtra("POSITION",position));
            }
        });


    }
    private void Addlist(){
        listView = (ListView) findViewById(R.id.listViewdanhsach);
        baiHatArrayList = new ArrayList<>();
        baiHatArrayList.add(new BaiHat("Bến tương tư - Quang Linh", R.raw.ben_tuong_tu_quang_linh));
        baiHatArrayList.add(new BaiHat("Buồn trong kỷ niệm - Đan Nguyên", R.raw.buon_trong_ky_niem_dan_nguyen));
        baiHatArrayList.add(new BaiHat("Cát bụi cuộc đời - Phi Nhung", R.raw.cat_bui_cuoc_doi_phi_nhung));
        baiHatArrayList.add(new BaiHat("Cho vừa lòng em - Như Nguyệt", R.raw.cho_vua_long_em_nhu_nguyet));
        baiHatArrayList.add(new BaiHat("Chuyến đò không em - Tuấn Vũ", R.raw.chuyen_do_khong_em_tuan_vu));

    }
}

    package com.myappnvt.truyendulieu;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.media.MediaPlayer;
import android.net.Uri;
import android.os.Bundle;
import android.widget.TextView;

import java.util.ArrayList;

public class SecondActivity extends AppCompatActivity {
    String bai_hat ="";
    int vi_tri =0;
    TextView txtlaydulieu;
    ArrayList<BaiHat> baiHatArrayList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        txtlaydulieu = (TextView) findViewById(R.id.textViewlaydulieu);
        Intent intent = getIntent();
        Bundle bundle = intent.getExtras();
        baiHatArrayList = (ArrayList) bundle.getParcelableArrayList("BAIHAT");
        bai_hat = baiHatArrayList.get(vi_tri).getTenBaiHat().toString();
        String songName = intent.getStringExtra("TENBAIHAT");
        txtlaydulieu.setText(songName);
        txtlaydulieu.setSelected(true);
        vi_tri = bundle.getInt("POSITION",0);
        Uri uri = Uri.parse(baiHatArrayList.get(vi_tri).toString());
        MediaPlayer  mediaPlayer = MediaPlayer.create(getApplicationContext(), uri);
        mediaPlayer.start();
    }
}

Answer 1

can you try to get item like this. instead of getting the list from the adapter you can also retrieve the source like belo. The error says nullpointer exception that means you are not getting the data from adapter. So instead try below once.

String ten_bai_hat = baiHatArrayList.get(position).toString();

Full code:

public class MainActivity extends AppCompatActivity {
BaiHatAdapter adapter;
ListView listView;
ArrayList<BaiHat> baiHatArrayList;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Addlist();
    adapter = new BaiHatAdapter(MainActivity.this, R.layout.dong_bai_hat, baiHatArrayList);
    listView.setAdapter(adapter);
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            String ten_bai_hat = baiHatArrayList.get(position).toString();
            startActivity(new Intent(getApplicationContext(),SecondActivity.class)
                    .putExtra("BAIHAT", baiHatArrayList)
            .putExtra("TENBAIHAT", ten_bai_hat)
            .putExtra("POSITION",position));
        }
    });


}
private void Addlist(){
    listView = (ListView) findViewById(R.id.listViewdanhsach);
    baiHatArrayList = new ArrayList<>();
    baiHatArrayList.add(new BaiHat("Bến tương tư - Quang Linh", R.raw.ben_tuong_tu_quang_linh));
    baiHatArrayList.add(new BaiHat("Buồn trong kỷ niệm - Đan Nguyên", R.raw.buon_trong_ky_niem_dan_nguyen));
    baiHatArrayList.add(new BaiHat("Cát bụi cuộc đời - Phi Nhung", R.raw.cat_bui_cuoc_doi_phi_nhung));
    baiHatArrayList.add(new BaiHat("Cho vừa lòng em - Như Nguyệt", R.raw.cho_vua_long_em_nhu_nguyet));
    baiHatArrayList.add(new BaiHat("Chuyến đò không em - Tuấn Vũ", R.raw.chuyen_do_khong_em_tuan_vu));

}

}

Answer 2

Try to implement your BaiHatAdapter like this:

public class BaiHatAdapter extends ArrayAdapter {
    public BaiHatAdapter(@NonNull Context context, int layout, int textViewId, @NonNull List<BaiHat> objects) {
        super(context, layout, textViewId, objects);
    }

    @Nullable
    @Override
    public Object getItem(int position) {
        BaiHat item = (BaiHat) super.getItem(position);
        return (item != null && item.getTenBaiHat() != null) ? item.getTenBaiHat() : "Default TenBaiHat Text";
    }
}

textViewId - id of TextView that will display the ten_bai_hat text. Here you override the getItem method to return the tenBaiHat value. If either the item is null or the tenBaiHat string is null, then return some default text.