从另一个活动中启动带有ListView的活动的麻烦

Question

**I have a strange problem (for me) when i launch an activity and this activity has a listview from another activity it didn't work but when i send a string to fill the listview it launches !! here is the activity from where i will launch the another activity (the problem is in the second intent which will go to show.class )

package com.hema.colornotes;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;


public class MainActivity extends ActionBarActivity {

    Button b1,b2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        b1 = (Button) findViewById(R.id.btAdd);
        b2 = (Button) findViewById(R.id.btShow);
        b1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent i =new Intent(MainActivity.this,add.class);
                startActivity(i);
            }
        });

        b2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent i = new Intent(MainActivity.this, show.class);
                startActivity(i);
            }
        });
    }




}

and here is the activity i need to launch

package com.hema.colornotes;
import android.content.Context;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;

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


public class show extends ActionBarActivity {
    Context context;
    List<String> tasks;
    ArrayAdapter<String> adapter;




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

        context = this;
        tasks = new ArrayList<String>();


        // second parameter is row layout,
        adapter = new ArrayAdapter<String>(context, android.R.layout.simple_list_item_1,tasks);
        ListView listView = (ListView) findViewById(R.id.listView);
        listView.setAdapter(adapter);
        Intent intent = getIntent();
        String value = intent.getExtras().getString("message");

        tasks.add(value);

        // this method will refresh your listview manually
        adapter.notifyDataSetChanged();


    }


}

and that's the activity which worked well and launched the show activity because i sent a string

package com.hema.colornotes;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;


public class add extends ActionBarActivity {
    EditText note;

    Button b1,b2;

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

        b1=(Button)findViewById(R.id.btSave);
        b2=(Button)findViewById(R.id.btCancel);


        b1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                note = (EditText) findViewById(R.id.etNote);
                String notes = note.getText().toString();
                Intent i = new Intent(add.this, show.class);
                i.putExtra("message", notes);
                startActivity(i);
            }
        });
        b2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(add.this, MainActivity.class);
                startActivity(i);

            }
        });


    }


}

thanks in advance !**

Answer 1

When you dont send bundle to show activity, It would have crashed the app with null pointer exception. That is why, it did not work. When you send the bundle, show activity gets the required data to display. So it works fine!

Answer 2

您的第二个活动被设置为从意图中获取“消息”，因此当您不输入任何字符串时，您的第二个活动将崩溃。