Question

我正在尝试创建一个＆＃34;新闻应用程序＆＃34;使用Android Studio。我的主要活动是ListView和floating button。在我的第二项活动中，我获得了two TextViews，two EditViews和one Button。我想做的是以下内容：首先，我按下主要活动中的floating Button。然后会弹出第二个活动，然后我在two EditText's上写下，然后按button。之后我应该回到主要活动那里，应该在ListView中添加一个新项目。新项目应填写第二个活动的EditTexts中的文本。

这是我的主要活动：

package news;

import android.content.Intent;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //Get subject value from addpost Activity
        final String new_value = getIntent().getStringExtra("newSubject");

        String [] new_subject = new String []
                {new_value};

        FloatingActionButton add_post_button = findViewById(R.id.post_btn);
        final ListView post_view = findViewById(R.id.news_feed);
        //Create Adapter to add the new subject to listview
        if (new_subject != null) {
            ArrayAdapter newslist_adapter = new ArrayAdapter(
                    MainActivity.this,
                    android.R.layout.simple_expandable_list_item_1, new_subject);
            post_view.setAdapter(newslist_adapter);
        }
        add_post_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, addpost_activity.class);
                MainActivity.this.startActivity(intent);
            }
        });
    }
}

这是第二个活动（addpost_activity）：

package news;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.content.Intent;
import android.widget.EditText;
import android.widget.ListView;

import java.util.ArrayList;


public class addpost_activity extends AppCompatActivity {

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

        Button post_button = findViewById(R.id.post_btn);
        final EditText subject_text = findViewById(R.id.subject_edit);

        post_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(subject_text.getText() != null) {
                    Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                    intent.putExtra("newSubject", subject_text.getText().toString());
                    startActivity(intent);
                }
            }
        });
    }
}

我的问题是，每次启动时应用程序都会崩溃。我也从日志中收到此消息：

java.lang.NullPointerException：尝试调用虚方法＆＃39; java.lang.String java.lang.Object.toString（）＆＃39;在空对象引用上

解决：感谢Munir，问题解决了。

以下是我的活动

MainActivity：

package news;

import android.app.Activity;
import android.content.Intent;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
    ArrayAdapter newslist_adapter;
    ArrayList<String> new_subject = new ArrayList<>();

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

        FloatingActionButton add_post_button = findViewById(R.id.post_btn);

        //Create Adapter to add the new subject to listview
        add_post_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, addpost_activity.class);
                startActivityForResult(intent, 1);

            }
        });

    }
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        final ListView post_view = findViewById(R.id.news_feed);
        if (requestCode == 1) {
            if(resultCode == Activity.RESULT_OK){
                String new_value = data.getStringExtra("newSubject");
                new_subject.add(new_value);
                newslist_adapter = new ArrayAdapter(
                        MainActivity.this,
                        android.R.layout.simple_expandable_list_item_1, new_subject);
                post_view.setAdapter(newslist_adapter);

            }
        }
    }
}

这是我的第二个活动（addpost_activity）：

package news;

import android.app.Activity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.content.Intent;
import android.widget.EditText;
import android.widget.ListView;

import java.util.ArrayList;


public class addpost_activity extends AppCompatActivity {

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

        Button post_button = findViewById(R.id.post_btn);
        final EditText subject_text = findViewById(R.id.subject_edit);

        post_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(subject_text.getText() != null) {
                    Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                    intent.putExtra("newSubject", subject_text.getText().toString());
                    setResult(Activity.RESULT_OK, intent);
                    finish();
                }
            }
        });
    }
}

Answer 1

为此，我建议使用Fragments代替两个Activities。然后MainActivity会保留您的所有数据。

请参阅Fragments guide。

Answer 2

从MainActivity使用addpost_activity方法调用startActivityForResult()而不是startActivity();

在addpost_activity中设置要返回FirstActivity

的数据

post_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(subject_text.getText() != null) {
                    Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                    intent.putExtra("newSubject", subject_text.getText().toString());
                    setResult(Activity.RESULT_OK,intent );
                   finish();

                }
            }
        });

在MainActivity通过覆盖onActivityResult()

访问结果

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    if (requestCode == 1) {
        if(resultCode == Activity.RESULT_OK){
            String result=data.getStringExtra("newSubject");
           //Add this value to your adapter and call notifyDataSetChanged();
        }

    }
}

Answer 3

如果你想在列表中保留旧条目，请使用String []的ArrayList，如下所示更改你的代码。

主要活动

将此变量声明为全局

ArrayAdapter newslist_adapter;
ArrayList<String> new_subject = new ArrayList<>();

设置适配器

newslist_adapter = new ArrayAdapter(
                    MainActivity.this,
                    android.R.layout.simple_expandable_list_item_1, new_subject);
            post_view.setAdapter(newslist_adapter);

将startActivity更改为startActivityForResult

 add_post_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, addpost_activity.class);
                startActivityForResult(intent,100);
            }
        });

在onActivity中覆盖此方法

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    if (requestCode == 100) {
            String result=data.getStringExtra("newSubject");
new_subject.add(result)
           //Add this value to your adapter and call newslist_adapter.notifyDataSetChanged();

    }
}

在你的addpost_activity中设置你想要返回的数据

post_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(subject_text.getText() != null) {
                    Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                    intent.putExtra("newSubject", subject_text.getText().toString());
                    setResult(100,intent );
                   finish();

                }
            }
        });

Answer 4

在MainActivity.java中，您应该覆盖名为“onActivityRecsult”的函数，当您从第二个活动返回到第一个活动时，将调用此方法。

添加新的ListView项并从另一个活动获取项的值

4 个答案: