如何在android中访问其他类中选择的列表项

Question

以下是我的类，当我运行应用程序时，它在选择时崩溃，并给我一个空错误。我正在设计一个修订应用程序，并试图做到这一点，而不必为涵盖的每个主题的类，任何帮助都会非常感谢。

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

import org.w3c.dom.Text;

public class TheoryMain extends Activity {
    TheoryTopicList ttl;
    TextView tv1;
    String choice;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.theory_layout);
        ttl = new TheoryTopicList();
        ttl.getChoice();
        tv1 = (TextView) findViewById(R.id.theory_tv);
        switch(choice){
            case("Photo-Electric effect"):
                tv1.setText(getString(R.string.photo_electric));
                break;
            case("Photons and Electrons"):
                tv1.setText(getString(R.string.photons_electrons));
                break;
            case("de Broglie wavelength"):
                tv1.setText(getString(R.string.de_broglie));
                break;
            case("Types of particles"):
                tv1.setText(getString(R.string.particles));
                break;
            case("Interactions"):
                tv1.setText(getString(R.string.interactions));
                break;
            case("Radiation"):
                tv1.setText(getString(R.string.radiation));
                break;
            case("Voltage, Current and Resistance(Ohms law)"):
                tv1.setText(getString(R.string.ohms_law));
                break;
            case("Circuits"):
                tv1.setText(getString(R.string.circuits));
                break;
            case("Power and Efficiency"):
                tv1.setText(getString(R.string.power_efficiency));
                break;
            case("Alternating Current and oscilloscope graphs"):
                tv1.setText(getString(R.string.ac_graphs));
                break;
            case("E.M.F and internal Resistance"):
                tv1.setText(getString(R.string.emf_resistance));
                break;
        }
    }
}






import android.app.ListActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;

public class TheoryTopicList extends ListActivity {
    String[] display = {"Photo-Electric effect", "Photons and Electrons", "de Broglie wavelength", "Types of particles",
                        "Interactions", "Radiation", "Voltage, Current and Resistance(Ohms law)", "Circuits",
                        "Power and Efficiency","Alternating Current and oscilloscope graphs", "E.M.F and internal Resistance"};
    String choice;
    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {

        super.onListItemClick(l, v, position, id);
        choice = display[position];
        Intent intent = new Intent("com.revisonapp.alec.alevelphysics.THEORYMAIN");
        startActivity(intent);
    }

    public String getChoice(){
        return choice;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setListAdapter(new ArrayAdapter<String>(TheoryTopicList.this,android.R.layout.simple_list_item_1,display));
    }
}

Answer 1

试着这样做

Intent intent=new Intent(this,AnotherClass.class);
intent.putExtra("yourStringVal",yourStringVal);
startActivity(intent);
and in your another class try 2 get these items by doing

  Bundle extras = getIntent().getExtras();
        if(extras!=null){
            String yourStringVal=extras.getString("yourStringVal");

        }

Answer 2

你对你处理活动的方式的理解有一点错误。

通常，您永远不会自己实例化一个活动。

您需要研究如何使用Intents、startActivity和onActivityResult。

首先，创建一个TheoryTopicList的Intent，然后设置结果数据并完成该Activity。然后从onActivityResult读取选定的选项。

Answer 3

在您的TheoryTopicList活动中，当您触发调用TheoryMain活动的意图时，您可以将数据与该意图一起发送。这些数据可以由TheoryMain类接收，并用于执行适当的操作。

在TheoryTopicList活动中向您的意图添加数据。

Intent intent = new Intent("com.revisonapp.alec.alevelphysics.THEORYMAIN");
intent.putExtra("choice_selected", choice);

//第一个参数是key，下一个参数是value//键，以便在下一个活动中检索该值。

startActivity(intent);

从TheoryMain，中删除活动实例化行

ttl = new TheoryTopicList(); // Not needed and not good
ttl.getChoice(); // Not needed and not good

现在检索上一个活动提供给您的数据。在你的onCreate of TheoryMain add中，

...

String choice = getIntent().getExtra("choice_selected"); // Using the key
switch(choice){

...
}