java - Android:创建意图后，无法从MainActivity打开UI

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  创建Intent方法后，无法从MainActivity访问UI。当我从MainActivity中选择SelectPLayer和SelectPLayer2活动时，我的应用程序崩溃。我也无法将Clickable ListViews转到GameEmulator中的指定TextView。

//MainActivity.java


        //Class to select player 1
        public class SelectPlayer extends Activity {


            //Public static array list with adapter to crete the array and reference for listview being sent by intent from AddPLayer class
            public static ArrayList<String> list = new ArrayList<>();
            public static ArrayAdapter<String> adapter;
            ListView selectView;
            TextView title;

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

                //Identify selectview ListView
                selectView = findViewById(R.id.selectview) ;
                selectView.setClickable(true);
                selectView.setVisibility(View.VISIBLE);

                //Using adapter for ListView menu
                adapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,list);
                selectView.setAdapter(adapter);
                selectView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                    @Override
                    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                        String item =(String) parent.getItemAtPosition(position); // finding the item which has been clicked by the user
                        Intent intent = new Intent(SelectPlayer.this, GameEmulator.class); //starting an intent to call GameEmulator Activity
                        intent.putExtra(GameEmulator.value, item);// Putting the value clicked by user in intent
                        startActivity(intent); // starting GameEmulator Activity
                    }
                });
                //Using intent to retrieve string from AddPlayer Activity
                    Intent i = getIntent();
                    String data = i.getExtras().getString("text_key");
                    list.add(data);
                    changeList();
            }
            public static void changeList()
            {

                adapter.notifyDataSetChanged();
            }
        }

//Addplayer.java

import android.app.Activity;
import android.view.View;
import android.widget.EditText;
import android.os.Bundle;
import android.widget.Button;
import android.content.Intent;

//Addplayer class to add players using intent to separate activities
public class AddPlayer extends Activity{

    //Declare buttons and edittext
    Button listButtonplayer1;
    Button listButtonplayer2;
    EditText playerIn;

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


        //Identify buttons and editext
        playerIn = (EditText) findViewById(R.id.inputname);

        listButtonplayer1 = (Button)findViewById(R.id.addbutton);
        listButtonplayer1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                //Using intent to send input to SelectPLayer Class
                Intent i = new Intent(AddPlayer.this,SelectPlayer.class);

                //Using string text_key to get and send string through intent
                i.putExtra("text_key", playerIn.getText().toString());


                //Start intent activity
                startActivity(i);
            }
        });

        //Second button I am trying to use for SelectPLayer2 activity only.
        listButtonplayer2 = (Button)findViewById(R.id.addbutton2);
        listButtonplayer2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                //Using intent to send input to SelectPLayer2 Class
                Intent in = new Intent(AddPlayer.this, SelectPlayer2.class);

                in.putExtra("text_key", playerIn.getText().toString());

                startActivity(in);
            }
        });
    }
}
Selectplayer1.java
//Class to select player 1
public class SelectPlayer extends Activity {


    //Public static array list with adapter to crete the array and reference for listview being sent by intent from AddPLayer class
    public static ArrayList<String> list = new ArrayList<>();
    public static ArrayAdapter<String> adapter;
    ListView selectView;
    TextView title;

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

        //Identify selectview ListView
        selectView = findViewById(R.id.selectview) ;
        selectView.setClickable(true);
        selectView.setVisibility(View.VISIBLE);

        //Using adapter for ListView menu
        adapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,list);
        selectView.setAdapter(adapter);
        selectView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String item =(String) parent.getItemAtPosition(position); // finding the item which has been clicked by the user
                Intent intent = new Intent(SelectPlayer.this, GameEmulator.class); //starting an intent to call GameEmulator Activity
                intent.putExtra(GameEmulator.value, item);// Putting the value clicked by user in intent
                startActivity(intent); // starting GameEmulator Activity
            }
        });
        //Using intent to retrieve string from AddPlayer Activity
            Intent i = getIntent();
            String data = i.getExtras().getString("text_key");
            list.add(data);
            changeList();
    }
    public static void changeList()
    {

        adapter.notifyDataSetChanged();
    }
}
//Selectplayer2.java

//Class to select player 2
public class SelectPlayer2 extends Activity {

    //Public static array list with adapter to crete the array and reference for listview being sent by intent from AddPLayer class
    public static ArrayList<String> list1 = new ArrayList<>();
    public static ArrayAdapter<String> adapter1;

    ListView selectView2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.selectplayer2);

        selectView2 = (ListView) findViewById(R.id.selectview1);
        selectView2.setClickable(true);
        selectView2.setVisibility(View.VISIBLE);

        //Using adapter for ListView menu
        adapter1 = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,list1);
        selectView2.setAdapter(adapter1);
        selectView2.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

                    String item =(String) parent.getItemAtPosition(position); // finding the item which has been clicked by the user
                    Intent in = new Intent(SelectPlayer2.this, GameEmulator.class); //starting an intent to call GameEmulator Activity
                    in.putExtra(GameEmulator.value, item);// Putting the value clicked by user in intent
                    startActivity(in); // starting GameEmulator Activity
            }
        });

        //Using intent to retrieve string from AddPlayer Activity
        Intent in = getIntent();
        String data = in.getExtras().getString("text_key");
        list1.add(data);
        changeList();
    }
    public static void changeList()
    {

        adapter1.notifyDataSetChanged();
    }
}

  我有单独的静态数组列表，但listView clickables仍显示在GameEmulator类的两个文本视图中。这是我的错误日志，试图单击SelectPLayer UI的按钮：

07-17 23:41:37.296 13468-13468/com.example.ryan_.ryansplayersapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.ryan_.ryansplayersapp, PID: 13468
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ryan_.ryansplayersapp/com.example.ryan_.ryansplayersapp.SelectPlayer}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
        at com.example.ryan_.ryansplayersapp.SelectPlayer.onCreate(SelectPlayer.java:47)
        at android.app.Activity.performCreate(Activity.java:7009)
        at android.app.Activity.performCreate(Activity.java:7000)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

最佳答案

 String data = i.getExtras().getString("text_key");

您的getExtras()返回null。确保正确初始化您的意图。