我正在尝试通过使用意图来填充列表视图 [重复]答案

【问题标题】：I'm trying to populate a listview by using an intent [duplicate]我正在尝试通过使用意图来填充列表视图 [重复]
【发布时间】：2017-12-22 19:33:35
【问题描述】：

我试图让用户将我的应用加载到包含列表视图的主 Activity，然后移动到第二个 Activity 并输入他们的“名称”和“cmets”，将其传递回主 Activity 并显示在敬酒，并将其放入列表视图中。但是，当我尝试时，我的应用程序崩溃了。为什么？

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorBackground"
    tools:context="com.example.MainActivity">


    <Button
        android:id="@+id/contact"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Contact Us"
        android:layout_marginRight="46dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintVertical_bias="0.982"
        android:layout_marginEnd="46dp" />

    <Button
        android:id="@+id/comments"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Add Comments"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/contact"
        android:layout_marginRight="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintHorizontal_bias="0.418"
        app:layout_constraintVertical_bias="0.982"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp" />

    <ListView
        android:id="@+id/ListViewMain"
        android:layout_width="207dp"
        android:layout_height="243dp"
        android:layout_marginLeft="8dp"
        android:textColor="@android:color/white"
        app:layout_constraintLeft_toLeftOf="parent"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintHorizontal_bias="0.503"
        app:layout_constraintVertical_bias="0.253" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:text="Comments"
        android:textColor="@android:color/white"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.062"
        app:layout_constraintHorizontal_bias="0.456" />

</android.support.constraint.ConstraintLayout>

MainActivity.java

package com.example;

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

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button comments;
    private Button contact;
    ListView lv;
    private String Name = "";
    private String Comments = "";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

         initViews();
        initListeners();
        lv = (ListView) findViewById(R.id.ListViewMain);
        Bundle extras = getIntent().getExtras();
                if (extras != null)
                {
                    Name = extras.getString("NAME");
                    Comments = extras.getString("COMMENTS");

                }


            String[] values = new String[] { Name, Comments};

            ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                    android.R.layout.simple_list_item_1, android.R.id.text1, values);

            lv.setAdapter(adapter);


    }

    private void initViews(){
        comments = (Button) findViewById(R.id.comments);
        contact = (Button) findViewById(R.id.contact);





    }

    private void initListeners(){
        comments.setOnClickListener(this);
        contact.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.comments:

                Context context = getApplicationContext();
                CharSequence text = "My name";
                int duration = Toast.LENGTH_SHORT;

                Toast toast = Toast.makeText(context, text, duration);
                toast.show();

                Intent intent2 = new Intent(getApplicationContext(), CommentsActivity.class);
                startActivity(intent2);

                break;
            case R.id.contact:
                Intent intent = new Intent(Intent.ACTION_SEND);
                intent.setType("text/plain");
                intent.putExtra(Intent.EXTRA_EMAIL, "myemail");
                intent.putExtra(Intent.EXTRA_SUBJECT, "Subject");
                intent.putExtra(Intent.EXTRA_TEXT, "Hello!");

                startActivity(Intent.createChooser(intent, "Send Email"));
                break;
        }
    }
}

activity_cmets.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorBackground"
    tools:context="com.example.CommentsActivity">

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:text="Comments:"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.378"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintHorizontal_bias="0.087" />

    <EditText
        android:id="@+id/COMMENTS"
        android:layout_width="316dp"
        android:layout_height="53dp"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:inputType="text"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.432" />

    <EditText
        android:id="@+id/NAME"
        android:layout_width="316dp"
        android:layout_height="53dp"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:inputType="text"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.174" />

    <Button
        android:id="@+id/submit"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Submit"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintVertical_bias="0.894" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Name:"
        android:textColor="@android:color/white"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        app:layout_constraintVertical_bias="0.14"
        android:layout_marginLeft="8dp"
        app:layout_constraintLeft_toLeftOf="parent"
        android:layout_marginRight="8dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintHorizontal_bias="0.079" />

</android.support.constraint.ConstraintLayout>

CommentsActivity.java

package com.example;

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

public class CommentsActivity extends AppCompatActivity implements View.OnClickListener {

    private Button submit;
    private EditText edNAME;
    private EditText edCOMMENTS;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_comments);
        initViews();
        initListeners();
         edNAME  = (EditText)findViewById(R.id.NAME);
        edCOMMENTS  = (EditText)findViewById(R.id.COMMENTS);

    }

    private void initListeners(){
        submit.setOnClickListener(this);

    }

    private void initViews(){
        submit = (Button) findViewById(R.id.submit);

    }

    @Override
    public void onClick(View v) {

        switch (v.getId()) {
            case R.id.submit:



                CharSequence text = edNAME.getText().toString() + " - "  + edCOMMENTS.getText().toString();
                int duration = Toast.LENGTH_SHORT;
                Toast toast = Toast.makeText(getApplicationContext(), text, duration);
                toast.show();

                Intent intent = new Intent(getBaseContext(), MainActivity.class);
                intent.putExtra("NAME",  edNAME.getText());
                intent.putExtra("COMMENTS",  edCOMMENTS.getText());
                startActivity(intent);

                break;
        }

    }
}

错误信息：

12-22 14:31:58.765 1253-1253/com.example E/AndroidRuntime: 致命例外：主要进程：com.example，PID：1253 java.lang.NullPointerException：尝试调用虚拟方法空对象上的“java.lang.String java.lang.Object.toString()” 参考在 android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:445) 在 android.widget.ArrayAdapter.getView(ArrayAdapter.java:407) 在 android.widget.AbsListView.obtainView(AbsListView.java:2372) 在 android.widget.ListView.makeAndAddView(ListView.java:2052) 在 android.widget.ListView.fillDown(ListView.java:786) 在 android.widget.ListView.fillFromTop(ListView.java:847) 在 android.widget.ListView.layoutChildren(ListView.java:1826) 在 android.widget.AbsListView.onLayout(AbsListView.java:2171) 在 android.view.View.layout(View.java:19586) 在 android.view.ViewGroup.layout(ViewGroup.java:6053) 在 android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1197) 在 android.view.View.layout(View.java:19586) 在 android.view.ViewGroup.layout(ViewGroup.java:6053) 在 android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) 在 android.widget.FrameLayout.onLayout(FrameLayout.java:261) 在 android.view.View.layout(View.java:19586) 在 android.view.ViewGroup.layout(ViewGroup.java:6053) 在 android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:434) 在 android.view.View.layout(View.java:19586) 在 android.view.ViewGroup.layout(ViewGroup.java:6053) 在 android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) 在 android.widget.FrameLayout.onLayout(FrameLayout.java:261) 在 android.view.View.layout(View.java:19586) 在 android.view.ViewGroup.layout(ViewGroup.java:6053) 在 android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791) 在 android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635) 在 android.widget.LinearLayout.onLayout(LinearLayout.java:1544) 在 android.view.View.layout(View.java:19586) 在 android.view.ViewGroup.layout(ViewGroup.java:6053) 在 android.widget.FrameLayout.layoutChildren(FrameLayout.java:323) 在 android.widget.FrameLayout.onLayout(FrameLayout.java:261) 在 com.android.internal.policy.DecorView.onLayout(DecorView.java:758) 在 android.view.View.layout(View.java:19586) 在 android.view.ViewGroup.layout(ViewGroup.java:6053) 在 android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2484) 在 android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2200) 在 android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1386) 在 android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6733) 在 android.view.Choreographer$CallbackRecord.run(Choreographer.java:911) 在 android.view.Choreographer.doCallbacks(Choreographer.java:723) 在 android.view.Choreographer.doFrame(Choreographer.java:658) 在 android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897) 在 android.os.Handler.handleCallback(Handler.java:789) 在 android.os.Handler.dispatchMessage(Handler.java:98) 在 android.os.Looper.loop(Looper.java:164) 在 android.app.ActivityThread.main(ActivityThread.java:6541) 在 java.lang.reflect.Method.invoke（本机方法）在 com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
                                                                       [ 12-22 14:31:58.766  1253: 1347 D/         ]
                                                                       SurfaceInterface::setAsyncMode: set async mode 1

【问题讨论】：

标签： java android android-layout listview nullpointerexception

【解决方案1】：

您在ArrayAdapter 中传递的数组中必须至少有一个空值。请在初始化ArrayAdapter 之前记录MainActivity 类中变量Name 和Comments 的值。

【讨论】：