所以我正在尝试将输入(数字)保存到ArrayList，然后在可点击的ListView上显示ArrayList。
当我尝试启动应用程序时，它不断崩溃，我哪里出错了？
public class MainActivity extends AppCompatActivity {
EditText numberIn;
ListView numberList;
Button saveBtn;
String numberToSave;
ArrayList<String> allNumbers = new ArrayList<String>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
numberIn = (EditText) findViewById(R.id.editText);
numberList = (ListView) findViewById(R.id.listView1);
saveBtn = (Button) findViewById(R.id.button);
myNumbers();
final ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, allNumbers);
numberList.setAdapter(arrayAdapter);
numberList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
String selectedNumber = arrayAdapter.getItem(position);
}
});
saveBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
numberToSave = numberIn.getText().toString();
myNumbers();
}
});
}
public void myNumbers(){
allNumbers.add(numberToSave);
}
}
日志猫：-
05-20 13:07:54.416 8449-8449/com.example.tsury.listex E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.tsury.listex, PID: 8449
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:409)
at android.widget.ArrayAdapter.getView(ArrayAdapter.java:371)
at android.widget.AbsListView.obtainView(AbsListView.java:2363)
at android.widget.ListView.makeAndAddView(ListView.java:1970)
at android.widget.ListView.fillDown(ListView.java:704)
at android.widget.ListView.fillFromTop(ListView.java:765)
at android.widget.ListView.layoutChildren(ListView.java:1744)
at android.widget.AbsListView.onLayout(AbsListView.java:2162)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1197)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:437)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1741)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1585)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1494)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at com.android.internal.policy.DecorView.onLayout(DecorView.java:726)
at android.view.View.layout(View.java:17637)
at android.view.ViewGroup.layout(ViewGroup.java:5575)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2346)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2068)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1254)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6337)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:874)
at android.view.Choreographer.doCallbacks(Choreographer.java:686)
at android.view.Choreographer.doFrame(Choreographer.java:621)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:860)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
您希望在膨胀布局之前获取 TextView，因此它是一个简单的 NullPointerExecption。更改您的代码：
公共类 ListViewDemo 扩展了 ListActivity {
ArrayList<String> listItems=new ArrayList<String>();
ArrayAdapter<String> adapter;
TextView theFact;
String shareFact;
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
theFact = (TextView) findViewById(R.id.tvContent)
shareFact = theFact.getText().toString();
adapter=new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1,
listItems);
setListAdapter(adapter);
listItems.add(shareFact);
adapter.notifyDataSetChanged();
}
} 如果你按下按钮，它将调用addItems()函数(正如你设置的那样)，但它还没有实现，所以它会生成另一个错误。
public void addItems(View v) {
listItems.add(shareFact);
adapter.notifyDataSetChanged();
}
在onCreate()方法下方添加此函数，并将listItems.add()行和adapter.notifyDataSetChanged()行移动到此新方法。
试试
saveBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if(numberIn.getText() != null){
numberToSave = numberIn.getText().toString();
myNumbers();
}
}
});
您尝试在此处对空对象调用.toString()：
numberToSave = numberIn.getText().toString();
您可能希望先添加空检查：
saveBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (numberIn.getText() != null {
numberToSave = numberIn.getText().toString();
myNumbers();
}
}
});