我為我的應(yīng)用程序創(chuàng)建了這個(gè)適配器：public class DeadlineAdapter extends ArrayAdapter<Deadline> { private Activity context; public List<Deadline> deadlineList; public DeadlineAdapter(Activity context, List<Deadline> deadlineList){ super(context, R.layout.item_deadline, deadlineList); this.context = context; this.deadlineList = deadlineList; } @NonNull @Override public View getView(int position, View convertView, ViewGroup parent) { LayoutInflater inflater = context.getLayoutInflater(); View listView = inflater.inflate(R.layout.item_deadline, parent, false); TextView displayDate = listView.findViewById(R.id.display_deadline_date_tv); TextView displayName = listView.findViewById(R.id.display_deadline_name_tv); Deadline deadline = deadlineList.get(position); displayDate.setText(deadline.getDate()); displayName.setText(deadline.getDeadlineName()); return listView; }}但是每次我嘗試運(yùn)行該應(yīng)用程序時(shí)，我的應(yīng)用程序都會崩潰。日志顯示以下內(nèi)容：原因：java.lang.NullPointerException：嘗試在空對象引用上調(diào)用虛擬方法 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)'我該如何解決這個(gè)問題？
1 回答
紅顏莎娜
TA貢獻(xiàn)1842條經(jīng)驗(yàn) 獲得超13個(gè)贊
檢查我的例子。我使用String的只是列表中的元素。
請記住，您應(yīng)該使用View Holder 模式（使用傳遞給此方法的回收視圖作為第二個(gè)參數(shù)）以實(shí)現(xiàn)更平滑的滾動(dòng)和節(jié)省內(nèi)存。
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
<ListView
android:id="@+id/list_view"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
ActivityMain.java
public class ActivityMain extends AppCompatActivity {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
setupUi();
}
private void setupUi() {
List<String> elements = new ArrayList<>(Arrays.asList(
"Red",
"Blue",
"Green",
"Black"
));
DeadlineAdapter adapter = new DeadlineAdapter(this, elements);
ListView listView = findViewById(R.id.list_view);
listView.setAdapter(adapter);
}
}
和適配器 - DeadlineAdapter.java：
public class DeadlineAdapter extends ArrayAdapter<String> {
private Activity context;
private List<String> deadlineList;
public DeadlineAdapter(Activity context, List<String> deadlineList) {
super(context, R.layout.item_deadline, deadlineList);
this.context = context;
this.deadlineList = deadlineList;
}
@NonNull
public View getView(int position, View convertView, @NotNull ViewGroup parent) {
LayoutInflater inflater = context.getLayoutInflater();
View listView = inflater.inflate(R.layout.item_deadline, parent, false);
TextView positionTv = listView.findViewById(R.id.row_id);
TextView textTv = listView.findViewById(R.id.row_text);
positionTv.setText(String.format("%s.", position + 1));
textTv.setText(deadlineList.get(position));
return listView;
}
}
和項(xiàng)目列表 - item_deadline：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="30dp">
<TextView
android:id="@+id/row_id"
android:layout_width="100dp"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/row_text"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</LinearLayout>
預(yù)期輸出：
檢查我的例子。我使用String的只是列表中的元素。
請記住，您應(yīng)該使用View Holder 模式（使用傳遞給此方法的回收視圖作為第二個(gè)參數(shù)）以實(shí)現(xiàn)更平滑的滾動(dòng)和節(jié)省內(nèi)存。
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
<ListView
android:id="@+id/list_view"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
ActivityMain.java
public class ActivityMain extends AppCompatActivity {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
setupUi();
}
private void setupUi() {
List<String> elements = new ArrayList<>(Arrays.asList(
"Red",
"Blue",
"Green",
"Black"
));
DeadlineAdapter adapter = new DeadlineAdapter(this, elements);
ListView listView = findViewById(R.id.list_view);
listView.setAdapter(adapter);
}
}
和適配器 - DeadlineAdapter.java：
public class DeadlineAdapter extends ArrayAdapter<String> {
private Activity context;
private List<String> deadlineList;
public DeadlineAdapter(Activity context, List<String> deadlineList) {
super(context, R.layout.item_deadline, deadlineList);
this.context = context;
this.deadlineList = deadlineList;
}
@NonNull
public View getView(int position, View convertView, @NotNull ViewGroup parent) {
LayoutInflater inflater = context.getLayoutInflater();
View listView = inflater.inflate(R.layout.item_deadline, parent, false);
TextView positionTv = listView.findViewById(R.id.row_id);
TextView textTv = listView.findViewById(R.id.row_text);
positionTv.setText(String.format("%s.", position + 1));
textTv.setText(deadlineList.get(position));
return listView;
}
}
和項(xiàng)目列表 - item_deadline：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="30dp">
<TextView
android:id="@+id/row_id"
android:layout_width="100dp"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/row_text"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</LinearLayout>
預(yù)期輸出：
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