如何解决去其他活动时的例外
我在转到其他活动时不断收到 NullPointerException 异常，而应用程序/模拟器只是崩溃了。
清单文件包含两个活动。
这是MainActivity：
public class MainActivity extends AppCompatActivity {
//Garbage Sorting V1
// GUI variables
private Button whereIsItems;
private Button addItem;
private TextView items;
private TextView listThings;
// Model: Database of items
private static ItemsDB itemsDB;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.garbage);
itemsDB = new ItemsDB();
itemsDB.fillItemsDB();
items = findViewById(R.id.items);
items.setText("Where to place them:");
listThings = findViewById(R.id.edit_text);
whereIsItems = findViewById(R.id.where_button);
whereIsItems.setonClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
items.setBackgroundColor(Color.parseColor("#FFFFFF"));
String str = listThings.getText().toString();
listThings.setText(itemsDB.getItemValue(str));
}
});
addItem = findViewById(R.id.add_button1);
assert addItem != null;
addItem.setonClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent(MainActivity.this,GarbageActivity.class);
startActivity(intent);
}
});
}
另一个活动 GarbageActivity（我正在尝试去的那个）：
public class GarbageActivity extends AppCompatActivity {
private static ItemsDB itemsDB;
private EditText whatText;
private EditText whereText;
private Button addItem;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.garbage_activity);
itemsDB = ItemsDB.get(GarbageActivity.this);
addItem = findViewById(R.id.add_button2);
addItem.setonClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
whatText = findViewById(R.id.what_text);
whereText = findViewById(R.id.where_text);
String str1 = whatText.getText().toString();
String str2 = whereText.getText().toString();
itemsDB.addItem(str1,str2);
whatText.setText("");
whereText.setText("");
}
});
}
public class ItemsDB {
private Map<String,Item> ItemsDB;
private static ItemsDB sItemsDB;
public ItemsDB() {
ItemsDB = new HashMap<>();
}
private ItemsDB(Context context) {
fillItemsDB();
}
public static ItemsDB get(Context context) {
if (sItemsDB == null) sItemsDB = new ItemsDB(context);
return sItemsDB;
}
public void addItem(String what,String where) {
Item it = new Item(what,where);
ItemsDB.put(it.getWhat(),it);
}
public String getItemValue(String itemName) {
Item item = ItemsDB.get(itemName);
if (item == null) {
return itemName + " not found";
} else {
return itemName + " should be placed in " + ItemsDB.get(itemName).getWhere();
}
}
public void fillItemsDB() {
addItem("Milk Carton","Cardboard");
addItem("Carrots","Food");
addItem("T-shirt","Not found");
addItem("Bread","Food");
addItem("Butter","Food");
}
garbage.xml：
<?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:orientation="vertical">
<!-- Textview for heading -->
<TextView
android:id="@+id/items"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
<!-- Text field to enter item -->
<EditText
android:id="@+id/edit_text"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:inputType="text"
android:layout_marginBottom="5dp"/>
<Button
android:id="@+id/where_button"
android:layout_width="wrap_content"
android:layout_gravity="left"
android:background="#39f208"
android:text="Where"
android:layout_height="80sp"/>
<Button
android:id="@+id/add_button1"
android:layout_width="wrap_content"
android:layout_gravity="left"
android:background="#39f208"
android:text="Add an item"
android:layout_height="80sp"/>
</LinearLayout>
garbage_activity.xml：
<?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:orientation="vertical">
<TableLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<TableRow
android:layout_width="match_parent"
android:layout_height="match_parent" >
<TextView
android:id="@+id/textView5"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="What Item?"
android:textAppearance="@style/TextAppearance.AppCompat.Large" />
<EditText
android:id="@+id/what_text"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:ems="10"
android:inputType="textPersonName" />
</TableRow>
<TableRow
android:layout_width="match_parent"
android:layout_height="match_parent" >
<TextView
android:id="@+id/textView6"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Where?"
android:textAppearance="@style/TextAppearance.AppCompat.Large" />
<EditText
android:id="@+id/where_text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:ems="10"
android:inputType="textPersonName" />
</TableRow>
<TableRow
android:layout_width="match_parent"
android:layout_height="match_parent" />
<TableRow
android:layout_width="match_parent"
android:layout_height="match_parent" >
<Button
android:id="@+id/add_button2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Add an Item" />
</TableRow>
</TableLayout>
</LinearLayout>
EDIT 这是调试器日志：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: dk.itu.sortinggarbageapp,PID: 8926
java.lang.RuntimeException: Unable to start activity ComponentInfo{dk.itu.sortinggarbageapp/dk.itu.sortinggarbageapp.GarbageActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.Map.put(java.lang.Object,java.lang.Object)' on a null object reference
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3449)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.Map.put(java.lang.Object,java.lang.Object)' on a null object reference
at dk.itu.sortinggarbageapp.ItemsDB.addItem(ItemsDB.java:27)
at dk.itu.sortinggarbageapp.ItemsDB.fillItemsDB(ItemsDB.java:40)
at dk.itu.sortinggarbageapp.ItemsDB.<init>(ItemsDB.java:17)
at dk.itu.sortinggarbageapp.ItemsDB.get(ItemsDB.java:21)
at dk.itu.sortinggarbageapp.GarbageActivity.onCreate(GarbageActivity.java:24)
at android.app.Activity.performCreate(Activity.java:8000)
at android.app.Activity.performCreate(Activity.java:7984)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3422)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.ja va:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
I/Process: Sending signal. PID: 8926 SIG: 9
disconnected from the target VM,address: 'localhost:63608',transport:
'socket'
解决方法
启动 Activity 时使用的 ItemsDB 构造函数调用 fillItemsDB()，将项目添加到 ItemsDB 字段。但是，该字段未在此构造函数中初始化。
你需要在那里初始化它，调用另一个构造函数，或者在你声明字段的地方初始化。
// initialize at field declaration
Map<String,Item> ItemsDB = new HashMap<>();
// initialize here
private ItemsDB(Context context) {
ItemsDB = new HashMap<>();
fillItemsDB();
}
// call the other constructor
public ItemsDB(Context context) {
this();
fillItemsDB();
}
这两种方法都行得通。
,你可以在你的垃圾类上试试这个
ItemsDB itemsDB = new ItemsDB();
itemsDB.get(GarbageActivity.this);
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