英文:
App keeps Stopping Process: com.example.test101, PID: 7666 java.lang.RuntimeException: Unable to instantiate activity
问题
以下是你提供的代码的翻译：
主要活动 (MainActivity.java)
package com.example.test101;
import androidx.appcompat.app.AppCompatActivity;
import androidx.constraintlayout.widget.ConstraintLayout;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import com.google.android.material.snackbar.Snackbar;
public class MainActivity extends AppCompatActivity {
ConstraintLayout constraintLayout;
DBCreate db;
Button button;
EditText user;
EditText pass;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
constraintLayout = findViewById(R.id.constraintLayout);
db = new DBCreate(this);
button = findViewById(R.id.button_reg);
user = findViewById(R.id.editTextTextPersonName);
pass = findViewById(R.id.editTextTextPassword);
regUser();
}
public void regUser() {
button.setOnKeyListener(new View.OnKeyListener() {
@Override
public boolean onKey(View view, int i, KeyEvent keyEvent) {
boolean isInserted = db.insertData(user.getText().toString(), pass.getText().toString());
if (isInserted) {
Snackbar datI = Snackbar.make(constraintLayout, "Registered", Snackbar.LENGTH_SHORT);
datI.show();
}
return true;
}
});
}
public void onReg(View view) {
if (user.getText().toString().equals("")) {
Snackbar snackbar1 = Snackbar.make(constraintLayout, "Please enter a valid username", Snackbar.LENGTH_SHORT);
snackbar1.show();
}
if (pass.getText().toString().equals("")) {
Snackbar snackbar1 = Snackbar.make(constraintLayout, "Please enter a valid password", Snackbar.LENGTH_SHORT);
snackbar1.show();
}
if (user.getText().toString().equals("ADMIN")) {
Snackbar snackbar1 = Snackbar.make(constraintLayout, "AAO PADHARO ADMIN SAAB", Snackbar.LENGTH_SHORT);
snackbar1.show();
if (!(pass.getText().toString().equals("ADMIN"))) {
Snackbar snackbar2 = Snackbar.make(constraintLayout, "ADMIN SAAB PASSWORD TOH THIK DALO", Snackbar.LENGTH_SHORT);
snackbar2.show();
}
}
}
}
数据库创建类 (DBCreate.java)
package com.example.test101;
import android.content.ContentValues;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DBCreate extends SQLiteOpenHelper {
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "testoneoone.db";
public static final String TABLE_NAME = "up";
public static final String ID = "ID";
public static final String NAME = "NAME";
public static final String PASS = "PASS";
public DBCreate(Context context) {
super(context, DATABASE_NAME, null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("create table " + TABLE_NAME + "(ID INTEGER PRIMARY KEY AUTOINCREMENT, NAME TEXT, PASS TEXT)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int i, int i1) {
db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
onCreate(db);
}
public boolean insertData(String name, String pass) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put(NAME, name);
contentValues.put(PASS, pass);
long res = db.insert(TABLE_NAME, null, contentValues);
return res != -1;
}
}
布局文件 (activity_main.xml)
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
tools:context=".MainActivity">
<TextView
android:id="@+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginBottom="476dp"
android:text="Password"
android:textSize="36sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginTop="96dp"
android:text="Username"
android:textSize="36sp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<EditText
android:id="@+id/editTextTextPassword"
android:layout_width="306dp"
android:layout_height="45dp"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:ems="10"
android:inputType="textPassword"
app:layout_constraintBottom_toTopOf="@+id/button_reg"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/textView2"
app:layout_constraintVertical_bias="0.0" />
<EditText
android:id="@+id/editTextTextPersonName"
android:layout_width="306dp"
android:layout_height="45dp"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginBottom="16dp"
android:ems="10"
android:inputType="textPersonName"
app:layout_constraintBottom_toTopOf="@+id/textView2"
app:layout_constraintStart_toStartOf="parent" />
<Button
android:id="@+id/button_reg"
android:layout_width="174dp"
android:layout_height="0dp"
android:layout_marginEnd="16dp"
android:layout_marginRight="16dp"
android:layout_marginBottom="4dp"
android:onClick="onReg"
android:text="REGISTER"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toBottomOf="@+id/textView3" />
<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="549dp"
android:layout_marginBottom="114dp"
android:text="Already have an account?"
app:layout_constraintBottom_toTopOf="@+id/button_reg"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
<details>
<summary>英文:</summary>
I made an app which has 1 Activity and 2 Classes, 1 Main and 1 for my Database.
But whenever i run my app on my virtual phone it stops
it says its prolly the Activity thingy, but i dont know much about it i m very begginer to android
thnx in advance,
this is wat i get in Log Cat
Process: com.example.test101, PID: 7666
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.test101/com.example.test101.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2843)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:159)
at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
at android.content.Context.obtainStyledAttributes(Context.java:675)
at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
at com.example.test101.MainActivity.<init>(MainActivity.java:20)
at java.lang.Class.newInstance(Native Method)
at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:69)
at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
at android.app.Instrumentation.newActivity(Instrumentation.java:1215)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2831)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2020-10-17 21:57:56.379 7666-7666/? I/Process: Sending signal. PID: 7666 SIG: 9
my code is <br>
**MAIN**
package com.example.test101;
import androidx.appcompat.app.AppCompatActivity;
import androidx.constraintlayout.widget.ConstraintLayout;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import com.google.android.material.snackbar.Snackbar;
public class MainActivity extends AppCompatActivity {
ConstraintLayout constraintLayout;
DBCreate db;
Button button = (Button) findViewById(R.id.button_reg);
EditText user = (EditText) findViewById(R.id.editTextTextPersonName);
EditText pass = (EditText) findViewById(R.id.editTextTextPassword);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
db = new DBCreate(this);
regUser();
}
public void regUser(){
button.setOnKeyListener(
new View.OnKeyListener() {
@Override
public boolean onKey(View view, int i, KeyEvent keyEvent) {
boolean isInserted = db.insertData(user.getText().toString() ,pass.getText().toString());
if(isInserted){
Snackbar datI = Snackbar.make(constraintLayout, "Registered", Snackbar.LENGTH_SHORT);
datI.show();
}
return true;
}
}
);
}
public void onReg(View view){
if(user.getText().toString() == ""){
Snackbar snackbar1 = Snackbar.make(constraintLayout, "Please enter a valid username", Snackbar.LENGTH_SHORT);
snackbar1.show();
}
if(pass.getText().toString() == ""){
Snackbar snackbar1 = Snackbar.make(constraintLayout, "Please enter a valid password", Snackbar.LENGTH_SHORT);
snackbar1.show();
}
if(user.getText().toString() == "ADMIN"){
Snackbar snackbar1 = Snackbar.make(constraintLayout, "AAO PADHARO ADMIN SAAB", Snackbar.LENGTH_SHORT);
snackbar1.show();
if(!(pass.getText().toString() == "ADMIN")){
Snackbar snackbar2 = Snackbar.make(constraintLayout, "ADMIN SAAB PASSWORD TOH THIK DALO", Snackbar.LENGTH_SHORT);
snackbar2.show();
}
}
}
}
and 2nd
package com.example.test101;
import android.content.ContentValues;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DBCreate extends SQLiteOpenHelper {
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "testoneoone.db";
public static final String TABLE_NAME = "up";
public static final String ID = "ID";
public static final String NAME = "NAME";
public static final String PASS = "PASS";
public DBCreate(Context context){
super(context,DATABASE_NAME,null,1);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("create table " + TABLE_NAME + "(ID INTEGER PRIMARY KEY AUTOINCREMENT, NAME TEXT, PASS TEXT)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int i, int i1) {
db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
onCreate(db);
}
public boolean insertData( String name, String pass){
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put(NAME, name);
contentValues.put(PASS, pass);
long res = db.insert(TABLE_NAME, null,contentValues);
return res != -1;
}
}
**my activity**
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
tools:context=".MainActivity">
<TextView
android:id="@+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginBottom="476dp"
android:text="Password"
android:textSize="36sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginTop="96dp"
android:text="Username"
android:textSize="36sp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<EditText
android:id="@+id/editTextTextPassword"
android:layout_width="306dp"
android:layout_height="45dp"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:ems="10"
android:inputType="textPassword"
app:layout_constraintBottom_toTopOf="@+id/button_reg"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/textView2"
app:layout_constraintVertical_bias="0.0" />
<EditText
android:id="@+id/editTextTextPersonName"
android:layout_width="306dp"
android:layout_height="45dp"
android:layout_marginStart="16dp"
android:layout_marginLeft="16dp"
android:layout_marginBottom="16dp"
android:ems="10"
android:inputType="textPersonName"
app:layout_constraintBottom_toTopOf="@+id/textView2"
app:layout_constraintStart_toStartOf="parent" />
<Button
android:id="@+id/button_reg"
android:layout_width="174dp"
android:layout_height="0dp"
android:layout_marginEnd="16dp"
android:layout_marginRight="16dp"
android:layout_marginBottom="4dp"
android:onClick="onReg"
android:text="REGISTER"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toBottomOf="@+id/textView3" />
<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="549dp"
android:layout_marginBottom="114dp"
android:text="Already have an account?"
app:layout_constraintBottom_toTopOf="@+id/button_reg"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
</details>
# 答案1
**得分**: 1
你不应立即分配视图引用，因为此时尚未填充活动的布局。请将其推迟到`onCreate`方法。
因此，
```java
public class MainActivity extends AppCompatActivity {
ConstraintLayout constraintLayout;
DBCreate db;
Button button;
EditText user;
EditText pass;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.button_reg);
user = (EditText) findViewById(R.id.editTextTextPersonName);
pass = (EditText) findViewById(R.id.editTextTextPassword);
db = new DBCreate(this);
regUser();
}
// ... 其他代码
}
英文:
You should not assign views references immediately, as the layout for the activity is not inflated at the moment. Defer it till onCreate method.
So,
public class MainActivity extends AppCompatActivity {
ConstraintLayout constraintLayout;
DBCreate db;
Button button;
EditText user;
EditText pass;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = (Button) findViewById(R.id.button_reg);
user = (EditText) findViewById(R.id.editTextTextPersonName);
pass = (EditText) findViewById(R.id.editTextTextPassword);
db = new DBCreate(this);
regUser();
}
// ... and so on
}
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