发生错误java.lang.RuntimeException：无法启动活动ComponentInfo。
代码错误：
java.lang.RuntimeException: Unable to start activity
ComponentInfo{firebase.app.ph/firebase.app.ph.LoginActivity}:
java.lang.NullPointerException: Attempt to invoke virtual method 'void
android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2757)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2818)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1557)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:163)
at android.app.ActivityThread.main(ActivityThread.java:6393)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:933)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void
android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at firebase.app.ph.LoginActivity.onCreate(LoginActivity.java:57)
at android.app.Activity.performCreate(Activity.java:6858)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2710)
... 9 more
代码应用： StartActivity：
public class StartActivity extends AppCompatActivity {
Button login , register;
FirebaseUser firebaseUser;
@Override
protected void onStart() {
super.onStart();
firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
if(firebaseUser!=null){
startActivity(new Intent(StartActivity.this,MainActivity.class));
finish();
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_start);
login = findViewById(R.id.login);
register = findViewById(R.id.register);
login.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(StartActivity.this,LoginActivity.class));
}
});
register.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(StartActivity.this,RegisterActivity.class));
}
});
}
}
登录活动：
public class LoginActivity extends AppCompatActivity {
EditText email, pass;
Button login;
TextView txtsignup;
FirebaseAuth auth;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
email = findViewById(R.id.email);
login = findViewById(R.id.login);
pass = findViewById(R.id.pass);
txt_signup = findViewById(R.id.txtsignup);
auth = FirebaseAuth.getInstance();
txt_signup.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(LoginActivity.this, RegisterActivity.class));
}
});
login.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
final ProgressDialog pd = new ProgressDialog(LoginActivity.this);
pd.setMessage("Please wait...");
pd.show();
String str_email = email.getText().toString();
String str_pass = pass.getText().toString();
if (TextUtils.isEmpty(str_email) || TextUtils.isEmpty(str_pass)) {
Toast.makeText(LoginActivity.this, "All Filed are required!", Toast.LENGTH_SHORT).show();
} else {
auth.signInWithEmailAndPassword(str_email, str_pass)
.addOnCompleteListener(LoginActivity.this, new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
if (task.isSuccessful()) {
DatabaseReference reference = FirebaseDatabase.getInstance().getReference().child("Users").child(auth.getCurrentUser().getUid());
reference.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
pd.dismiss();
Intent intent = new Intent(LoginActivity.this, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(intent);
finish();
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
pd.dismiss();
}
});
} else {
pd.dismiss();
Toast.makeText(LoginActivity.this, "!", Toast.LENGTH_SHORT).show();
}
}
});
}
}
});
}
}
activity_login.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
android:gravity="center"
android:padding="10dp"
android:background="@drawable/b228"
tools:context=".LoginActivity">
<ImageView
android:layout_width="wrap_content"
android:layout_height="150dp"
android:src="@drawable/image"></ImageView>
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/edittext_background"
android:hint="Email"
android:layout_marginTop="15dp"
android:id="@+id/email"
android:inputType="textEmailAddress"
android:padding="10dp"/>
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/edittext_background"
android:hint="Password"
android:layout_marginTop="15dp"
android:id="@+id/pass"
android:inputType="textPass"
android:padding="10dp"/>
<Button
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/login"
android:text="Log in"
android:textColor="@color/colorPrimary"
android:background="@drawable/button_background"
android:layout_marginTop="30dp"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="!"
android:id="@+id/txt_signup"
android:textColor="@color/colorWhite"
android:layout_marginTop="15dp"/>
我也有activity_register.xml和RegisterActivity，但是它们与Login不同，它们工作得很好。 我刚刚开始使用Java和Android Studio进行编程，因此，很高兴听到您的回答！
答案 0 :(得分：0)
此错误是由于您在两个活动中的视图命名约定中均存在错误做法所致。代码语法正确，但错误是：
activity_start.xml和activity_login.xml activity_start.xml 和 activity_login.xml 中的按钮具有与相同的 id 登录” 。因此，当您在 LoginActivity 中键入 login = findViewById(R.id.login); 时，它指向 StartActivity 中的视图，该视图引发了 NullPointerException
这很简单，只需更改两个布局中的按钮的 id ，以使其唯一。