Question

我正在为android创建移动应用程序，但在Google下载连接后遇到问题

应用程序崩溃。有人可以给我一个理由以及如何附加吗？

主要活动触手可及。

public class MainActivity extends AppCompatActivity {
GoogleSignInClient mGoogleSignInClient;
private int RC_SIGN_IN = 3;
SignInButton signInButton;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    signInButton = findViewById(R.id.sign_in_button);

    signInButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            switch (view.getId()) {
                case R.id.sign_in_button:
                    signIn();
                    break;
                // ...
            }
        }
    });
    GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestEmail()
            .build();

    mGoogleSignInClient = GoogleSignIn.getClient(this, gso);

}
private void signIn() {
    Intent signInIntent = mGoogleSignInClient.getSignInIntent();
    startActivityForResult(signInIntent, RC_SIGN_IN);
}
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    // Result returned from launching the Intent from GoogleSignInClient.getSignInIntent(...);
    if (requestCode == RC_SIGN_IN) {
        // The Task returned from this call is always completed, no need to attach
        // a listener.
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        handleSignInResult(task);
    }
}
private void handleSignInResult(Task<GoogleSignInAccount> completedTask) {
    try {
        GoogleSignInAccount account = completedTask.getResult(ApiException.class);

        Intent intent = new Intent(MainActivity.this, MenuActivity.class);
        startActivity(intent);
    } catch (ApiException e) {
        // The ApiException status code indicates the detailed failure reason.
        // Please refer to the GoogleSignInStatusCodes class reference for more information.
        Log.w("TAG", "signInResult:failed code=" + e.getStatusCode());
        // updateUI(null);
    }
}

}

登录后的目标活动：

public class MenuActivity extends AppCompatActivity  {

GoogleSignInClient mGoogleSignInClient;
Button logoutBtn;
TextView userName;
ImageView profileImage;
private GoogleSignInOptions gso;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
            .requestEmail()
            .build();

    mGoogleSignInClient = GoogleSignIn.getClient(this, gso);

    logoutBtn=(Button)findViewById(R.id.button_wyl);
    profileImage=(ImageView)findViewById(R.id.profileImage);
    userName = findViewById(R.id.name);
    logoutBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            switch (view.getId()) {
                // ...
                case R.id.button_wyl:
                    signOut();
                    break;
                // ...
            }
        }
    });

    GoogleSignInAccount acct = GoogleSignIn.getLastSignedInAccount(this);
    if (acct != null) {
        String personName = acct.getDisplayName();
        Uri personPhoto = acct.getPhotoUrl();

        userName.setText(personName);
        Glide.with(this).load(String.valueOf(personPhoto)).into(profileImage);

    }
}

private void signOut() {
    mGoogleSignInClient.signOut()
            .addOnCompleteListener(this, new OnCompleteListener<Void>() {
                @Override
                public void onComplete(@NonNull Task<Void> task) {
                    Toast.makeText(MenuActivity.this, "Signed out Successfully", Toast.LENGTH_LONG).show();
                    finish();
                }
            });
}

}

例外：

E / Android运行时：致命异常：主要流程：com.example.goodmath，PID：1780 java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.goodmath / com.example.goodmath.MenuActivity}： java.lang.NullPointerException：尝试调用虚拟方法'void android.widget.Button.setOnClickListener（android.view.View $ OnClickListener）' 在空对象引用上在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2913）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3048）在android.app.servertransaction.LaunchActivityItem.execute（LaunchActivityItem.java:78）在android.app.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）在android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1808）在android.os.Handler.dispatchMessage（Handler.java:106）在android.os.Looper.loop（Looper.java:193）在android.app.ActivityThread.main（ActivityThread.java:6669）在java.lang.reflect.Method.invoke（本机方法）在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run（RuntimeInit.java:493）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:858）原因：java.lang.NullPointerException：尝试调用虚拟方法'void android.widget.Button.setOnClickListener（android.view.View $ OnClickListener）' 在空对象引用上在com.example.goodmath.MenuActivity.onCreate（MenuActivity.java:46）在android.app.Activity.performCreate（Activity.java:7136）在android.app.Activity.performCreate（Activity.java:7127）在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1271）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2893）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3048）在android.app.servertransaction.LaunchActivityItem.execute（LaunchActivityItem.java:78）在android.app.servertransaction.TransactionExecutor.executeCallbacks（TransactionExecutor.java:108）在android.app.servertransaction.TransactionExecutor.execute（TransactionExecutor.java:68）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1808）在android.os.Handler.dispatchMessage（Handler.java:106）在android.os.Looper.loop（Looper.java:193）在android.app.ActivityThread.main（ActivityThread.java:6669）在java.lang.reflect.Method.invoke（本机方法）在com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run（RuntimeInit.java:493）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:858） I / Process：正在发送信号。 PID：1780 SIG：9

Answer 1

您不会在setContentView()的{{1}}中调用onCreate()，因此您的MenuActivity查找将失败。结果，findViewById()是logoutBtn，因此当您尝试在其上调用方法时，会崩溃null。

Answer 2

实际错误是：

java.lang.NullPointerException: Attempt to invoke virtual method 'void 
android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' 
on a null object

因此findViewById失败。

在super.onCreate（）之后直接进行setContentView(R.layout.activity_menu)

如果这行不通，您确定所引用的视图在activity_menu布局文件中吗？

无法启动活动ComponentInfo-Android

2 个答案: