尝试在空对象引用上调用虚拟方法 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)'
Attempt to invoke virtual method 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)' on a null object reference
我花了很多时间寻找解决方案。我不知道为什么这是一个空对象，我也不确定我错过了什么。如果我尝试对代码进行任何更改，它会抛出错误并且让我无处可去。有人知道我可能会错过什么吗？
这是抛出的错误：
03-25 11:32:47.068 4802-4802/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.evilcorporation.employeetimeclock, PID: 4802
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.evilcorporation.employeetimeclock/com.evilcorporation.employeetimeclock.ui.login.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)' on a null object reference
at com.evilcorporation.employeetimeclock.ui.login.LoginActivity.onCreate(LoginActivity.java:99)
at android.app.Activity.performCreate(Activity.java:6251)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)```
这是我的 LoginActivity.java:
package com.evilcorporation.employeetimeclock.ui.login;
import android.app.Activity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProviders;
import android.os.Bundle;
import androidx.annotation.Nullable;
import androidx.annotation.StringRes;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelStore;
import androidx.lifecycle.ViewModelStoreOwner;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.KeyEvent;
import android.view.View;
import android.view.inputmethod.EditorInfo;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
import com.evilcorporation.employeetimeclock.R;
import com.evilcorporation.employeetimeclock.ui.login.LoginViewModel;
import com.evilcorporation.employeetimeclock.ui.login.LoginViewModelFactory;
public class LoginActivity extends AppCompatActivity {
private LoginViewModel loginViewModel;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
loginViewModel = ViewModelProviders.of(this, new LoginViewModelFactory())
.get(LoginViewModel.class);
final EditText usernameEditText = findViewById(R.id.username);
final EditText passwordEditText = findViewById(R.id.password);
final Button loginButton = findViewById(R.id.login);
final ProgressBar loadingProgressBar = findViewById(R.id.loading);
loginViewModel.getLoginFormState().observe(this, new Observer<LoginFormState>() {
@Override
public void onChanged(@Nullable LoginFormState loginFormState) {
if (loginFormState == null) {
return;
}
loginButton.setEnabled(loginFormState.isDataValid());
if (loginFormState.getUsernameError() != null) {
usernameEditText.setError(getString(loginFormState.getUsernameError()));
}
if (loginFormState.getPasswordError() != null) {
passwordEditText.setError(getString(loginFormState.getPasswordError()));
}
}
});
loginViewModel.getLoginResult().observe(this, new Observer<LoginResult>() {
@Override
public void onChanged(@Nullable LoginResult loginResult) {
if (loginResult == null) {
return;
}
loadingProgressBar.setVisibility(View.GONE);
if (loginResult.getError() != null) {
showLoginFailed(loginResult.getError());
}
if (loginResult.getSuccess() != null) {
updateUiWithUser(loginResult.getSuccess());
}
setResult(Activity.RESULT_OK);
//Complete and destroy login activity once successful
finish();
}
});
TextWatcher afterTextChangedListener = new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
// ignore
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
// ignore
}
@Override
public void afterTextChanged(Editable s) {
loginViewModel.loginDataChanged(usernameEditText.getText().toString(),
passwordEditText.getText().toString());
}
};
usernameEditText.addTextChangedListener(afterTextChangedListener);
passwordEditText.addTextChangedListener(afterTextChangedListener);
passwordEditText.setOnEditorActionListener(new TextView.OnEditorActionListener() {
@Override
public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
if (actionId == EditorInfo.IME_ACTION_DONE) {
loginViewModel.login(usernameEditText.getText().toString(),
passwordEditText.getText().toString());
}
return false;
}
});
loginButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
loadingProgressBar.setVisibility(View.VISIBLE);
loginViewModel.login(usernameEditText.getText().toString(),
passwordEditText.getText().toString());
}
});
}
private void updateUiWithUser(LoggedInUserView model) {
String welcome = getString(R.string.welcome) + model.getDisplayName();
// TODO : initiate successful logged in experience
Toast.makeText(getApplicationContext(), welcome, Toast.LENGTH_LONG).show();
}
private void showLoginFailed(@StringRes Integer errorString) {
Toast.makeText(getApplicationContext(), errorString, Toast.LENGTH_SHORT).show();
}
}
如果需要更多信息来帮助解决此问题，请告诉我，我很乐意提供更多信息。谢谢！
您在方法内声明 editText、按钮和进度条并使其为空或无法通过其他方法访问的方式。所以，你需要这样做：
public class LoginActivity extends AppCompatActivity {
private LoginViewModel loginViewModel;
protected EditText usernameEditText;
protected EditText passwordEditText;
protected Button loginButton;
protected ProgressBar loadingProgressBar;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
loginViewModel = ViewModelProviders.of(this, new LoginViewModelFactory())
.get(LoginViewModel.class);
usernameEditText = findViewById(R.id.username);
passwordEditText = findViewById(R.id.password);
loginButton = findViewById(R.id.login);
loadingProgressBar = findViewById(R.id.loading);
我花了很多时间寻找解决方案。我不知道为什么这是一个空对象，我也不确定我错过了什么。如果我尝试对代码进行任何更改，它会抛出错误并且让我无处可去。有人知道我可能会错过什么吗？
这是抛出的错误：
03-25 11:32:47.068 4802-4802/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.evilcorporation.employeetimeclock, PID: 4802
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.evilcorporation.employeetimeclock/com.evilcorporation.employeetimeclock.ui.login.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)' on a null object reference
at com.evilcorporation.employeetimeclock.ui.login.LoginActivity.onCreate(LoginActivity.java:99)
at android.app.Activity.performCreate(Activity.java:6251)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)```
这是我的 LoginActivity.java:
package com.evilcorporation.employeetimeclock.ui.login;
import android.app.Activity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProviders;
import android.os.Bundle;
import androidx.annotation.Nullable;
import androidx.annotation.StringRes;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.ViewModelStore;
import androidx.lifecycle.ViewModelStoreOwner;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.KeyEvent;
import android.view.View;
import android.view.inputmethod.EditorInfo;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
import com.evilcorporation.employeetimeclock.R;
import com.evilcorporation.employeetimeclock.ui.login.LoginViewModel;
import com.evilcorporation.employeetimeclock.ui.login.LoginViewModelFactory;
public class LoginActivity extends AppCompatActivity {
private LoginViewModel loginViewModel;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
loginViewModel = ViewModelProviders.of(this, new LoginViewModelFactory())
.get(LoginViewModel.class);
final EditText usernameEditText = findViewById(R.id.username);
final EditText passwordEditText = findViewById(R.id.password);
final Button loginButton = findViewById(R.id.login);
final ProgressBar loadingProgressBar = findViewById(R.id.loading);
loginViewModel.getLoginFormState().observe(this, new Observer<LoginFormState>() {
@Override
public void onChanged(@Nullable LoginFormState loginFormState) {
if (loginFormState == null) {
return;
}
loginButton.setEnabled(loginFormState.isDataValid());
if (loginFormState.getUsernameError() != null) {
usernameEditText.setError(getString(loginFormState.getUsernameError()));
}
if (loginFormState.getPasswordError() != null) {
passwordEditText.setError(getString(loginFormState.getPasswordError()));
}
}
});
loginViewModel.getLoginResult().observe(this, new Observer<LoginResult>() {
@Override
public void onChanged(@Nullable LoginResult loginResult) {
if (loginResult == null) {
return;
}
loadingProgressBar.setVisibility(View.GONE);
if (loginResult.getError() != null) {
showLoginFailed(loginResult.getError());
}
if (loginResult.getSuccess() != null) {
updateUiWithUser(loginResult.getSuccess());
}
setResult(Activity.RESULT_OK);
//Complete and destroy login activity once successful
finish();
}
});
TextWatcher afterTextChangedListener = new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {
// ignore
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
// ignore
}
@Override
public void afterTextChanged(Editable s) {
loginViewModel.loginDataChanged(usernameEditText.getText().toString(),
passwordEditText.getText().toString());
}
};
usernameEditText.addTextChangedListener(afterTextChangedListener);
passwordEditText.addTextChangedListener(afterTextChangedListener);
passwordEditText.setOnEditorActionListener(new TextView.OnEditorActionListener() {
@Override
public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
if (actionId == EditorInfo.IME_ACTION_DONE) {
loginViewModel.login(usernameEditText.getText().toString(),
passwordEditText.getText().toString());
}
return false;
}
});
loginButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
loadingProgressBar.setVisibility(View.VISIBLE);
loginViewModel.login(usernameEditText.getText().toString(),
passwordEditText.getText().toString());
}
});
}
private void updateUiWithUser(LoggedInUserView model) {
String welcome = getString(R.string.welcome) + model.getDisplayName();
// TODO : initiate successful logged in experience
Toast.makeText(getApplicationContext(), welcome, Toast.LENGTH_LONG).show();
}
private void showLoginFailed(@StringRes Integer errorString) {
Toast.makeText(getApplicationContext(), errorString, Toast.LENGTH_SHORT).show();
}
}
如果需要更多信息来帮助解决此问题，请告诉我，我很乐意提供更多信息。谢谢！
您在方法内声明 editText、按钮和进度条并使其为空或无法通过其他方法访问的方式。所以，你需要这样做：
public class LoginActivity extends AppCompatActivity {
private LoginViewModel loginViewModel;
protected EditText usernameEditText;
protected EditText passwordEditText;
protected Button loginButton;
protected ProgressBar loadingProgressBar;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
loginViewModel = ViewModelProviders.of(this, new LoginViewModelFactory())
.get(LoginViewModel.class);
usernameEditText = findViewById(R.id.username);
passwordEditText = findViewById(R.id.password);
loginButton = findViewById(R.id.login);
loadingProgressBar = findViewById(R.id.loading);