嘗試在空對象引用上調用虛擬方法“void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)”

Question

我花了很多時間尋找解決方案。 我不知道為什么這是一個空對象，我不確定我錯過了什么。 如果我嘗試對代碼進行任何更改，它會拋出錯誤並使我無處可去。 有人知道我可能會錯過什么嗎？

這是被拋出的錯誤：

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)```

這是我的 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();
    }
}

如果需要更多信息來幫助解決此問題，請告訴我，我很樂意提供更多信息。 謝謝！

Answer 1

您在方法中聲明editText、button 和progressbar 並使其為null 或無法通過其他方法訪問的方式。 所以，你需要這樣做：

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);