错误 - 尝试在空对象引用上调用虚拟方法___________setOnClickListener(android.view.View$OnClickListener)'

Question

我找不到任何未初始化的东西，并且无法进一步工作。 同样的位适用于我编写的另一段代码，两者完全相同，我无法弄清楚问题所在。 完整的错误代码是 - Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' 在一个空对象引用上。

日志文件——

2021-07-30 21:29:33.613 17989-17989/com.elc.hostel E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.elc.hostel, PID: 17989
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.elc.hostel/com.elc.hostel.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.elc.hostel.LoginActivity.onCreate(LoginActivity.java:50)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
2021-07-30 21:29:35.004 17989-18005/com.elc.hostel W/System: A resource failed to call close. 
 

代码-

public class LoginActivity extends AppCompatActivity {

    EditText mEmail, mPassword;
    Button mLogin;
    ProgressBar mProgressBar;
    FirebaseAuth fAuth;

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

        requestWindowFeature(Window.FEATURE_NO_TITLE);
        this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
        Objects.requireNonNull(getSupportActionBar()).hide();

        mEmail = findViewById(R.id.LoginEmail);
        mPassword = findViewById(R.id.LoginPassword);
        mLogin = findViewById(R.id.LoginButton);

        mProgressBar = findViewById(R.id.progressBarLog);
        fAuth = FirebaseAuth.getInstance();

        mLogin.setOnClickListener(new View.OnClickListener() {------------------Error
            @Override
            public void onClick(View v) {
                String email = mEmail.getText().toString().trim();
                String password = mPassword.getText().toString().trim();

                if (TextUtils.isEmpty(email)){
                    mEmail.setError("Email is required");
                    return;
                }
                if (TextUtils.isEmpty(password)){
                    mPassword.setError("Password is required");
                    return;
                }
                if (password.length() < 6){
                    mPassword.setError("Password must be at least 6 characters");
                    return;
                }
                mProgressBar.setVisibility(View.VISIBLE);

Answer 1

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.elc.hostel.LoginActivity.onCreate(LoginActivity.java:50)

堆栈上方表示 mLogin 为空。

findViewById() 返回 null 的少数情况是：

1. findViewById() 在 setContentView() 之前调用。
2. 在 findViewById() 中传递的 id 在 setContentView() 中设置的布局中不存在。

在这种情况下，不会调用 setContentView()。 在访问视图之前添加 setContentView() 将解决问题。