android studio 中沒有顯示錯誤，但 setOnClickListener 僅在一個按鈕上有效，而在另一個按鈕上無效

Question

我的 java 代碼：

public class StartActivity extends AppCompatActivity {

    private Button mRegBtn;
    private Button mLoginBtn;

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

        mRegBtn = (Button) findViewById(R.id.start_reg_btn);
        mLoginBtn = (Button) findViewById(R.id.start_login_btn);

        mRegBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent reg_intent = new Intent(StartActivity.this, RegisterActivity.class);
                startActivity(reg_intent);
            }
        });

        mLoginBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent login_intent = new Intent(StartActivity.this, LoginActivity.class);
                startActivity(login_intent);

            }
        });
    }
}

LogCat 顯示了這一點：

2020-04-30 12:53:42.391 31407-31407/com.example.chattting E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.chattting, PID: 31407
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.chattting/com.example.chattting.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2947)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3012)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1716)
        at android.os.Handler.dispatchMessage(Handler.java:110)
        at android.os.Looper.loop(Looper.java:232)
        at android.app.ActivityThread.main(ActivityThread.java:6802)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1103)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:964)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at com.example.chattting.LoginActivity.onCreate(LoginActivity.java:43)
        at android.app.Activity.performCreate(Activity.java:6974)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2900)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3012) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1716) 
        at android.os.Handler.dispatchMessage(Handler.java:110) 
        at android.os.Looper.loop(Looper.java:232) 
        at android.app.ActivityThread.main(ActivityThread.java:6802) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1103) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:964) 

Answer 1

我給出的第一個答案。 Based on your log, the line "Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference" says, that無法設置 AppBar 標題。 我敢打賭，在您嘗試從按鈕開始的一項活動中，您有某種錯誤。 如果您需要更多幫助，請告訴失敗的按鈕是什么，然后發送其他兩個活動代碼。