Question

我正在尝试在Login类的构造函数中查找ViewById，但它会在对象创建和崩溃应用程序上抛出错误。

错误仅在Login类中的构造上抛出而不在侦听器上抛出。单击按钮后，应调用loginAction（）。

我该怎么做才能解决这个问题（这是我的第一个应用程序）？

错误：

11-26 09:13:57.592 5926-5926/com.wolfriders E/AndroidRuntime: FATAL EXCEPTION: main
                                                          Process: com.wolfriders, PID: 5926
                                                          java.lang.RuntimeException: Unable to start activity ComponentInfo{com.wolfriders/com.wolfriders.Main}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
                                                              at android.app.ActivityThread.-wrap11(Unknown Source:0)
                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
                                                              at android.os.Handler.dispatchMessage(Handler.java:105)
                                                              at android.os.Looper.loop(Looper.java:164)
                                                              at android.app.ActivityThread.main(ActivityThread.java:6541)
                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                              at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
                                                           Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
                                                              at android.app.Activity.findViewById(Activity.java:2563)
                                                              at com.wolfriders.Login.<init>(Login.java:13)
                                                              at com.wolfriders.Main.onCreate(Main.java:18)
                                                              at android.app.Activity.performCreate(Activity.java:6975)
                                                              at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
                                                              at android.app.ActivityThread.-wrap11(Unknown Source:0) 
                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
                                                              at android.os.Handler.dispatchMessage(Handler.java:105) 
                                                              at android.os.Looper.loop(Looper.java:164) 
                                                              at android.app.ActivityThread.main(ActivityThread.java:6541) 
                                                              at java.lang.reflect.Method.invoke(Native Method) 
                                                              at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

主：

package com.wolfriders;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

public class Main extends AppCompatActivity {

private boolean logged = false;

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

    if (logged == false) {
        setContentView(R.layout.login_from);

        final Login login = new Login();

        login.login_btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                login.loginAction();
            }

        });
    } else { setContentView(R.layout.main_activity); }
}
}

登录：

package com.wolfriders;

import android.app.Activity;
import android.widget.Button;
import android.widget.EditText;

public class Login extends Activity {

public EditText login_phone, login_password;
public Button login_btn;

Login() {
    this.login_phone = (EditText)findViewById(R.id.inputloginpassword);
    this.login_password = (EditText)findViewById(R.id.inputloginpassword);
    this.login_btn = (Button)findViewById(R.id.btnlogin);
}

public void loginAction() {
    // Do something after login button clicked
}

}

Answer 1

如果您的登录类需要是一个活动，您需要覆盖onCreate函数，因为它是活动，然后您需要一个.xml文件来将该活动类与所表示的ui相关联。你是如何创建这个Login类的（我的意思是作为一个活动或只是一个类）？关于活动的更详细的阅读和解释，你可以在这里阅读：

https://developer.android.com/guide/components/activities/intro-activities.html

另一个类中的findViewById（）给出了错误

1 个答案: