java - 无法解决 Android Studio 2.1 上的运行时错误

Question

我是一名 Android 初学者，我正在构建一个测试应用程序来了解我能做什么和不能做什么，我正在尝试解决此运行时错误，但不知道该怎么做。 有人可以帮我吗？我发布我的代码和错误

MainActivity.java:

package com.example.android.myapplication;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {

    EditText uname;

    {
        uname = (EditText) findViewById(R.id.username);
    }

    EditText pword;

    {
        pword = (EditText) findViewById(R.id.password);
    }

    Button btn;

    {
        btn = (Button) findViewById(R.id.button);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        OnClickButtonListener();
    }
    public void OnClickButtonListener() {
        btn.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        Intent intent = new Intent("com.example.android.myapplication.Activity2");
                        startActivity(intent);
                    }
                }
        );
    }
}

日志:

05-01 14:32:21.725 32378-32378/com.example.android.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.myapplication, PID: 32378
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.myapplication/com.example.android.myapplication.MainActivity}: 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:2555)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2773)
    at android.app.ActivityThread.access$900(ActivityThread.java:177)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1434)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5930)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)
 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:2172)
    at com.example.android.myapplication.MainActivity.<init>(MainActivity.java:12)
    at java.lang.reflect.Constructor.newInstance(Native Method)
    at java.lang.Class.newInstance(Class.java:1690)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2545)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2773) 
    at android.app.ActivityThread.access$900(ActivityThread.java:177) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1434) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5930) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200) 

Answer 1

像这样移动所有三个语句:

uname = (EditText) findViewById(R.id.username);

在 setContentView() 调用之后进入 onCreate()，原因有两个:

1. 一般来说，在调用 super.onCreate() 之前，您无法安全地调用从 Activity 继承的方法

2. 在实际拥有 View 之前，您不能调用 findViewById()，例如通过调用 setContentView()