android - 当我尝试通过 ListView 项目开始新 Activity 时应用程序崩溃；由按钮实例化引起...？

Question

尝试构建我的第一个应用程序，但遇到了一些困难；现在只是想测试我的应用程序，但点击 ListView 项会使它崩溃。

这是日志:

10-12 22:09:17.868  28134-28134/com.example.naomikoo.classalert E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.naomikoo.classalert/com.example.naomikoo.classalert.pickSection}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
        at android.app.ActivityThread.access$600(ActivityThread.java:141)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5039)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at android.app.Activity.findViewById(Activity.java:1839)
        at com.example.naomikoo.classalert.pickSection.<init>(pickSection.java:25)
        at java.lang.Class.newInstanceImpl(Native Method)
        at java.lang.Class.newInstance(Class.java:1319)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1054)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5039)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

pickSection.java 目前看起来像这样:

public class pickSection extends AppCompatActivity {
    String course;
    String number;
    String section;

    Button button = (Button) findViewById(R.id.favouriteButton);
    TextView courseInfo = (TextView) findViewById(R.id.courseinfo);
    TextView capacity = (TextView) findViewById(R.id.capacity);

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        Intent intent2 = getIntent();
        course = intent2.getStringExtra("com.naomi.classAlert.classList.course");
        number = intent2.getStringExtra("com.naomi.classAlert.classList.number");
        section = intent2.getStringExtra("com.naomi.classAlert.classList.section");

        courseInfo.setText(course+number+" "+section);
        capacity.setText("dummy text");
        setContentView(R.layout.activity_pick_section);
    }

    public void onClick(View v) {
    }

}

第 25 行是按钮 button = (Button) findViewById(R.id.favouriteButton);

但不确定为什么这是错误的？如果有人能指出我正确的方向......

谢谢!

Answer 1

在setContentView之后调用findViewById

setContentView(R.layout.activity_pick_section);

Button button = (Button) findViewById(R.id.favouriteButton);
TextView courseInfo = (TextView) findViewById(R.id.courseinfo);
TextView capacity = (TextView) findViewById(R.id.capacity);