java - 空指针异常。无法创建 Activity

Question

我不知道哪里出了问题。这是堆栈跟踪:

08-16 18:00:36.455: E/AndroidRuntime(22834): FATAL EXCEPTION: main
08-16 18:00:36.455: E/AndroidRuntime(22834): Process: org.example.calcu, PID: 22834
08-16 18:00:36.455: E/AndroidRuntime(22834): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.example.calcu/org.example.calcu.Menu}: java.lang.NullPointerException
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2224)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2283)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.ActivityThread.access$800(ActivityThread.java:144)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1205)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.os.Handler.dispatchMessage(Handler.java:102)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.os.Looper.loop(Looper.java:136)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.ActivityThread.main(ActivityThread.java:5158)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at java.lang.reflect.Method.invokeNative(Native Method)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at java.lang.reflect.Method.invoke(Method.java:515)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:796)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:612)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:132)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at dalvik.system.NativeStart.main(Native Method)
08-16 18:00:36.455: E/AndroidRuntime(22834): Caused by: java.lang.NullPointerException
08-16 18:00:36.455: E/AndroidRuntime(22834):    at org.example.calcu.Menu.onCreate(Menu.java:79)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.Activity.performCreate(Activity.java:6084)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
08-16 18:00:36.455: E/AndroidRuntime(22834):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2181)
08-16 18:00:36.455: E/AndroidRuntime(22834):    ... 12 more

这是我尝试在 MainActivity 中开始 Activity 的地方:

empty 是 Button(相当长而且是空的 - 里面没有文本)。我向左滑动手指以创建新 Intent - 菜单

empty = (Button) findViewById(R.id.empty);
        empty.setOnTouchListener(new OnSwipeTouchListener(this){

             @Override
                public void onSwipeLeft() {
                Intent ent;
                ent = new Intent(MainActivity.this, Menu.class);
                startActivity(ent);
             }
        });

这是我尝试创建的 Activity ，菜单:

package org.example.calcu;


import android.app.Activity;
import android.content.Context;
import android.graphics.Color;
import android.graphics.PorterDuff;
import android.os.Bundle;
import android.text.Layout;
import android.util.Log;
import android.view.GestureDetector;
import android.view.MotionEvent;
import android.view.View;
import android.view.GestureDetector.SimpleOnGestureListener;
import android.view.View.OnClickListener;
import android.view.View.OnTouchListener;
import android.widget.Button;




public class Menu extends MainActivity{

    Button blueTheme;
    View menu;

    class OnSwipeTouchListener implements OnTouchListener{

         private final GestureDetector gestureDetector;

            public OnSwipeTouchListener(Context context) {
                gestureDetector = new GestureDetector(context, new GestureListener());
            }

            public void onSwipeLeft() {
            }

            public void onSwipeRight() {
            }

            public boolean onTouch(View v, MotionEvent event) {
                return gestureDetector.onTouchEvent(event);
            }

            private final class GestureListener extends SimpleOnGestureListener {

                private static final int SWIPE_DISTANCE_THRESHOLD = 100;
                private static final int SWIPE_VELOCITY_THRESHOLD = 100;

                @Override
                public boolean onDown(MotionEvent e) {
                    return true;
                }

                @Override
                public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
                    float distanceX = e2.getX() - e1.getX();
                    float distanceY = e2.getY() - e1.getY();
                    if (Math.abs(distanceX) > Math.abs(distanceY) && Math.abs(distanceX) > SWIPE_DISTANCE_THRESHOLD && Math.abs(velocityX) > SWIPE_VELOCITY_THRESHOLD) {
                        if (distanceX > 0)
                            onSwipeRight();
                        else
                            onSwipeLeft();
                        return true;
                    }
                    return false;
                }
            }
        }   




    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d("LOG", "menu opened!");
        setContentView(R.layout.menu);
        menu = (View) findViewById(R.layout.menu);
        menu.setOnTouchListener(new OnSwipeTouchListener(this){ /* this is line 79 */

            @Override
            public void onSwipeRight(){
                setContentView(R.layout.activity_main);
            }


        });
        blueTheme = (Button) findViewById(R.id.blueTheme);
        blueTheme.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {



            one.setBackgroundColor(Color.parseColor("#90CAF9"));    


            }
        });



}
}

感谢您的帮助，抱歉我是编程初学者。

Answer 1

menu = (View) findViewById(R.layout.menu);

findViewById 将 id 作为参数而不是布局。

可能应该是:

menu = (View) findViewById(R.id.menu);

但是，当然你还应该在你的 xml layout 中有一个带有 id menu 的 view。

要完成您的 Activity 并返回上一个 Activity :

menu.setOnTouchListener(new OnSwipeTouchListener(this){
    @Override public void onSwipeRight()
    {
        Menu.this.finish();
    }
}