Android Null对象参考

Question

好的，所以我有两个活动，并且我想打开活动“单击按钮时插入”。 但是它发出了一条消息，不幸的是，活动已意外停止或发生了任何事情。

来自主活动的部分代码，其中按钮正在调用意图：

//The intent typed earlier in the activity
 Intent myIntent = new Intent(MainActivity.this, Insert.class);

private OnClickListener onClick() {
    return new OnClickListener() {

        @Override
        public void onClick(View v) {

            switch (v.getId()) {
                case R.id.addcolumnbutton:
                    if(value==k||value==10)
                    {
                        Toast.makeText(getApplicationContext(), "Maximum number of Columns created",
                                Toast.LENGTH_SHORT).show();
                    }
                    else {
                        k++;
                        mLayout.addView(createNewLayout());
                    }
                    break;
                case R.id.createtablebutton:
                    if(value==k) {

                       MainActivity.this.startActivity(myIntent);

                    }
                    else
                    {
                        Toast.makeText(getApplicationContext(), "Incorrect number of Columns created",
                                Toast.LENGTH_SHORT).show();

                    }
                    break;
            }
        }


    };
}

我的LogCat在这里：

    03-02 17:21:34.342  12789-12789/com.cambiopune.tosdatabase D/AndroidRuntime﹕       Shutting down VM
  03-02 17:21:34.345  12789-12789/com.cambiopune.tosdatabase E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.cambiopune.tosdatabase, PID: 12789
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.cambiopune.tosdatabase/com.cambiopune.tosdatabase.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
        at android.app.ActivityThread.access$800(ActivityThread.java:144)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        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:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
           at   android.content.ContextWrapper.getPackageName(ContextWrapper.java:131)
         at android.content.ComponentName.<init>(ComponentName.java:77)
         at android.content.Intent.<init>(Intent.java:3996)
         at com.cambiopune.tosdatabase.MainActivity.<init>(MainActivity.java:34)
        at java.lang.reflect.Constructor.newInstance(Native Method)
        at java.lang.Class.newInstance(Class.java:1572)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1065)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199)
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)  

Answer 1

只需在代码中删除此行：

private OnClickListener onClick() {
    return new OnClickListener() {

它应该只是：

   @Override
    public void onClick(View v) {

        switch (v.getId()) {
            case R.id.addcolumnbutton:
                if(value==k||value==10)
                {
                    Toast.makeText(getApplicationContext(), "Maximum number of Columns created",
                            Toast.LENGTH_SHORT).show();
                }
                else {
                    k++;
                    mLayout.addView(createNewLayout());
                }
                break;
            case R.id.createtablebutton:
                if(value==k) {

                   MainActivity.this.startActivity(myIntent);

                }
                else
                {
                    Toast.makeText(getApplicationContext(), "Incorrect number of Columns created",
                            Toast.LENGTH_SHORT).show();

                }
                break;
        }
    }