Question

我正在尝试执行这些步骤（下面给出）来调用我的活动中的片段方法，而不使用任何片段ID和标记。但我得到NullpointerException。这对我来说非常重要...这些正确的步骤是否从活动中调用片段方法？ 如果是，我做得对吗？如果没有，有没有更好的方法从我的活动中调用片段方法？和我的MainActivty和Fragmet在下面给出... plz help，thanks

_STEPS_WHICH_I_AM_TRYING_TO_DO

1步骤）首先创建一个界面

public interface MyInterface {
    void myAction() ;
}

您的片段必须实现此接口。

public MyFragment extends Fragment implements MyInterface

2 STEP）在您的活动中，定义MyInterface类型的字段：

 private MyInterface listener ;

 public void setListener(MyInterface listener) {
   this.listener = listener ;
}

3 STEP）创建片段并添加片段时：

setListener(myFragment);

最后，当你想要调用Fragment方法的condtion时，只需调用：

   listener.myAction() ; // this will call the implementation in your   MyFragment class

MainActivity.java

public class MainActivity extends AppCompatActivity
    implements  NavigationView.OnNavigationItemSelectedListener{ 

 Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
private hell listener;
   ClaimFragment claimFragment;
    setListener(claimFragment);
     viewPager = (ViewPager) findViewById(R.id.viewpager);
    setupViewPager(viewPager);

   Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

        @Override
        public void onPageSelected(int position) {
            listener.willwork();
        }

        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2) { }

        @Override
        public void onPageScrollStateChanged(int arg0) { }
    });
}
       tabLayout.setupWithViewPager(viewPager);

  //THIS IS MY INTERFACE IN MAINACTIVITY

public interface hell {
    void willwork();
}

public void setListener(hell listener) {
   this.listener = listener ;
}

ClaimFragment.java

public class ClaimFragment extends Fragment implements SwipeRefreshLayout.OnRefreshListener , MainActivity.hell {

    @Override
    public void willwork() {
        Toast.makeText(getContext(), "this is ClaimFrag", Toast.LENGTH_SHORT).show();
    }

}

MY_LOG

04-06 18:31:23.104 20843-20843/? E/AndroidRuntime: FATAL EXCEPTION: main
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime: Process: earn.free.,        PID: 20843
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:        java.lang.NullPointerException: Attempt to invoke interface method 'void                earn.free.cashback.snapcash.MainActivity$hell.willwork()' on a null object reference
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at   earn.free.cashback.snapcash.MainActivity$2.onPageSelected(MainActivity.java:152)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at       android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1856)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at   android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:625)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at      android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:609)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at    android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:570)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at  android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:551)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at    android.support.design.widget.TabLayout$ViewPagerOnTabSelectedListener.onTabSele cted(TabLayout.java:2008)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at  android.support.design.widget.TabLayout.selectTab(TabLayout.java:1025)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at android.support.design.widget.TabLayout.selectTab(TabLayout.java:995)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at  android.support.design.widget.TabLayout$Tab.select(TabLayout.java:1272)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at android.support.design.widget.TabLayout$TabView.performClick(TabLayout.java:1377 )
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at   android.view.View$PerformClick.run(View.java:19866)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at        android.os.Handler.handleCallback(Handler.java:739)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at    android.os.Handler.dispatchMessage(Handler.java:95)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:135)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at    android.app.ActivityThread.main(ActivityThread.java:5254)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at  java.lang.reflect.Method.invoke(Native Method)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at  java.lang.reflect.Method.invoke(Method.java:372)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at  com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
04-06 18:31:23.104 20843-20843/? E/AndroidRuntime:     at  com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
04-06 18:31:23.105 747-1255/? W/ActivityManager:   Force finishing  activity 1 earn.free.cashback.snapcash/.MainActivity

使用接口NullPointerException从MainActivity调用Fragment方法

0 个答案: