片段中的按钮单击实现后应用程序崩溃

Question

我的应用包含导航抽屉。其中有一个项目片段。在那个片段中，我尝试实现三个按钮单击事件来打开 3 个活动。但是，每当我单击导航抽屉中的片段时，应用程序就会因 nullPointerAcception 而崩溃。调试后，它显示以下包含 onClickListener 的代码存在问题：

itemButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent itemIntent=new Intent(getActivity(),ItemsActivity.class);
                startActivity(itemIntent);
            }
        });

这是我的片段代码。

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;


/**
 * A simple {@link Fragment} subclass.
 */
public class ItemsFragment extends Fragment {

    Button itemButton;
    Button categoryButton;
    Button discountButton;

    public ItemsFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {


        itemButton=(Button) container.findViewById(R.id.items_button);
        categoryButton=(Button) container.findViewById(R.id.category_button);
        discountButton=(Button) container.findViewById(R.id.discount_button);

        itemButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent itemIntent=new Intent(getActivity(),ItemsActivity.class);
                startActivity(itemIntent);
            }
        });

        setHasOptionsMenu(false);
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_items, container, false);
    }

}

// Logcat 错误

E/AndroidRuntime: FATAL EXCEPTION: main
                  java.lang.NullPointerException
                      at com.soumya.possystem.ItemsFragment.onCreateView(ItemsFragment.java:36)
                      at android.support.v4.app.Fragment.performCreateView(Fragment.java:2080)
                      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1108)
                      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1290)
                      at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:801)
                      at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1677)
                      at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:536)
                      at android.os.Handler.handleCallback(Handler.java:725)
                      at android.os.Handler.dispatchMessage(Handler.java:92)
                      at android.os.Looper.loop(Looper.java:137)
                      at android.app.ActivityThread.main(ActivityThread.java:5041)
                      at java.lang.reflect.Method.invokeNative(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:511)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
                      at dalvik.system.NativeStart.main(Native Method)

Answer 1

在膨胀视图和创建视图之前，您试图找到控件的实例。并在 onCreateView 中返回膨胀视图

另一种方法是在onViewCreated或onActivityCreated中获取实例(findviewById)

第二个问题是您试图从 container 获取控件对象，而不是您需要从膨胀视图中获取。

View v=inflater.inflate(R.layout.fragment_items, container, false);

下面我保留了更改的代码。

import android.content.Intent;
    import android.os.Bundle;
    import android.support.v4.app.Fragment;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.Button;


    /**
     * A simple {@link Fragment} subclass.
     */
    public class ItemsFragment extends Fragment {

        Button itemButton;
        Button categoryButton;
        Button discountButton;

        public ItemsFragment() {
            // Required empty public constructor
        }


        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
    View v=inflater.inflate(R.layout.fragment_items, container, false);
    //check in place of container here you need to use inflated view (`v`) instance
            itemButton=(Button) v.findViewById(R.id.items_button);
            categoryButton=(Button) v.findViewById(R.id.category_button);
            discountButton=(Button) v.findViewById(R.id.discount_button);

            itemButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Intent itemIntent=new Intent(getActivity(),ItemsActivity.class);
                    startActivity(itemIntent);
                }
            });

            setHasOptionsMenu(false);
            // Inflate the layout for this fragment
            return v;
        }

    }

Answer 2

您必须根据要求初始化视图：
Create a Fragment Class

所以你必须这样做
View view = inflater.inflate(R.layout.YOUR_LAYOUT, container, false);
然后
itemButton=(Button) view.findViewById(R.id.items_button);

Answer 3

    public class ItemsFragment extends Fragment {

    Button itemButton;
    Button categoryButton;
    Button discountButton;

    public ItemsFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
    /** Inflate rootView of fragment **/
    View rootView = inflater.inflate(R.layout.fragment_items, container, false);

      //replace "container" with "rootView" **/
        itemButton=(Button) rootView.findViewById(R.id.items_button);
        categoryButton=(Button) rootView.findViewById(R.id.category_button);
        discountButton=(Button) rootView.findViewById(R.id.discount_button);

        itemButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent itemIntent=new Intent(getActivity(),ItemsActivity.class);
                startActivity(itemIntent);
            }
        });

        setHasOptionsMenu(false);
        // Inflate the layout for this fragment
        return rootView; // Return the view object you inflated
    }

}

Answer 4

试试这个

    View rootView = inflater.inflate(R.layout.fragment_id, container, false);
    itemButton=(Button) container.findViewById(R.id.items_button);
    categoryButton=(Button) container.findViewById(R.id.category_button);
    discountButton=(Button) container.findViewById(R.id.discount_button);
.
.
.

    return rootView;

Answer 5

试试这个：

public class ItemsFragment extends Fragment {

Button itemButton;
Button categoryButton;
Button discountButton;

public ItemsFragment() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View v=inflater.inflate(R.layout.fragment_items, container, false);
    setHasOptionsMenu(false);
    return v;
}

 @Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
   itemButton=(Button) view.findViewById(R.id.items_button);
    categoryButton=(Button) view.findViewById(R.id.category_button);
    discountButton=(Button) view.findViewById(R.id.discount_button);

    itemButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent itemIntent=new Intent(getActivity(),ItemsActivity.class);
            startActivity(itemIntent);
        }
    });

}
}