java.lang.NullPointerException：尝试在空对象引用上调用虚方法’android.view.View android.view.View.findViewById（int）’
我有一个BlankFragment
package com.hfad.addingafragmenttoframelayout; import android.os.Bundle; import android.support.v4.app.Fragment; import android.util.Log; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.TextView; import org.w3c.dom.Text; /** * A simple {@link Fragment} subclass. */ public class BlankFragment extends Fragment { public BlankFragment() { // Required empty public constructor } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.d("message","onCreateView() of BlankFragment"); // Inflate the layout for this fragment return inflater.inflate(R.layout.fragment_blank, container, false); } public void setText(String textToBeSet){ TextView tv = (TextView)this.getView().findViewById(R.id.fragmentText); tv.setText(textToBeSet); } }
它的布局是：
这是MainActivity
package com.hfad.addingafragmenttoframelayout; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.View; import android.widget.TextView; public class MainActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); //BlankFragment bf = new BlankFragment(); //getSupportFragmentManager().beginTransaction().add(R.id.fragment_container, bf).commit(); BlankFragment bf = new BlankFragment(); bf.setText("hello world"); getSupportFragmentManager().beginTransaction().add(R.id.fragment_container, bf).commit(); } }
这是MainActivity的布局
在onCreate活动方法中，我将片段添加到布局中。 这工作正常，除非我尝试使用setText方法更改文本。
我收到这个错误：
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference at com.hfad.addingafragmenttoframelayout.BlankFragment.setText(BlankFragment.java:35) at com.hfad.addingafragmenttoframelayout.MainActivity.onStart(MainActivity.java:29)
指的是这条线
TextView tv = (TextView)this.getView().findViewById(R.id.fragmentText);
有人可以解释导致这种exception的原因吗？
别
@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.d("message","onCreateView() of BlankFragment"); // Inflate the layout for this fragment return inflater.inflate(R.layout.fragment_blank, container, false); } public void setText(String textToBeSet){ TextView tv = (TextView)this.getView().findViewById(R.id.fragmentText); tv.setText(textToBeSet); }
做
@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View RootView = inflater.inflate(R.layout.fragment_blank, container, false); TextView tv = (TextView)RootView.findViewById(R.id.fragmentText); tv.setText("HI"); return RootView; }
这里：
bf.setText("hello world");
导致问题的行，因为在Fragment中添加Fragment之前调用Fragment的setText方法。
添加bf片段后调用setText方法：
getSupportFragmentManager().beginTransaction(). add(R.id.fragment_container, bf).commit(); bf.setText("hello world"); // call here
TextView tv = (TextView)this.getView().findViewById(R.id.fragmentText);
在onCreateView（）中执行;
在尚未创建视图时，可以使用可以保存String的临时变量。
private TextView tv; private String holder = ""; @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_blank, container,false); tv = (TextView)v.findViewById(R.id.fragmentText); return v; } @Override public void onViewCreated(View view, @Nullable Bundle savedInstanceState) { tv.setText(holder); } public void setText(String textToBeSet){ if(tv != null){ tv.setText(textToBeSet); }else{ holder = textToBeSet; } }
我的假设是您希望使用一些初始文本实例化BlankFragment ，并允许稍后从MainActivity设置文本。
在这种情况下，我会首先更改片段以接受如下的初始文本：
public class BlankFragment extends Fragment { private static final String ARG_INITIAL_TEXT = "arg_initial_text"; public static BlankFragment newInstance(CharSequence initialText) { final BlankFragment fragment = new BlankFragment(); final Bundle args = new Bundle(); args.putCharSequence(ARG_INITIAL_TEXT, initialText); fragment.setArguments(args); return fragment; } public BlankFragment() { // Required empty public constructor } private CharSequence mInitialText; private TextView mText; @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); mInitialText = getArguments().getCharSequence(ARG_INITIAL_TEXT); } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.d("message", "onCreateView() of BlankFragment"); // Inflate the layout for this fragment return inflater.inflate(R.layout.fragment_blank, container, false); } @Override public void onViewCreated(View view, @Nullable Bundle savedInstanceState) { super.onViewCreated(view, savedInstanceState); mText = view.findViewById(R.id.fragmentText); mText.setText(mInitialText); } public void setText(CharSequence textToBeSet) { mText.setText(textToBeSet); } }
然后，Activity代码如下所示：
public class MainActivity extends AppCompatActivity { private BlankFragment mBlankFragment; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); if (savedInstanceState == null) { final Fragment bf = BlankFragment.newInstance("hello world"); getSupportFragmentManager() .beginTransaction() .add(R.id.fragment_container, bf, BlankFragment.class.getName()) .commit(); } mBlankFragment = (BlankFragment) getSupportFragmentManager() .findFragmentByTag(BlankFragment.class.getName()); } private void someTimeLaterWhenWantingToSetNewTextToFragmentFromActivity(CharSequence newText) { if (mBlankFragment != null) { mBlankFragment.setText(newText); } } }
请注意， savedInstanceState的空值检查是为了保护Activity-recreation，例如由于配置更改。
问题是FragmentTransaction默认是异步的
FragmentTransaction ft = getSupportFragmentManager().beginTransaction(); ft.add(R.id.fragment_container, BlankFragment.newInstance); ft.disallowBackStack(); ft.commitNowAllowingStateLoss();
然后你仍然无法获得Fragment，你必须从FragmentTransaction获得它
BlankFragment bf = (BlankFragment) getSupportFragmentManager().getFragmentById(R.id.fragment_container); bf.setText("hello world");
这篇文章帮助我更好地理解了这个问题