尝试getText时获取空指针错误

Question

我是Android新手，所以请原谅我的无知。我试过搜索论坛，但一直没能找到任何东西让这个工作。我试图使用.getText（）。toString（）（见下文），但得到一个java.lang.NullPointerException错误。我更改了我的setContentView，以便查看fragment_main.xml，但仍然为我的不同组件返回null。这个目标非常简单，只是为了改变TextView上的文本，因为我刚刚开始。任何帮助将不胜感激。

package com.example.app;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_main);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }

        Button changeTxt = (Button) findViewById(R.id.btnChangeText);
        final TextView textView = (TextView) findViewById(R.id.txtMessage);

        changeTxt.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String text = textView.getText().toString();

                if (text.contains("World")) {
                    textView.setText("Hello Android!");
                }
                else {
                    textView.setText(("Hello World!"));
                }
            }
        });
    }


    public void onStart(){
        super.onStart();
    }

    public void onResume(){
        super.onResume();
    }

    public void onPause(){
        super.onPause();
    }

    public void onStop(){
        super.onStop();
    }

    public void onDestroy(){
        super.onDestroy();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

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

这是xml：

<LinearLayout
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Large Text"
        android:id="@+id/txtMessage"
        android:layout_gravity="center_horizontal" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Change Text"
        android:id="@+id/btnChangeText"
        android:layout_gravity="center_horizontal" />
</LinearLayout>

Fllo，更新代码：

XML fragment_main.xml

<LinearLayout
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Large Text"
        android:id="@+id/txtMessage1"
        android:layout_gravity="center_horizontal" />

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Change Text"
            android:id="@+id/btnChangeText1"
            android:layout_gravity="center" />
    </FrameLayout>
</LinearLayout>

主要活动

package com.example.app;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_main);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }


    }


    public void onStart(){
        super.onStart();
    }

    public void onResume(){
        super.onResume();
    }

    public void onPause(){
        super.onPause();
    }

    public void onStop(){
        super.onStop();
    }

    public void onDestroy(){
        super.onDestroy();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);

            Button changeTxt = (Button) findViewById(R.id.btnChangeText);
            final TextView textView = (TextView) findViewById(R.id.txtMessage);

            changeTxt.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view)
                {
                    String text = textView.getText().toString();

                    if (text.contains("World"))
                    {
                        textView.setText("Hello Android!");
                    }
                    else
                    {
                        textView.setText(("Hello World!"));
                    }
                }
            });

            return rootView;
        }
    }

}

Answer 1

出现 NullPointerException ，因为您的Activity和Fragment似乎有相同的布局：

 // Activity
 setContentView(R.layout.fragment_main);

 // Fragment
 View rootView = inflater.inflate(R.layout.fragment_main, container, false);
 return rootView;  

您需要检查您的观看次数（TextView＆amp; Button）是否在正确的布局中，而不是另外（我猜activity_main）。

两个解决方案：

• 将fragment_main内的观点复制/移至activity_main并更改setContentView(R.layout.activity_main)。

• 或者让你的观点在fragment_main中，并使用以下代码在Fragment中找到你的观点，如下所示：

  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container,
          Bundle savedInstanceState) {
      View rootView = inflater.inflate(R.layout.fragment_main, container, false);
      Button changeTxt = (Button) rootView.findViewById(R.id.btnChangeText);
      final TextView textView = (TextView) rootView.findViewById(R.id.txtMessage);
  
      changeTxt.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
              String text = textView.getText().toString();
  
              if (text.contains("World")) {
                  textView.setText("Hello Android!");
              }
              else {
                  textView.setText(("Hello World!"));
              }
         }
      });
      return rootView;
  }
  

希望这会有所帮助。

---

更新：

1。移除fragment_main中的 FrameLayout 。创建一个名为activity_main的新布局，并将 FrameLayout 添加到其中（使用 id ）：

<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" 
    android:layout_height="match_parent"
    android:id="@+id/container" />

2。将onCreate方法中的此布局设置为 Activity ，如：

// onCreate method Activity
setContentView(R.layout.activity_main); 

3。然后，删除 Fragment 声明中的static，这应该是：

public class PlaceholderFragment ... { } 

4。最后，当您尝试在onCreateView方法中查找ID元素时， 始终 将膨胀视图用作：

rootView.findViewById(R.id.some_id_example);  

瞧！这应该现在有效。