Question

我是java的新手并且正在做一些练习。我的问题是代码中的java.lang.NullPointerException。我读了很多问题，但我无法找到答案。所有字符串都添加了strings.xml

logcat错误：

06-13 19:08:07.690: D/AndroidRuntime(865): Shutting down VM
06-13 19:08:07.690: W/dalvikvm(865): threadid=1: thread exiting with uncaught exception group=0xb2ab1ba8)
06-13 19:08:07.720: E/AndroidRuntime(865): FATAL EXCEPTION: main
06-13 19:08:07.720: E/AndroidRuntime(865): Process: com.example.fpscalculator, PID: 865
06-13 19:08:07.720: E/AndroidRuntime(865): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fpscalculator/com.example.fpscalculator.MainActivity}: java.lang.NullPointerException

我的xml代码：

<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/scrollView1"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="499dp" >

<EditText
    android:id="@+id/bbweight"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="34dp"
    android:ems="10"
    android:inputType="number"
    android:labelFor="@id/bbweight"
    android:text="@string/bbweight" />

<EditText
    android:id="@+id/bbfps"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/bbweight"
    android:layout_below="@+id/bbweight"
    android:layout_marginTop="34dp"
    android:ems="10"
    android:inputType="number" 
    android:text="@string/bbfps"
    android:labelFor="@id/bbfps"
    />

<Button
    android:id="@+id/calc"
    style="?android:attr/buttonStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/bbfps"
    android:layout_alignParentRight="true"
    android:layout_marginBottom="9dp"
    android:layout_marginRight="18dp"
    android:text="@string/calc" />


<TextView
    android:id="@+id/tenergy"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/bbfps"
    android:layout_below="@+id/bbfps"
    android:layout_marginTop="25dp"
    android:text="@string/tenergy" />

<TextView
    android:id="@+id/fps20"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/tenergy"
    android:layout_below="@+id/tenergy"
    android:layout_marginTop="30dp"
    android:text="@string/fps20" 
    />

<TextView
    android:id="@+id/fps25"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/fps20"
    android:layout_below="@+id/fps20"
    android:layout_marginTop="30dp"
    android:text="@string/fps25" />

<TextView
    android:id="@+id/fps28"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/fps25"
    android:layout_below="@+id/fps25"
    android:layout_marginTop="30dp"
    android:text="@string/fps28" />



<TextView
    android:id="@+id/fps30"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/fps28"
    android:layout_below="@+id/fps28"
    android:layout_marginTop="30dp"
    android:text="@string/fps30" />

<TextView
    android:id="@+id/fps40"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/fps30"
    android:layout_below="@+id/fps30"
    android:layout_marginTop="30dp"
    android:text="@string/fps40" />

</RelativeLayout>

java代码：

package com.example.fpscalculator;

import android.app.Activity;
import android.app.ActionBar;
import android.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.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.os.Build;
import java.lang.Math;

public class MainActivity extends Activity {

double fps,energy;

double bb20=0.20,bb25=0.25,bb28=0.28,bb30=0.30,bb40=0.40;


EditText bbweight;

EditText bbfps;

Button calc;

TextView displayenergy, fps20, fps25, fps28, fps30, fps40;




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

    fps=0;
    bbweight = (EditText) findViewById(R.id.bbweight);
    bbfps = (EditText) findViewById(R.id.bbfps);
    displayenergy = (TextView) findViewById(R.id.tenergy);
    fps20 = (TextView) findViewById(R.id.fps20);
    fps25 = (TextView) findViewById(R.id.fps25);
    fps28 = (TextView) findViewById(R.id.fps28);
    fps30 = (TextView) findViewById(R.id.fps30);
    fps40 = (TextView) findViewById(R.id.fps40);
    calc = (Button) findViewById(R.id.calc);

    calc.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            double BBweight =Double.valueOf(bbweight.getText().toString());

            double BBfps =Double.valueOf(bbfps.getText().toString());

            fps = 0;                
            energy = 0;

            bb20=0.20;bb25=0.25;bb28=0.28;bb30=0.30;bb40=0.40;

            energy = 0.5 * (0.001 * BBweight) * (Math.pow((0.3048 * BBfps),2));
            displayenergy.setText("BB energy = " + energy + " joule");

            fps = BBfps * (Math.sqrt(BBweight/bb20));
            fps20.setText("0.20g BB fps = " + fps);

            fps = BBfps * (Math.sqrt(BBweight/bb25));
            fps25.setText("0.25g BB fps = " + fps);

            fps = BBfps * (Math.sqrt(BBweight/bb28));
            fps28.setText("0.28g BB fps = " + fps);

            fps = BBfps * (Math.sqrt(BBweight/bb30));
            fps30.setText("0.30g BB fps = " + fps);

            fps = BBfps * (Math.sqrt(BBweight/bb40));
            fps40.setText("0.40g BB fps = " + fps);  

        }
    });

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

@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;
    }
}

  }

Answer 1

您已将代码放置在托管片段的Activity内，因为您需要在onCreateView() PlaceholderFragment片段中编写该代码。我假设您发布的xml属于PlaceholderFragment片段而非Activity类。从MainActivity类中删除代码段并替换如下，

 public class MainActivity extends Activity {

    double fps,energy;    
    double bb20=0.20,bb25=0.25,bb28=0.28,bb30=0.30,bb40=0.40;  
    EditText bbweight;    
    EditText bbfps;    
    Button calc;    
    TextView displayenergy, fps20, fps25, fps28, fps30, fps40;    

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

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

        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);

            fps=0;
            bbweight = (EditText)rootView.findViewById(R.id.bbweight);
            bbfps = (EditText)rootView.findViewById(R.id.bbfps);
            displayenergy = (TextView)rootView.findViewById(R.id.tenergy);
            fps20 = (TextView)rootView.findViewById(R.id.fps20);
            fps25 = (TextView)rootView.findViewById(R.id.fps25);
            fps28 = (TextView)rootView.findViewById(R.id.fps28);
            fps30 = (TextView)rootView.findViewById(R.id.fps30);
            fps40 = (TextView)rootView.findViewById(R.id.fps40);
            calc = (Button)rootView.findViewById(R.id.calc);

            calc.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    double BBweight =Double.valueOf(bbweight.getText().toString());

                    double BBfps =Double.valueOf(bbfps.getText().toString());

                    fps = 0;                
                    energy = 0;

                    bb20=0.20;bb25=0.25;bb28=0.28;bb30=0.30;bb40=0.40;

                    energy = 0.5 * (0.001 * BBweight) * (Math.pow((0.3048 * BBfps),2));
                    displayenergy.setText("BB energy = " + energy + " joule");

                    fps = BBfps * (Math.sqrt(BBweight/bb20));
                    fps20.setText("0.20g BB fps = " + fps);

                    fps = BBfps * (Math.sqrt(BBweight/bb25));
                    fps25.setText("0.25g BB fps = " + fps);

                    fps = BBfps * (Math.sqrt(BBweight/bb28));
                    fps28.setText("0.28g BB fps = " + fps);

                    fps = BBfps * (Math.sqrt(BBweight/bb30));
                    fps30.setText("0.30g BB fps = " + fps);

                    fps = BBfps * (Math.sqrt(BBweight/bb40));
                    fps40.setText("0.40g BB fps = " + fps);  

                }
            });


                return rootView;
            }
        }
}

Answer 2

请检查您是否初始化了代码中使用的每个控件和变量。因为它是空指针异常的主要原因。

带有EditText的java.lang.NullPointerException

2 个答案: