Question

我是android开发的初学者。我正在尝试制作一个将米转换为cm的转换器应用程序。但是在onClick方法中它没有从spinner获取所选项目。除了应用程序停止。

MainActivity.java

package com.example.spinnertext;

import java.util.ArrayList;

import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.Toast;

public class MainActivity extends Activity {
    Spinner sp;
    EditText et;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        sp=(Spinner) findViewById(R.id.spinner1);
        ArrayAdapter<CharSequence> adapter= ArrayAdapter.createFromResource(this, R.array.items, android.R.layout.simple_spinner_item);
        sp.setAdapter(adapter);
        sp.setOnItemSelectedListener(new CustomListener());
        Button b=(Button) findViewById(R.id.button1);
        b.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                EditText et=(EditText) findViewById(R.id.editText1);
                sp=(Spinner) findViewById(R.id.spinner1);
                Double d = null,s=Double.valueOf(et.getText().toString());
                if(arg0.getId()==R.id.spinner1)
                {
                    Log.d("TEST", "From Main");
                    if(sp.getSelectedItemPosition()==0)
                    {
                        d=s*100;
                        Log.d("TEST", "Meter From Main");
                    }
                    else
                    {
                        Log.d("TEST", "Cm From Main");
                        d=s;
                    }
                }
                Toast.makeText(arg0.getContext(), d.toString(), Toast.LENGTH_SHORT).show();
            }
        });
    }


}

CustomListener.java

package com.example.spinnertext;

import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;

public class CustomListener implements OnItemSelectedListener {

    @Override
    public void onItemSelected(AdapterView<?> arg0, View arg1, int arg2,
            long arg3) {
        // TODO Auto-generated method stub
        Log.d("TEST", arg0.getSelectedItem().toString()+"selected");

    }

    @Override
    public void onNothingSelected(AdapterView<?> arg0) {
        // TODO Auto-generated method stub

    }

}

布局文件：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginTop="31dp"
        android:ems="10" />

    <Spinner
        android:id="@+id/spinner1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/editText1"
        android:layout_alignParentRight="true"
        android:layout_alignTop="@+id/editText1"
        android:layout_toRightOf="@+id/editText1" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/button1"
        android:layout_below="@+id/button1"
        android:layout_marginTop="42dp"
        android:textSize="25sp" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/editText1"
        android:layout_below="@+id/editText1"
        android:layout_marginTop="47dp"
        android:text="Button" />

</RelativeLayout>

LogCat显示以下错误：

03-24 13:22:05.357: D/libEGL(1505): loaded /system/lib/egl/libEGL_genymotion.so
03-24 13:22:05.361: D/(1505): HostConnection::get() New Host Connection established 0xb89c08e8, tid 1505
03-24 13:22:05.385: D/libEGL(1505): loaded /system/lib/egl/libGLESv1_CM_genymotion.so
03-24 13:22:05.385: D/libEGL(1505): loaded /system/lib/egl/libGLESv2_genymotion.so
03-24 13:22:05.465: W/EGL_genymotion(1505): eglSurfaceAttrib not implemented
03-24 13:22:05.465: E/OpenGLRenderer(1505): Getting MAX_TEXTURE_SIZE from GradienCache
03-24 13:22:05.497: E/OpenGLRenderer(1505): Getting MAX_TEXTURE_SIZE from Caches::initConstraints()
03-24 13:22:05.497: D/OpenGLRenderer(1505): Enabling debug mode 0
03-24 13:22:05.525: D/TEST(1505): Meterselected
03-24 13:22:10.481: D/AndroidRuntime(1505): Shutting down VM
03-24 13:22:10.485: W/dalvikvm(1505): threadid=1: thread exiting with uncaught exception (group=0xa4bd5648)
03-24 13:22:10.489: E/AndroidRuntime(1505): FATAL EXCEPTION: main
03-24 13:22:10.489: E/AndroidRuntime(1505): java.lang.NullPointerException
03-24 13:22:10.489: E/AndroidRuntime(1505):     at com.example.spinnertext.MainActivity$1.onClick(MainActivity.java:50)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at android.view.View.performClick(View.java:4240)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at android.view.View$PerformClick.run(View.java:17721)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at android.os.Handler.handleCallback(Handler.java:730)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at android.os.Handler.dispatchMessage(Handler.java:92)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at android.os.Looper.loop(Looper.java:137)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at android.app.ActivityThread.main(ActivityThread.java:5103)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at java.lang.reflect.Method.invokeNative(Native Method)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at java.lang.reflect.Method.invoke(Method.java:525)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
03-24 13:22:10.489: E/AndroidRuntime(1505):     at dalvik.system.NativeStart.main(Native Method)

来自CustomListener类的LogCat显示消息，但不显示OnClick方法日志。

Answer 1

你的吐司中有一个空指针异常。

Toast.makeText(arg0.getContext(), d.toString(), Toast.LENGTH_SHORT).show();

可能是arg0 == null或d == null。

检查这两个值pls

Answer 2

arg0.getId()==R.id.spinner1将始终返回false，因为arg0是对您按钮的引用。这意味着您的d引用永远不会被初始化（您只在if子句中执行），因此调用d.toString()会导致NullPointerException。

Answer 3

取出：

         et=(EditText) findViewById(R.id.editText1);
         sp=(Spinner) findViewById(R.id.spinner1);

从onClick到onCreate()并将它们声明为最终的全局变量：

         private final EditText et;
         private final Sinner sp;

然后您可以在onClick()

中使用它们

希望这有帮助！

微调器中的java.lang.nullpointerException

3 个答案: