安卓应用: NullPointerException

Question

我正在做一个测试，以便在Eclipse中进行Android开发。当按下calculate按钮时，我的应用程序崩溃。看起来问题出在AlertDialog上，但我找到的所有例子都是一样的。

package mobile.ildu.AndroidTest;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class AndroidTestActivity extends Activity {
    private EditText text1;
    private EditText text2;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }

    public void myClickHandler(View view)
    {
        switch(view.getId())
        {
            case R.id.calculate:
                if(text1.getText().length()==0)
                {
                    callDialog("Error","You must enter a number in the first box");
                    return;
                }
                if(text2.getText().length()==0)
                {
                    callDialog("Error","You must enter a number in the second box");
                    return;
                }
                float number1 = Float.parseFloat(text1.getText().toString());
                float number2 = Float.parseFloat(text2.getText().toString());
                float answer = number1*number2;
                callDialog("Answer",number1+"*"+number2+" = "+answer);
                break;
        }
    }

    private void callDialog(String title, String message)
    {
        AlertDialog alertDialog= new AlertDialog.Builder(AndroidTestActivity.this).create();
        alertDialog.setTitle(title);   
        alertDialog.setMessage(message);
        alertDialog.setButton("Close", new DialogInterface.OnClickListener() 
        {         
            public void onClick(DialogInterface dialog, int which) 
            {
                dialog.dismiss();
            }
        });
        alertDialog.show();
    }
}

错误日志：

11-22 10:41:45.092: E/AndroidRuntime(675): FATAL EXCEPTION: main
11-22 10:41:45.092: E/AndroidRuntime(675): java.lang.IllegalStateException: Could not execute method of the activity
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.view.View$1.onClick(View.java:3019)
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.view.View.performClick(View.java:3460)
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.view.View$PerformClick.run(View.java:13955)
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.os.Handler.handleCallback(Handler.java:605)
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.os.Handler.dispatchMessage(Handler.java:92)
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.os.Looper.loop(Looper.java:137)
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.app.ActivityThread.main(ActivityThread.java:4340)
11-22 10:41:45.092: E/AndroidRuntime(675):  at java.lang.reflect.Method.invokeNative(Native Method)
11-22 10:41:45.092: E/AndroidRuntime(675):  at java.lang.reflect.Method.invoke(Method.java:511)
11-22 10:41:45.092: E/AndroidRuntime(675):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
11-22 10:41:45.092: E/AndroidRuntime(675):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
11-22 10:41:45.092: E/AndroidRuntime(675):  at dalvik.system.NativeStart.main(Native Method)
11-22 10:41:45.092: E/AndroidRuntime(675): Caused by: java.lang.reflect.InvocationTargetException
11-22 10:41:45.092: E/AndroidRuntime(675):  at java.lang.reflect.Method.invokeNative(Native Method)
11-22 10:41:45.092: E/AndroidRuntime(675):  at java.lang.reflect.Method.invoke(Method.java:511)
11-22 10:41:45.092: E/AndroidRuntime(675):  at android.view.View$1.onClick(View.java:3014)
11-22 10:41:45.092: E/AndroidRuntime(675):  ... 11 more
11-22 10:41:45.092: E/AndroidRuntime(675): Caused by: java.lang.NullPointerException
11-22 10:41:45.092: E/AndroidRuntime(675):  at mobile.ildu.AndroidTest.AndroidTestActivity.myClickHandler(AndroidTestActivity.java:27)
11-22 10:41:45.092: E/AndroidRuntime(675):  ... 14 more

谢谢你的帮助！

Answer 1

试试这个：

package mobile.ildu.AndroidTest;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class AndroidTestActivity extends Activity {
    private EditText text1;
    private EditText text2;
    private Button button;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        text1 = (EditText)findViewById(R.id.text1);
        text2 = (EditText)findViewById(R.id.text2);
        button = (Button) findViewById(R.id.calculate);

        button.setOnClickListener(new  OnClickListener() {

            public void onClick(View arg0) {

                if(text1.getText().length()==0)
                {
                    callDialog("Error","You must enter a number in the first box");
                    return;
                }
                if(text2.getText().length()==0)
                {
                    callDialog("Error","You must enter a number in the second box");
                    return;
                }
                float number1 = Float.parseFloat(text1.getText().toString());
                float number2 = Float.parseFloat(text2.getText().toString());
                float answer = number1*number2;
                callDialog("Answer",number1+"*"+number2+" = "+answer);
            }
        });
    }

    private void callDialog(String title, String message)
    {
        AlertDialog alertDialog= new AlertDialog.Builder(AndroidTestActivity.this).create();
        alertDialog.setTitle(title);   
        alertDialog.setMessage(message);
        alertDialog.setButton("Close", new DialogInterface.OnClickListener() 
        {         
            public void onClick(DialogInterface dialog, int which) 
            {
                dialog.dismiss();
            }
        });
        alertDialog.show();
    }
}

Answer 2

您从不初始化EditText成员变量，因此在执行此操作时：

text1.getText().length()

你会得到一个空指针异常。在调用setContentView之后，将此代码添加到onCreate方法中

text1 = (EditText)findViewById(R.id.idOfWidget);
text2 = (EditText)findViewById(R.id.idOfWidget2);

(当然，用正确的常量替换这两个ids )。

Answer 3

除非您将它们设置在此处未显示的其他位置，否则text1和text2不包含任何引用，因此当您尝试对它们调用方法时，它们会崩溃。