Question

 package viewbrosers.mehdi.home.mytestappviewbrowser;

import android.app.Activity;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

import java.util.Random;


public class MainActivity extends Activity {

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

    Button getAnswer = (Button) findViewById(R.id.getAnswerButton);
    getAnswer.setOnClickListener(getAnswerListener);
}
private View.OnClickListener getAnswerListener = new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Random randomnum = new Random();
        int numToast = randomnum.nextInt(19);
        numToast++;

        CharSequence text = "";
        switch (numToast) {
            case 1:
                text = getString(R.string.answer1);
                break;
            case 2:
                text = getString(R.string.answer2);
                break;
            case 3:
                text = getString(R.string.answer3);
                break;
            case 4:
                text = getString(R.string.answer4);
                break;
            case 5:
                text = getString(R.string.answer5);
                break;
            case 6:
                text = getString(R.string.answer6);
                break;
            case 7:
                text = getString(R.string.answer7);
                break;

        }
        Context context = getApplicationContext();
        int duration = Toast.LENGTH_LONG;
        Toast toast = Toast.makeText(context, text, duration);
        toast.show();
    }
};

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.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();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

我的错误是：

Unfortunately Application has stopped and  06-05
11:31:44.131  13711-13711/? I/art﹕ Not late-enabling -Xcheck:jni
(already on) 06-05 11:31:44.208  13711-13711/? D/AndroidRuntime﹕
Shutting down VM 06-05 11:31:44.208  13711-13711/? E/AndroidRuntime﹕
FATAL EXCEPTION: main
    Process: viewbrosers.mehdi.home.mytestappviewbrowser, PID: 13711
    java.lang.RuntimeException: Unable to start activity ComponentInfo{viewbrosers.mehdi.home.mytestappviewbrowser/viewbrosers.mehdi.home.mytestappviewbrowser.MainActivity}:
java.lang.NullPointerException: Attempt to invoke virtual method
'boolean java.lang.String.equals(java.lang.Object)' on a null object
reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on
a null object reference
            at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:715)
            at android.view.LayoutInflater.rInflate(LayoutInflater.java:806)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:504)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:414)
            at android.view.LayoutInflater.inflate(LayoutInflater.java:365)
            at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:378)
            at android.app.Activity.setContentView(Activity.java:2145)
            at viewbrosers.mehdi.home.mytestappviewbrowser.MainActivity.onCreate(MainActivity.java:23)
            at android.app.Activity.performCreate(Activity.java:5990)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 06-05
11:31:44.218  13711-13718/? I/art﹕ Debugger is no longer active 06-05
11:32:28.690  13711-13718/viewbrosers.mehdi.home.mytestappviewbrowser
W/art﹕ Suspending all threads took: 12.748ms

Answer 1

将text变量的创建和初始化移到onClick（）方法之外。请尝试以下代码。

public class MainActivity extends Activity {

CharSequence text;

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

    text = "";

    Button getAnswer = (Button) findViewById(R.id.getAnswerButton);
    getAnswer.setOnClickListener(getAnswerListener);
}
private View.OnClickListener getAnswerListener = new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Random randomnum = new Random();
        int numToast = randomnum.nextInt(19);
        numToast++;

        switch (numToast) {
            case 1:
                text = getString(R.string.answer1);
                break;
            case 2:
                text = getString(R.string.answer2);
                break;
            case 3:
                text = getString(R.string.answer3);
                break;
            case 4:
                text = getString(R.string.answer4);
                break;
            case 5:
                text = getString(R.string.answer5);
                break;
            case 6:
                text = getString(R.string.answer6);
                break;
            case 7:
                text = getString(R.string.answer7);
                break;

        }
        Context context = getApplicationContext();
        int duration = Toast.LENGTH_LONG;
        Toast toast = Toast.makeText(context, text, duration);
        toast.show();
    }
};

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.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();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

Answer 2

您使用equals string上的null函数，'boolean java.lang.String.equals(java.lang.Object)' on a null object reference。查看例外：{{1}}

Answer 3

尝试一下，可能就是这样。

public class MainActivity extends Activity implements OnClickListener
{

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

    Button getAnswer = (Button) findViewById(R.id.getAnswerButton);
    getAnswer.setOnClickListener(this);
}

@Override
    public void onClick(View v) {

        switch (v.getId()) 
        {
case R.id.getAnswerButton:

            //Your code

            break;

           }

}
}

我在android studio中有简单的Android应用程序，它不起作用

3 个答案: