为啥我不能在同一类的受保护方法中调用方法[重复]
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【中文标题】为啥我不能在同一类的受保护方法中调用方法[重复]【英文标题】：Why can't i call a method within a protected method of the same class [duplicate]为什么我不能在同一类的受保护方法中调用方法[重复] 【发布时间】：2018-04-06 11:33:05 【问题描述】：在 MainActivity 类中，我有两种方法 - 一种是公共的，另一种是受保护的。我从受保护的方法调用公共方法。我正在运行应用程序并且应用程序崩溃。如果我在受保护的方法中剪切/粘贴代码一切正常。一种可能性 - 它没有被正确调用。为什么会崩溃？
代码---
public void generateQuestion()
Random rand = new Random();
int a = rand.nextInt(21);
int b = rand.nextInt(21);
question.setText(Integer.toString(a) + "+" + Integer.toString(b));
int locationOfCorrectAnswer = rand.nextInt(4);
int incorrectAnswer;
for (int i = 0; i < 4; i++)
if (i == locationOfCorrectAnswer)
answers.add(a + b);
else
incorrectAnswer = rand.nextInt(41);
while (incorrectAnswer == a + b)
incorrectAnswer = rand.nextInt(41);
answers.add(incorrectAnswer);
button1.setText(Integer.toString(answers.get(0)));
button2.setText(Integer.toString(answers.get(1)));
button3.setText(Integer.toString(answers.get(2)));
button4.setText(Integer.toString(answers.get(3)));
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
goButton = (Button) findViewById(R.id.goButton);
question = (TextView) findViewById(R.id.question);
pagetimer = (TextView) findViewById(R.id.pagetimer);
noIndiacator = (TextView) findViewById(R.id.noIndiacator);
resulttext = (TextView) findViewById(R.id.noIndiacator);
Button button1 = (Button) findViewById(R.id.button1);
Button button2 = (Button) findViewById(R.id.button2);
Button button3 = (Button) findViewById(R.id.button3);
Button button4 = (Button) findViewById(R.id.button4);
generateQuestion();
LogCat---
10-25 17:17:57.688 13344-13344/? I/zygote: Not late-enabling -Xcheck:jni (already on)
10-25 17:17:57.699 13344-13344/? W/zygote: Unexpected CPU variant for X86 using defaults: x86
10-25 17:17:58.277 13344-13344/com.example.saurabhdutta.brainteser I/InstantRun: starting instant run server: is main process
10-25 17:17:58.650 13344-13355/com.example.saurabhdutta.brainteser I/zygote: Background concurrent copying GC freed 3526(1172KB) AllocSpace objects, 0(0B) LOS objects, 68% free, 693KB/2MB, paused 1.023ms total 141.970ms
10-25 17:17:58.739 13344-13344/com.example.saurabhdutta.brainteser D/androidRuntime: Shutting down VM
--------- beginning of crash
10-25 17:17:58.741 13344-13344/com.example.saurabhdutta.brainteser E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.saurabhdutta.brainteser, PID: 13344
java.lang.RuntimeException: Unable to start activity ComponentInfocom.example.saurabhdutta.brainteser/com.example.saurabhdutta.brainteser.MainActivity: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setText(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setText(java.lang.CharSequence)' on a null object reference
at com.example.saurabhdutta.brainteser.MainActivity.generateQuestion(MainActivity.java:50)
at com.example.saurabhdutta.brainteser.MainActivity.onCreate(MainActivity.java:95)
at android.app.Activity.performCreate(Activity.java:6975)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
【问题讨论】：
你需要查看堆栈跟踪以了解崩溃并学习使用调试器 请分享logcat 分享您的代码，包括这两种方法（公共的和受保护的）以及您的 logcat 错误 发布您的代码，不是屏幕截图。发布你的 logcat。 正如它在 logcat 中所说，question 为空。
【参考方案1】：
您正在onCreate 方法中创建button1、button2 等的局部变量。这样，generateQuestion 方法不知道这些变量并使用具有相同名称的类变量（未包含在您的代码中，但我想您可能在某处声明它们可能在您的活动类之上）因此未初始化你得到NullPointerException。
尝试将您的generateQuestion 更改为：
public void generateQuestion(Button button1, Button button2, Button button3, Button button4)
Random rand = new Random();
int a = rand.nextInt(21);
int b = rand.nextInt(21);
question.setText(Integer.toString(a) + "+" + Integer.toString(b));
int locationOfCorrectAnswer = rand.nextInt(4);
int incorrectAnswer;
for (int i = 0; i < 4; i++)
if (i == locationOfCorrectAnswer)
answers.add(a + b);
else
incorrectAnswer = rand.nextInt(41);
while (incorrectAnswer == a + b)
incorrectAnswer = rand.nextInt(41);
answers.add(incorrectAnswer);
button1.setText(Integer.toString(answers.get(0)));
button2.setText(Integer.toString(answers.get(1)));
button3.setText(Integer.toString(answers.get(2)));
button4.setText(Integer.toString(answers.get(3)));
并像这样从onCreate 调用它：
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
goButton = (Button) findViewById(R.id.goButton);
question = (TextView) findViewById(R.id.question);
pagetimer = (TextView) findViewById(R.id.pagetimer);
noIndiacator = (TextView) findViewById(R.id.noIndiacator);
resulttext = (TextView) findViewById(R.id.noIndiacator);
Button button1 = (Button) findViewById(R.id.button1);
Button button2 = (Button) findViewById(R.id.button2);
Button button3 = (Button) findViewById(R.id.button3);
Button button4 = (Button) findViewById(R.id.button4);
generateQuestion(button1, button2, button3, button4);
这样您将在generateQuestion 方法中传递局部变量。
【讨论】：
谢谢 :)以上是关于为啥我不能在同一类的受保护方法中调用方法[重复]的主要内容，如果未能解决你的问题，请参考以下文章
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