为什么我不能在同一个 class 的受保护方法中调用方法
Why can't i call a method within a protected method of the same class
在 MainActivity 中 class 我有两种方法 - 一种 public 另一种 protected.I 正在从受保护的 method.I 调用 public 方法 运行 应用程序和应用程序 crashes.If 我 cut/paste 受保护方法中的代码一切正常 fine.One 可能性 - 它没有被调用 properly.Why 它崩溃了吗？
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 ComponentInfo{com.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)
您正在 onCreate 方法中创建 button1、button2 等局部变量。这样，generateQuestion 方法不知道这些变量并使用具有相同名称的 class 变量（不包含在您的代码中，但我想您可能在某处声明了它们可能在您的 activity class) 没有初始化，因此你得到 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)));
}
并从 onCreate 调用它，如下所示：
@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。
在 MainActivity 中 class 我有两种方法 - 一种 public 另一种 protected.I 正在从受保护的 method.I 调用 public 方法 运行 应用程序和应用程序 crashes.If 我 cut/paste 受保护方法中的代码一切正常 fine.One 可能性 - 它没有被调用 properly.Why 它崩溃了吗？
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 ComponentInfo{com.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)
您正在 onCreate 方法中创建 button1、button2 等局部变量。这样，generateQuestion 方法不知道这些变量并使用具有相同名称的 class 变量（不包含在您的代码中，但我想您可能在某处声明了它们可能在您的 activity class) 没有初始化，因此你得到 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)));
}
并从 onCreate 调用它，如下所示：
@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。