尝试在 android studio 中打印随机值时出现 NullPointerException
NullPointerException when trying to print random values in android studio
我在第 99 行和第 77 行不断收到空指针异常，这是我尝试打印由随机 class（第 99 行）创建的随机值的地方，也是我调用我的 setQuestion 方法（第 77 行。randomNum 到 layout_game xml 文件。我不确定为什么我得到一个空值，因为当我将这些值记录到控制台时，它说正在生成值。
package com.grantsolutions.mathgamechapter1;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import java.util.Random;
public class GameActivity extends Activity implements OnClickListener {
int correctAnswer;
Button buttonObjectChoice1;
Button buttonObjectChoice2;
Button buttonObjectChoice3;
TextView textObjectPartA //it says that textObjectPartA and B are never used;
TextView textObjectPartB;
TextView textObjectScore;
TextView textObjectLevel;
int currentScore;
int currentLevel = 1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layout_game);
//declaring variables for game
//no longer being declared
/*int partA = 2;
int partB = 2;
correctAnswer = partA * partB;
int wrongAnswer1 = correctAnswer - 1;
int wrongAnswer2 = correctAnswer + 1;
/*this is where we link the variables
from the java code to the button UI in
activity_game xml itself
*/
TextView textObjectPartA = (TextView) findViewById(R.id.holdTextA);
TextView textObjectPartB = (TextView) findViewById(R.id.holdTextB);
textObjectScore = (TextView) findViewById(R.id.textLevel);
textObjectLevel = (TextView) findViewById(R.id.textScore);
buttonObjectChoice1 =
(Button) findViewById(R.id.choice1);
buttonObjectChoice2 =
(Button) findViewById(R.id.choice2);
buttonObjectChoice3 =
(Button) findViewById(R.id.choice3);
/*Use the setText method of the class on our objects
to show our variable values on the UI elements.
Similar to outputting to the console in the exercise,
only using the setText method*/
//telling the console to listen for button clicks from the user
buttonObjectChoice1.setOnClickListener(this);
buttonObjectChoice2.setOnClickListener(this);
buttonObjectChoice3.setOnClickListener(this);
setQuestion();
}
void setQuestion() {
Random randomNum;
randomNum = new Random();
int partA = 0;
int numberRange = currentLevel * 3;
partA = randomNum.nextInt(numberRange) + 1;
//removes possibility of a zero value
int partB = randomNum.nextInt(numberRange) + 1;
correctAnswer = partA * partB;
Log.i("info", ""+partA);
Log.i("info", ""+partB);
Log.d("info", ""+partA);
int wrongAnswer1 = correctAnswer - 2;
int wrongAnswer2 = correctAnswer + 2;
Log.i("info", ""+partA);
Log.i("info", ""+partB);
textObjectPartA.setText(""+partA); //error here
textObjectPartB.setText(""+partB);
//case for setting multiple choice buttons
int buttonLayout = randomNum.nextInt(3);
switch (buttonLayout) {
case 0:
buttonObjectChoice1.setText("" + correctAnswer);
buttonObjectChoice2.setText("" + wrongAnswer1);
buttonObjectChoice3.setText("" + wrongAnswer2);
break;
case 1:
buttonObjectChoice2.setText("" + correctAnswer);
buttonObjectChoice1.setText("" + wrongAnswer1);
buttonObjectChoice3.setText("" + wrongAnswer2);
break;
case 2:
buttonObjectChoice3.setText("" + correctAnswer);
buttonObjectChoice1.setText("" + wrongAnswer1);
buttonObjectChoice2.setText("" + wrongAnswer2);
break;
}
}
void updateScoreAndLevel(int answerGiven) {
if (isCorrect(answerGiven)) {
for (int i = 1; i <= currentLevel; i++) {
currentScore = currentScore + i;
}
} else {
currentLevel = 1;
currentScore = 0;
}
currentLevel++;
}
boolean isCorrect(int answerGiven) {
boolean correctTrueOrFalse = true;
if (answerGiven == correctAnswer) {//YAY!
Toast.makeText(getApplicationContext(), "Well Done", Toast.LENGTH_LONG).show();
}
else {//If wrong print this instead
Toast.makeText(getApplicationContext(), "You're wrong", Toast.LENGTH_SHORT).show();
} //case for each button being made
correctTrueOrFalse = false;
return correctTrueOrFalse;
}
@Override
public void onClick (View view){
//declaring a new int to use in all cases
int answerGiven;
switch (view.getId()) {
//case for each button being made
case R.id.choice1:
//button1 information goes here
//initialize a new integer with the value
//contained in buttonObjectChoice1
answerGiven = Integer.parseInt("" + buttonObjectChoice1.getText());
//check if the right answer
break;
case R.id.choice2:
//button2 information goes here
answerGiven = Integer.parseInt("" + buttonObjectChoice2.getText());
break;
case R.id.choice3:
//button 3 information goes here
answerGiven = Integer.parseInt("" + buttonObjectChoice3.getText());
break;
}
}
}
此处为第 99 行和第 77 行的代码片段
第 77 行
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layout_game);
TextView textObjectPartA = (TextView) findViewById(R.id.holdTextA);
TextView textObjectPartB = (TextView) findViewById(R.id.holdTextB);
textObjectScore = (TextView) findViewById(R.id.textLevel);
textObjectLevel = (TextView) findViewById(R.id.textScore);
buttonObjectChoice1 =
(Button) findViewById(R.id.choice1);
buttonObjectChoice2 =
(Button) findViewById(R.id.choice2);
buttonObjectChoice3 =
(Button) findViewById(R.id.choice3);
/*Use the setText method of the class on our objects
to show our variable values on the UI elements.
Similar to outputting to the console in the exercise,
only using the setText method*/
//telling the console to listen for button clicks from the user
buttonObjectChoice1.setOnClickListener(this);
buttonObjectChoice2.setOnClickListener(this);
buttonObjectChoice3.setOnClickListener(this);
setQuestion(); //code error shown in this line
}
第 99 行
Log.i("info", ""+partA);
Log.i("info", ""+partB);
textObjectPartA.setText(""+partA); //error here
textObjectPartB.setText(""+partB);
layout_game 文件
<?xml version="1.0" encoding="utf-8"?>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="2"
android:id="@+id/holdTextA"
android:layout_marginTop="44dp"
android:textSize="50sp"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginLeft="44dp"
android:layout_marginStart="44dp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="X"
android:id="@+id/textView4"
android:layout_alignTop="@+id/holdTextA"
android:layout_centerHorizontal="true"
android:textSize="50sp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="4"
android:id="@+id/holdTextB"
android:textSize="50sp"
android:layout_above="@+id/textView6"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_marginRight="53dp"
android:layout_marginEnd="53dp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="="
android:id="@+id/textView6"
android:layout_marginTop="118dp"
android:layout_below="@+id/textView4"
android:layout_alignLeft="@+id/textView4"
android:layout_alignStart="@+id/textView4"
android:textSize="60sp" />
<Button
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/choice1"
android:text="2"
android:textSize="40sp"
android:layout_alignParentBottom="true"
android:layout_alignRight="@+id/holdTextA"
android:layout_alignEnd="@+id/holdTextA"
android:layout_marginBottom="37dp" />
<Button
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="4"
android:id="@+id/choice2"
android:layout_alignBottom="@+id/choice1"
android:layout_alignLeft="@+id/holdTextB"
android:layout_alignStart="@+id/holdTextB"
android:textSize="40sp" />
<Button
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="3"
android:id="@+id/choice3"
android:layout_alignBottom="@+id/choice1"
android:layout_centerHorizontal="true"
android:layout_alignTop="@+id/choice1"
android:textSize="40sp" />
在你的
onCreateView()
您已声明
TextView textObjectPartA = (TextView) findViewById(R.id.holdTextA);
这样你的全局变量 textObjectPartA 还没有设置，因此空点异常。
我在第 99 行和第 77 行不断收到空指针异常，这是我尝试打印由随机 class（第 99 行）创建的随机值的地方，也是我调用我的 setQuestion 方法（第 77 行。randomNum 到 layout_game xml 文件。我不确定为什么我得到一个空值，因为当我将这些值记录到控制台时，它说正在生成值。
package com.grantsolutions.mathgamechapter1;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import java.util.Random;
public class GameActivity extends Activity implements OnClickListener {
int correctAnswer;
Button buttonObjectChoice1;
Button buttonObjectChoice2;
Button buttonObjectChoice3;
TextView textObjectPartA //it says that textObjectPartA and B are never used;
TextView textObjectPartB;
TextView textObjectScore;
TextView textObjectLevel;
int currentScore;
int currentLevel = 1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layout_game);
//declaring variables for game
//no longer being declared
/*int partA = 2;
int partB = 2;
correctAnswer = partA * partB;
int wrongAnswer1 = correctAnswer - 1;
int wrongAnswer2 = correctAnswer + 1;
/*this is where we link the variables
from the java code to the button UI in
activity_game xml itself
*/
TextView textObjectPartA = (TextView) findViewById(R.id.holdTextA);
TextView textObjectPartB = (TextView) findViewById(R.id.holdTextB);
textObjectScore = (TextView) findViewById(R.id.textLevel);
textObjectLevel = (TextView) findViewById(R.id.textScore);
buttonObjectChoice1 =
(Button) findViewById(R.id.choice1);
buttonObjectChoice2 =
(Button) findViewById(R.id.choice2);
buttonObjectChoice3 =
(Button) findViewById(R.id.choice3);
/*Use the setText method of the class on our objects
to show our variable values on the UI elements.
Similar to outputting to the console in the exercise,
only using the setText method*/
//telling the console to listen for button clicks from the user
buttonObjectChoice1.setOnClickListener(this);
buttonObjectChoice2.setOnClickListener(this);
buttonObjectChoice3.setOnClickListener(this);
setQuestion();
}
void setQuestion() {
Random randomNum;
randomNum = new Random();
int partA = 0;
int numberRange = currentLevel * 3;
partA = randomNum.nextInt(numberRange) + 1;
//removes possibility of a zero value
int partB = randomNum.nextInt(numberRange) + 1;
correctAnswer = partA * partB;
Log.i("info", ""+partA);
Log.i("info", ""+partB);
Log.d("info", ""+partA);
int wrongAnswer1 = correctAnswer - 2;
int wrongAnswer2 = correctAnswer + 2;
Log.i("info", ""+partA);
Log.i("info", ""+partB);
textObjectPartA.setText(""+partA); //error here
textObjectPartB.setText(""+partB);
//case for setting multiple choice buttons
int buttonLayout = randomNum.nextInt(3);
switch (buttonLayout) {
case 0:
buttonObjectChoice1.setText("" + correctAnswer);
buttonObjectChoice2.setText("" + wrongAnswer1);
buttonObjectChoice3.setText("" + wrongAnswer2);
break;
case 1:
buttonObjectChoice2.setText("" + correctAnswer);
buttonObjectChoice1.setText("" + wrongAnswer1);
buttonObjectChoice3.setText("" + wrongAnswer2);
break;
case 2:
buttonObjectChoice3.setText("" + correctAnswer);
buttonObjectChoice1.setText("" + wrongAnswer1);
buttonObjectChoice2.setText("" + wrongAnswer2);
break;
}
}
void updateScoreAndLevel(int answerGiven) {
if (isCorrect(answerGiven)) {
for (int i = 1; i <= currentLevel; i++) {
currentScore = currentScore + i;
}
} else {
currentLevel = 1;
currentScore = 0;
}
currentLevel++;
}
boolean isCorrect(int answerGiven) {
boolean correctTrueOrFalse = true;
if (answerGiven == correctAnswer) {//YAY!
Toast.makeText(getApplicationContext(), "Well Done", Toast.LENGTH_LONG).show();
}
else {//If wrong print this instead
Toast.makeText(getApplicationContext(), "You're wrong", Toast.LENGTH_SHORT).show();
} //case for each button being made
correctTrueOrFalse = false;
return correctTrueOrFalse;
}
@Override
public void onClick (View view){
//declaring a new int to use in all cases
int answerGiven;
switch (view.getId()) {
//case for each button being made
case R.id.choice1:
//button1 information goes here
//initialize a new integer with the value
//contained in buttonObjectChoice1
answerGiven = Integer.parseInt("" + buttonObjectChoice1.getText());
//check if the right answer
break;
case R.id.choice2:
//button2 information goes here
answerGiven = Integer.parseInt("" + buttonObjectChoice2.getText());
break;
case R.id.choice3:
//button 3 information goes here
answerGiven = Integer.parseInt("" + buttonObjectChoice3.getText());
break;
}
}
}
此处为第 99 行和第 77 行的代码片段
第 77 行
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layout_game);
TextView textObjectPartA = (TextView) findViewById(R.id.holdTextA);
TextView textObjectPartB = (TextView) findViewById(R.id.holdTextB);
textObjectScore = (TextView) findViewById(R.id.textLevel);
textObjectLevel = (TextView) findViewById(R.id.textScore);
buttonObjectChoice1 =
(Button) findViewById(R.id.choice1);
buttonObjectChoice2 =
(Button) findViewById(R.id.choice2);
buttonObjectChoice3 =
(Button) findViewById(R.id.choice3);
/*Use the setText method of the class on our objects
to show our variable values on the UI elements.
Similar to outputting to the console in the exercise,
only using the setText method*/
//telling the console to listen for button clicks from the user
buttonObjectChoice1.setOnClickListener(this);
buttonObjectChoice2.setOnClickListener(this);
buttonObjectChoice3.setOnClickListener(this);
setQuestion(); //code error shown in this line
}
第 99 行
Log.i("info", ""+partA);
Log.i("info", ""+partB);
textObjectPartA.setText(""+partA); //error here
textObjectPartB.setText(""+partB);
layout_game 文件
<?xml version="1.0" encoding="utf-8"?>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="2"
android:id="@+id/holdTextA"
android:layout_marginTop="44dp"
android:textSize="50sp"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginLeft="44dp"
android:layout_marginStart="44dp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="X"
android:id="@+id/textView4"
android:layout_alignTop="@+id/holdTextA"
android:layout_centerHorizontal="true"
android:textSize="50sp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="4"
android:id="@+id/holdTextB"
android:textSize="50sp"
android:layout_above="@+id/textView6"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_marginRight="53dp"
android:layout_marginEnd="53dp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="="
android:id="@+id/textView6"
android:layout_marginTop="118dp"
android:layout_below="@+id/textView4"
android:layout_alignLeft="@+id/textView4"
android:layout_alignStart="@+id/textView4"
android:textSize="60sp" />
<Button
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/choice1"
android:text="2"
android:textSize="40sp"
android:layout_alignParentBottom="true"
android:layout_alignRight="@+id/holdTextA"
android:layout_alignEnd="@+id/holdTextA"
android:layout_marginBottom="37dp" />
<Button
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="4"
android:id="@+id/choice2"
android:layout_alignBottom="@+id/choice1"
android:layout_alignLeft="@+id/holdTextB"
android:layout_alignStart="@+id/holdTextB"
android:textSize="40sp" />
<Button
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="3"
android:id="@+id/choice3"
android:layout_alignBottom="@+id/choice1"
android:layout_centerHorizontal="true"
android:layout_alignTop="@+id/choice1"
android:textSize="40sp" />
在你的
onCreateView()
您已声明
TextView textObjectPartA = (TextView) findViewById(R.id.holdTextA);
这样你的全局变量 textObjectPartA 还没有设置，因此空点异常。