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- What is a NullPointerException, and how do I fix it? 12 answers
什么是NullPointerException,我该如何解决? 12个答案
I hope it isn't too much trouble if I just post all the code I have. If anyone would be willing to take a look at it, that would be awesome. Sorry that I'm kind of clueless.. :/ I tried the suggestions from the answers but to be honest, I don't think I did it right... so... maybe I'm just making a simple mistake.
如果我发布所有代码,我希望不是太麻烦。如果有人愿意看一眼,那就太棒了。对不起,我有点无能......:/我尝试了答案中的建议,但说实话,我认为我做得不对......所以...也许我只是犯了一个简单的错误。
public class Enemy {
/**
* This field
*/
private Gun gun;
/**
* This field holds the amount of hitpoints the enemy has, which is 5.
*/
private int hitPoints;
GameEngine ge = new GameEngine();
/**
* This method determines whether the enemy shot hit or not.
*/
public boolean enemyShotSuccess(){
return gun.shoot(ge.enemySpawn());
}
/**
* This method determines what the enemy drops if it dies.
*/
public String Item_Drop(){
Random rdm_Numbers = new Random();
int Random_Item = rdm_Numbers.nextInt(100);
String Dropped_Item = null;
if (Random_Item >= 0 && Random_Item < 30)
Dropped_Item = "Health";
if (Random_Item >= 30 && Random_Item < 100)
Dropped_Item = "Ammo";
return Dropped_Item;
}
/**
* This is the default constructor that holds the 5 hitpoints of the enemy, as well as the gun it spawns with.
*/
public Enemy(Gun chosenGun){
gun = chosenGun;
hitPoints = 5;
}
/**
* When the enemy is hit, this method updates the hit points.
*/
public int dmgTaken(int dmg){
hitPoints -= dmg;
return hitPoints;
}
/**
* This is a getter for the remaining hp of the enemy.
*/
public int getHP(){
return hitPoints;
}
/**
* This method calls the Item_Drop method when the enemy dies.
*/
public boolean die(){
this.Item_Drop();
return true;
}
}
next class
public class GameEngine {
/**
* These are the 3 enemies that can appear, stored in variables.
*/
public static enum OPPONENT_LEVEL {THUG, SPECIALIST, BERSERKER};
private Player player;
private Enemy enemy;
/**
* This method creates a new character and lets the player choose a gun.
* @param playerGun
* @param playerHP
* @param enemyGun
* @param enemyHP
*/
public Player NewGame(Gun playerGun){
return player = new Player(playerGun);
}
/**
* This method determines whether the player was successful in shooting or not.
*/
public boolean playerTurn(Gun playerGun){
player.playerShotSuccess(playerGun);
return player.playerShotSuccess(playerGun);
}
/**
* This method holds the chance for the player to escape.
*/
public boolean attemptEscape(){
Random random = new Random();
boolean success;
int escape = random.nextInt(100);
if (escape < 10)
success = true;
else success = false;
return success;
}
/**
* This method gets the amount of ammo left in the clip.
* @return
*/
public int ammoRemaining(){
return player.ammoRemaining();
}
/**
* This method determines the type of enemy that spawns, and gives it its respective weapon.
* @return
*/
public Gun enemySpawn(){
Random random = new Random();
OPPONENT_LEVEL ol = null;
int enemySpawned = random.nextInt(100);
Gun enemyGun = null;
if (enemySpawned < 50){
enemyGun = new Gun(75, 1, 15);
Enemy enemy = new Enemy(enemyGun);
OPPONENT_LEVEL enemyName = ol.THUG;
System.out.println("A " + enemyName + " has spawned! He has a pistol and 5 hp.");
}
if (enemySpawned >= 50 && enemySpawned < 85){
enemyGun = new Gun(65, 2, 10);
Enemy enemy = new Enemy(enemyGun);
OPPONENT_LEVEL enemyName = ol.SPECIALIST;
System.out.println("A " + enemyName + " has spawned! He has a rifle and 5 hp.");
}
if (enemySpawned >= 85 && enemySpawned < 100){
enemyGun = new Gun(40, 5, 5);
Enemy enemy = new Enemy(enemyGun);
OPPONENT_LEVEL enemyName = ol.BERSERKER;
System.out.println("A " + enemyName + " has spawned! He has a shotgun and 5 hp.");
}
return enemyGun;
}
/**
* This method holds the remaining hp of the enemy.
* @return
*/
public int enemyRemainingHP(){
return enemy.dmgTaken(enemy.getHP());
}
}
next class
public class Gun {
/**
* This method will return true if, when shot, the shot is a hit.
* If it is a miss, it will return false.
*/
public boolean shoot(Gun playerGun){
boolean hit = false;
Random Random_Numbers = new Random();
int chanceOfShooting = Random_Numbers.nextInt(100);
if (chanceOfShooting < accuracy){
hit = true;
}
return hit;
}
/**
* This field holds the the maximum amount of bullets one clip can hold.
*/
private int clipSize;
/**
* This field holds the accuracy of the gun.
*/
private int accuracy;
/**
* This field holds the damage of the gun.
*/
private int damage;
/**
* This field holds the amount of remaining bullets in the clip.
*/
private int ammoRemaining;
/**
* This field gets the amount of bullets remaining in the clip.
* @return
*/
public int getAmmoRemaining(){
ammoRemaining = clipSize - 1;
return ammoRemaining;
}
/**
* This is the default constructor for the Gun, holding accuracy, damage and size of the clip.
*/
public Gun(int gunAccuracy, int gunDMG, int clip){
accuracy = gunAccuracy;
damage = gunDMG;
clipSize = clip;
}
/**
* This method gets the damage of the gun.
*/
public int getDMG(){
return damage;
}
}
next class
public class Player {
private Gun gun;
/**
* This method determines whether the player's shot was a hit or miss.
*/
public boolean playerShotSuccess(Gun playerGun){
gun.shoot(playerGun);
return gun.shoot(playerGun);
}
/**
* This field holds the amount of hitpoints that the player spawns with.
*/
private int hitPoints;
/**
* This is the default constructor which says that the player has 20 hitpoints, and
* also displays the player's gun choice.
*/
public Player(Gun chosenGun){
hitPoints = 20;
chosenGun = gun;
}
/**
* This method updates the player's hp with the damage received by the enemy.
*/
public int dmgTaken(int dmg){
hitPoints -= dmg;
return hitPoints;
}
/**
* This method gets the damage of the gun the player is currently using.
* @return
*/
public int getDMG(){
return gun.getDMG();
}
/**
* This field updates the amount of ammo remaining whenever a shot is fired.
* @return
*/
public int ammoRemaining(){
return gun.getAmmoRemaining();
}
}
next class
public class UI {
public static void main(String[] args){
UI ui = new UI();
ui.startGame();
}
/**
*This field is for user input
*/
private Scanner keyboard;
GameEngine ge = new GameEngine();
/**
* This method executes the game by calling all the associated methods.
*/
public void startGame(){
keyboard = new Scanner(System.in);
welcomeMessage();
Gun playerGun = weaponChoice(welcomeMessage());
ge.NewGame(playerGun);
takeStep(playerGun, ge.NewGame(playerGun));
keyboard.nextLine();
}
/**
* This method prints the welcoming message to the player.
*/
public int welcomeMessage(){
System.out.println("Welcome to escape the dungeon! You are 10 steps away from the exit.");
System.out.println("Be wary of enemies! \nThe pistol has 15 bullets, 75% accuracy, and does one damage. "
+ "\nThe rifle has 10 bullets, 65% accuracy, and does 2 damage. "
+ "\nThe shotgun has 5 bullets, 40% accuracy, and does 5 damage.");
System.out.println("Choose your weapon. \nPress 1 for pistol. "
+ "\nPress 2 for rifle. "
+ "\nPress 3 for shotgun.");
Scanner keyboard = new Scanner(System.in);
int weaponChoice = keyboard.nextInt();
return weaponChoice;
}
/**
* This method allows the player to choose his or her weapon.
*/
public Gun weaponChoice(int weaponChooser){
Gun playerGun = new Gun(0, 0, 0);
switch (weaponChooser){
case 1:
playerGun = new Gun(75, 1, 15);
System.out.println("You have selected the pistol!");
break;
case 2:
playerGun = new Gun(65, 2, 10);
System.out.println("You have selected the rifle!");
break;
case 3:
playerGun = new Gun(40, 5, 5);
System.out.println("You have selected the shotgun!");
break;
default:
playerGun = new Gun(0, 0, 0);
}
System.out.println(playerGun);
return playerGun;
}
/**
* This is the main loop of the game. The player advances by pressing enter and, if met with an enemy, engages or attemps an escape.
* @return
*/
public void takeStep(Gun playerGun, Player player){
for (int oneStep = 1; oneStep <= 10; oneStep++){
randomEncounter();
if (randomEncounter()){
Gun enemyGun = ge.enemySpawn();
System.out.println("Press 1 to attack. \nPress 2 to attempt an escape and move back one step.");
do {
int Decision = keyboard.nextInt();
if (Decision ==1){
ge.playerTurn(playerGun);
System.out.println("You have " + ge.ammoRemaining() + " bullets left.");
}
if (Decision ==2){
ge.attemptEscape();
if (ge.attemptEscape()){
oneStep -= 2;
System.out.println("You have succesfully escaped! You have moved back one step.");
}
else System.out.println("Sorry, you failed to escape and lost your turn.");
}
} while (ge.enemyRemainingHP() > 0);
}
System.out.println("You have taken one step! Only " + (10 - oneStep) + " more to go!");
keyboard.nextLine();
System.out.println("Press enter to take another step.");
keyboard.nextLine();
}
}
/**
* This method holds the random encounter. There is a 15% chance to encounter an enemy.
*/
public boolean randomEncounter(){
Random random = new Random();
int randomEncounter = random.nextInt(100);
if (randomEncounter < 15)
return true;
else return false;
}
}
and those are all of them. I do have a main to run the program.
这些都是他们。我有一个主程序来运行该程序。
1 个解决方案
#1
The stacktrace tells that the NPE occurs in player.playerShotSuccess(playerGun) So, player is null. Debug to see if weaponChoice() is returning a valid Gun object. Put a logger/system.out after the Gun playerGun = weaponChoice() line to see the initialized Gun object.
堆栈跟踪告诉NPE发生在player.playerShotSuccess(playerGun)中,因此,播放器为空。调试以查看weaponChoice()是否返回有效的Gun对象。在Gun playerGun = weaponChoice()行之后放置一个logger / system.out来查看初始化的Gun对象。
#1
The stacktrace tells that the NPE occurs in player.playerShotSuccess(playerGun) So, player is null. Debug to see if weaponChoice() is returning a valid Gun object. Put a logger/system.out after the Gun playerGun = weaponChoice() line to see the initialized Gun object.
堆栈跟踪告诉NPE发生在player.playerShotSuccess(playerGun)中,因此,播放器为空。调试以查看weaponChoice()是否返回有效的Gun对象。在Gun playerGun = weaponChoice()行之后放置一个logger / system.out来查看初始化的Gun对象。