HELP! Java Programming.....

[Deleted User]

Hi all, hoping someone can help a java noob here.

Learning Java in my college course, but I've come across a little snag.

I have to create a little app to draw some orbs on a display, but I always get "NullPointerException" error when I enter a value.

I understand that it's because I'm not initialising my target integer, but I cannot for the love of me understand WHY I'm not initialising it.

Anyone wanna point out to me what I'm doing and why?

Main

Spoiler

import javax.swing.*;

Action ac;

void setup(){
size(1920,1080);
noStroke();
frameRate(144);
fill(255); //background white (for now)
rect(0,0,1920,1080); //Right screen edge block
fill(0,120,200); //Edge data block colors, light blue
rect(0,0,550,1080); //Left data block
rect(1370,0,1920,1080); //Right data block
textSize(60);
text("BlitzBall v0.1", 95, 60);
int flotsome = Integer.parseInt(JOptionPane.showInputDialog("How many balls to spawn ", "2"));
ac.setAmount(flotsome);
}

void draw(){
ac.display();
print(ac.amount);
}

Action class

Spoiler

class Action{
private int amount;
int xLoc[];
int yLoc[];
//============
//Constructors
//============
Action(){
}

Action(int amount){
this.amount = amount;
print("Constructor check " + amount);
}

Action(int xLoc, int yLoc){
//Dummy for now
}

//=======
//Methods
//=======
void display(){
if(amount == 1){
ellipse(900,540,50,50);
}
else if(amount == 2){
ellipse(800,540,50,50);
ellipse(900,540,50,50);
}
else if(amount == 3){
ellipse(800,540,50,50);
ellipse(900,540,50,50);
ellipse(1000,540,50,50);
}
else{
}
}

//=======
//Setters
//=======
public void setAmount(int amount){
if((amount > 0) && (amount <= 3)){
this.amount = amount;
print("Setter Check " +amount);
}
}

/*
public void setLocs(int passX, int passY){
if((passX >= 0) && (passX <= 1920)){
//this.xLoc = passX;
}
else{
}
if((passY >= 0) && (passY <= 1080)){
//this.yLoc = passY;
}
else{
}
}
*/

//=======
//Getters
//=======
}

 

[Wuigi]

Is ac initialized when you do "setAmount(flotsome);"?

 

[Deleted User]

Is ac initialized when you do "setAmount(flotsome);"?

afaik, no. actually, when it tries to execute that line of code, is when I get the NullPointerException error.

 

[Wuigi]

@StarGazerTom Then maybe try ac = new Action(); in your setup method of you main class.

 

[Deleted User]

@StarGazerTom Then maybe try ac = new Action(); in your setup method of you main class.

Nope, get an error "The class ac does not exist"..... which it clearly does.

 

[KleinesSinchen]

Nope, get an error "The class ac does not exist"..... which it clearly does.

Action ac = new Action();

 

[Deleted User]

Action ac = new Action();

That got it! Now to try and understand WHY that worked.

Thanks for the help

 

[KleinesSinchen]

That got it! Now to try and understand WHY that worked.

Thanks for the help

Your class is named "Action"
You want a new Object. Object type: "Action" Name of the Object/ the variable: ac

Action ac;
Java uses references (similar to pointers). Where does it point now? Nowhere. We don't have any Object of the type "Action" yet. So we create a new one.
ac = new Action(); // make new object of type "Action" and use the constructor without arguments
All objects must be created. You could as well do this with any primitive datatype (although it makes no sense)
int i = new Integer(9); // int i=9;

 

[Deleted User]

Your class is named "Action"
You want a new Object. Object type: "Action" Name of the Object/ the variable: ac

Action ac;
Java uses references (similar to pointers). Where does it point now? Nowhere. We don't have any Object of the type "Action" yet. So we create a new one.
ac = new Action(); // make new object of type "Action" and use the constructor without arguments
All objects must be created. You could as well do this with any primitive datatype (although it makes no sense)
int i = new Integer(9); // int i=9;

Ahhhhh that explains it nice and clearly. Thanks mate, appreciate it, was tearing my hair out trying to figure it out for the past 2 days now.