Why I get NullPointerException?

Question

I keep getting NullPointerException in the following code. The exception appears at this line System.out.println(myHeros.toString());

public class test {

public static void main(String[] args) {

    Hero A = new Hero("Archer");
    Hero B = new Hero("Bacca");
    Hero C = new Hero("Chester");
    Hero D = new Hero("Teemo");
    Hero E = new Hero("Garen");

    MyStack myHeros = new MyStack(10);

    System.out.println(!(A instanceof Hero)); // Output: false;

    boolean debug = true;
    try {

        myHeros.push(A);
        if(debug) {
            System.out.println("lalala");
        }
    }
    catch(notAHeroException e) {
        //e.printStackTrace();
    }

    System.out.println(myHeros.toString());   //This is the line that throws nullPointerException;

    System.out.println(myHeros.getTop()); //(Debug) Output: 1, the Hero was actually pushed into myHeroStack;           
}

}

This is the class containing the toString method. I really cannot see where possibly can cause a NullPointerException.

public class MyStack implements Stack{

public Hero[] myHeroStack; //instance field; can be from this class to the whole world;
int top;                   //The combination of these instance fields represent all you know about your object and all you can use about your object;

public MyStack() {
    myHeroStack = new Hero[5];
    top = 0;

}

public MyStack(int numOfHero) {
    myHeroStack = new Hero[numOfHero];  
    top = 0;
}

@Override
public void push(Hero h) throws notAHeroException{
    //check the input;
    boolean checkType = !(h instanceof Hero);
    if (checkType) {        //if (!(h instanceof Hero))
        throw new notAHeroException("Check again what did you add?!");
    }

    else {
        myHeroStack[top] = h;
        top = top + 1;
    }

}

@Override
public void pull(Hero g) throws emptyStackException { //check whether the element you want to throw actually exist;
    //check whether the element exist
    if(myHeroStack[top - 1] == null) {
        throw new emptyStackException("You do have this hero in you stack!");

    }
    else {
        Hero heroToPull = myHeroStack[top - 1];
        System.out.println(heroToPull.getName());
        myHeroStack[top - 1] = null;
        top = top -1;

    }

}

@Override
public void peek(Hero p) {
    System.out.println(p.getName());
}

public int getTop() {
    return top;
}

public String toString() {
    //check whether the stack is null
    if(top == 0) {
        return ("You hero stack is empty");
    }
    else {
        String AllMyHero = "";
        for(Hero hero: myHeroStack) {           
            AllMyHero = AllMyHero + "All my hero is " + hero.getName()+ "%n"; //hero is null point
        }
        return AllMyHero;
    }


}

}

This is the Hero class, which is quite simple.

public class Hero {

    private String name;

    public Hero(String name) {
        this.name = name;
    }
}

Thank you for all who takes a look at this question!

Answer 1

You are initializing myHeroStack by doing myHeroStack = new Hero[5]; which basically means to prepare 5 places for 5 Hero objects. This will be the look of your stack at initialization.

myHeroStack :

null 
null
null
null
null

Then in your main you added Hero A = new Hero("Archer"); by doing this myHeros.push(A); thus the look of your stack will be.

myHeroStack :

A (The Hero you added)
null
null
null
null

Now in your print method you used for(Hero hero: myHeroStack) { which basically means to iterate each element of your stack. Thus when the second iteration occurs the hero in your loop will be null. That is why NullPointerException occurs.

You should update your print method to include a checking for null values. Refer to the code below.

public String toString() {
    //check whether the stack is null
    if(top == 0) {
        return ("You hero stack is empty");
    }
    else {
        String AllMyHero = "";
        for(Hero hero: myHeroStack) {  
            if (hero != null) {
                AllMyHero = AllMyHero + "All my hero is " + hero.getName()+ "%n"; //hero is null point
            }         
        }    
    }
        return AllMyHero;
}

Answer 2

You can change the for loop in your toString() method to the code below so it won't print the null values in your array:

for (int i = 0; i < top; i++) {
    // ...
}