CODEWITHSUNDEEP
javaclassnullpointerexception
Yuval Simon
Yuval Simon

Reputation: 335

solving NullPointerExceptions

I have been studying Java since last week (I have basic knowledge in C#) and now I am practicing classes. I don't understand why I get this exception, I tried to change the code many times and nothing works. This is the code:

package javaapplication1;

import java.util.Scanner;

public class JavaApplication1 {
    static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) {
        Competitor[] competitors;
        System.out.println("Enter number of competitors");
        competitors = Create(sc.nextInt());
        for(int i = 0;i<competitors.length - 1;i++)
            for(int j=i+1;j<competitors.length;j++)
                if(competitors[i].GetHeight()==competitors[j].GetHeight())
                    System.out.println(competitors[i]+" and "
                            +competitors[j] + "bounced to height"
                            +competitors[i].GetHeight());
    }
    static Competitor[] Create(int n) {
        Competitor[] competitors = new Competitor[n];
        for(Competitor c : competitors) {
            c = new Competitor();
            System.out.println("Enter name, id and bounce height");
            c.SetName(sc.next());
            c.SetId(sc.next());
            c.SetHeight(sc.nextFloat());
        }
        return competitors;
    }
}
class Competitor {
    private String name;
    private String id;
    private float height;
    public Competitor() {
    }
    public Competitor(String name, String id, float height) {
        this.name = name;
        this.id = id;
        this.height = height;
    }
    public String GetName() {
        return this.name;
    }
    public String GetId() {
        return this.id;
    }
    public Float GetHeight() {
        return this.height;
    }
    public void SetName(String name) {
        this.name = name;
    }
    public void SetId(String id) {
        this.id = id;
    }
    public void SetHeight(Float height) {
        this.height = height;
    }
}

I get the exception in this line: if(competitors[i].GetHeight()==competitors[j].GetHeight())

What is the problem here?

Thanks very much!

Upvotes: 1

Views: 118

Answers (3)

Doodad
Doodad

Reputation: 1518

You're doing this

Competitor[] competitors = new Competitor[n];
for(Competitor c : competitors) {
   c = new Competitor();
   System.out.println("Enter name, id and bounce height");
   c.SetName(sc.next());
   c.SetId(sc.next());
   c.SetHeight(sc.nextFloat());
}
return competitors;

But like that you just assign a new Competitor to "c". You should take off that c = new Competitor() so it's like this

Competitor[] competitors = new Competitor[n];
for(Competitor c : competitors) {
   System.out.println("Enter name, id and bounce height");
   c.SetName(sc.next());
   c.SetId(sc.next());
   c.SetHeight(sc.nextFloat());
}
return competitors;

Your other problem with SetHeight() was because SetHeight() was picking a primitive value and putting it in a Class type Float (automatically thanks to auto-boxing). The class type internally store the float primitive in a field. When you used ==, it compared the two objects and determined that they didn't point to the same place, even though their fields pointed to a float primitive with the same value.

That's why when comparing non-primitive types we should always use .equals() method instead of ==, unless you're trying to test it against null. .equals() will make sure it doesn't compare the place where the variable is pointing but their internal fields.

Take a look at this answer https://stackoverflow.com/a/73021/2576857

I think it will help you to understand both problems you had.

Upvotes: 0

johnchen902
johnchen902

Reputation: 9601

This enhanced for loop:

for(Competitor c : competitors) {
    c = new Competitor();
    System.out.println("Enter name, id and bounce height");
    c.SetName(sc.next());
    c.SetId(sc.next());
    c.SetHeight(sc.nextFloat());
}

is equivalent to:

for(int i = 0; i < competitors.length; i++) {
    Competitor c = competitors[i];
    c = new Competitor();
    System.out.println("Enter name, id and bounce height");
    c.SetName(sc.next());
    c.SetId(sc.next());
    c.SetHeight(sc.nextFloat());
}

So you modified c, but you never modified competitors

Java have different conventions with C#. See here.

Methods should be verbs, in mixed case with the first letter lowercase, with the first letter of each internal word capitalized.

Upvotes: 0

Fedy2
Fedy2

Reputation: 3207

The Create method initializes the competitors values in the wrong way. Try this:

    for(int i = 0;i<competitors.length;i++){
        competitors[i] = new Competitor();
        System.out.println("Enter name, id and bounce height");
        competitors[i].SetName(sc.next());
        competitors[i].SetId(sc.next());
        competitors[i].SetHeight(sc.nextFloat());
    }

One more suggestion. While in C# methods name starts with one upper case char, in Java they start in lower case.

Upvotes: 2

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