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Thread: Total noob needing help - Arrays and scanning.

  1. #1
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    Default Total noob needing help - Arrays and scanning.

    So I get more or less how arrays work, and I made them work in simple programs with numbers and what not.
    But now I wanna get more complex and say I want to make a small (fleeting) database of people, so I wrote a class for it...

    public class Person {
    	public static int count;
    	private int id;
     
    	// define variables for each object
    	private String name;
    	private String surname;
    	private int age;
    	private int weight;
     
    	public Person() {
    		// set the id equal to the total class count
    		this.id = count;
    		// increase the count
    		count++;
    	}
     
    	public void setName(String name) {
    		this.name = name;
    	}
     
    	public void setSurname(String surname) {
    		this.surname = surname;
    	}
     
    	public void setAge(int age) {
    		this.age = age;
    	}
     
    	public void setWeight(int weight) {
    		this.weight = weight;
    	}
     
    	public int getId() {
    		return id;
    	}
     
    	public String getName() {
    		return name;
    	}
     
    	public String getSurname() {
    		return surname;
    	}
     
    	public int getAge() {
    		return age;
    	}
     
    	public int getWeight() {
    		return weight;
    	}
    }

    I'm assuming the class is fine and that I'm doing something wrong as far as scanning the people into the array...

    Please don't mind the crude programming, I've only been learning for a week.

    import java.util.Scanner;
     
     
    public class Application {
     
    	public static void main(String[] args) {
     
    		Scanner scan = new Scanner(System.in);
    		System.out.println("Amount of people?");
    		int n = scan.nextInt();
    		Person[] people = new Person[n];
    		int x = n;
     
    		for (int i=0 ; i < x ; i++ ) {
    			System.out.println("Insert Person number " + (i+1) );
    			String name = scan.nextLine();
    			people[i].setName(name);
    		}
     
    		for (int i=0 ; i < x ; i++ ) {
    			System.out.print(people[i].getName() + " ");
    		}
     
     
     
    	}
    }

    I'm getting

    "Amount of people?
    2
    Insert Person number 1
    Exception in thread "main" java.lang.NullPointerException
    at Application.main(Application.java:34)"

    Of course this line is total garbage:
    people[i].setName(name);

    What am I doing wrong?


  2. #2
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    Default Re: Total noob needing help - Arrays and scanning.

    Many n00bs make this mistake. Here's a little story:

    I want to go out and pick some apples. Before that I make a carton to hold the apples. After I have finished making the carton how many apples does it hold?

    Hopefully your answer was zero. Well it's the same for arrays of objects. Once you have declared the array it does not automagically contain any objects.
    Improving the world one idiot at a time!

  3. #3
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    Default Re: Total noob needing help - Arrays and scanning.

    You will also run into the "Scanner Bug". Google it.

    When you try to read in a String after you've taken a numerical input from the Scanner object, you're going to have another 'scratch your head' moment trust me.

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    Default Re: Total noob needing help - Arrays and scanning.

    Ok I think I got your suggestions...

    So here's how the new code looks

    Scanner scan = new Scanner(System.in);
    System.out.println("Amount of people?");
    int n = scan.nextInt();
    scan.nextLine(); // scanner bug, flushing out the Enter
    Person[] people = new Person[n];
    int x = n;
     
    for (int i=0 ; i < x ; i++ ) {
    	System.out.println("Insert Person " + (i+1) + " name");
    	String name = scan.nextLine();
    	people[i] = new Person(); //filling the basket with apples
    	people[i].setName(name);
    }
     
    for (int i=0 ; i < x ; i++ ) {
    	System.out.print(people[i].getName() + " ");
    }


    It seems to be working, did I get it right?
    Will a new empty scan.nextLine(); be necessary after each field?

  5. #5
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    Default Re: Total noob needing help - Arrays and scanning.

    That seems like it will work. Testing your code will be the best way to find out.

    You only need to call nextLine between reading a single token (eg an int value) and reading a full line or whenever you want to flush the buffer.
    Improving the world one idiot at a time!

  6. The Following User Says Thank You to Junky For This Useful Post:

    MayhemCode (October 9th, 2013)

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