Null issue needs an explanation

Question

I am a newbie at programming and Java and this is my first null, I am a little bit confused because I do not know what happened is that kind of errors in coding? Or any thing else? Kindly needs your explanation in this situation and about null at overall in a simple way

public static void main(String[] args) {
    Scanner input = new Scanner (System.in);
    System.out.println("Enter grades size :");

    int Size = input.nextInt();

    String[] y = new String[Size];
    int[] x = new int[Size];

    int Max = 0;
    int Min = x[0];

    String Max_studen = y[0];
    String Min_studen = y[0];

    for (int i = 0 ; i < Size ; i++) {
        System.out.println("Enter student name #" + (i));
        y[i] = input.next();

        System.out.println("Enter student grade : #" + (i));
        x[i] = input.nextInt();

        if (x[i] > Max) {
            Max = x[i];
            Max_studen = y[i];
        } else if (x[i] < Min) {
            Min = x[i];
            Min_studen = y[i];
        }
    }

    System.out.println("Max Student is: " + Max_studen);
    System.out.println("Max grade is: ");
    System.out.println(Max);
    System.out.println("Min Student is: " + Min_studen);
    System.out.println("Min grade is : ");
    System.out.println(Min);
}

Output is:

Enter grades size :
2
Enter student name #0
Sam
Enter student grade : #0
85
Enter student name #1
Samy
Enter student grade : #1
95
Max Student is: Samy
Max grade is:
95
Min Student is: null
Min grade is :
0 

Answer 1

For a beginner, this is a tricky one. Let us walk through this code together.

The first important lines are those:

String[] y = new String[Size];
int[] x = new int[Size];

There is a major difference between those two arrays. x is an array of primitives, while y is an array of objects. Primitives have a well-defined default values in Java (typically 0), so the array x is initialized with all cells containing a 0. For objects there is no such thing as a default value. Since you can write your own class, Java does not know what a good default value for, e.g., a Car is. Therefore the default value for objects is null. For the moment, let us define null as a value signaling "This cell is empty" ¹. So each cell of y contains null as its value.

Now to the next important lines:

int Min = x[0];
[...]
String Min_studen = y[0];

As explained earlier, x is initialized with all 0. Concluding from this, Min is now 0. We have a problem here because in a typical scenario we will never enter a grade < 0 and therefore Min will never be overwritten. In genereal, if we are looking for some maximum or minimum, we should initialize them with Integer.MIN_VALUE (for the maximum) or Integer.MAX_VALUE (for the minimum) respectively. There are similar constants for long, float and double. Theses constants typically avoid problems like the given one. Since Min is never overwritten, Min_studen is never overwritten either. The initial values of y are nulls, as is Min_studen's value.

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Some remarks on your code:

• Variable names should be written in camelCase.
• The [] of an array is typically written following the type, without a blank: String[] instead of String [].
• After a {, there should always be a linebreak.
• Variable names should be meaningful and descriptive. One could use studentGrades and studentNames instead of x and y.

There are many sites explaining coding conventions and style guides, some of them are here and here ². You might not want to look at them right now, but keep them in mind for later, if you got the basics of Java down and read them.

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_{¹The actual meaning of null is quite close to the description I gave, but the reasoning is a little bit more complex and requires the reader to have knowledge about heap- and stackmemory and what is stored where.}

_{²There is no such thing as "the styleguid". You are of course free to define your own. Style guides are a means to make code easier to read. Please keep this in mind whenever you change your style guide or parts of it.}