A Better Way to Convert Integer (May Be Null) to Int in Java

A better way to convert Integer (may be null) to int in Java?

Avoiding an exception is always better.

int i = integer != null ? integer.intValue() : -1;

Conversion from null to int possible?

Let's look at the line:

return y == null ? null : y.intValue();

In a ? : statement, both sides of the : must have the same type. In this case, Java is going to make it have the type Integer. An Integer can be null, so the left side is ok. The expression y.intValue() is of type int, but Java is going to auto-box this to Integer (note, you could just as well have written y which would have saved you this autobox).

Now, the result has to be unboxed again to int, because the return type of the method is int. If you unbox an Integer that is null, you get a NullPointerException.

Note: Paragraph 15.25 of the Java Language Specification explains the exact rules for type conversions with regard to the ? : conditional operator.

Assigning a null value to an int

That code will give a NullPointerException when you run it. It's basically equivalent to:

Integer a = null;
int b = a.intValue();

... which makes it clearer that it will indeed fail.

You're not really assigning a null value to an int - you're trying and failing.

It's fine to use null as a value for an Integer; indeed often Integer is used instead of int precisely as a "nullable equivalent`.

Convert a nullable integer value in Java (Short, Long etc.)

Plain simple "casting" and the "ternary conditional operator" would be the minimal and fastest solution.

Short shortVal = null;
Long longVal = shortVal == null ? null : (long) shortVal;

Since you don't seem to know about casting, here's an example that does not work - and how to fix it. Mind that the problem of null is completely ignored here - this is just a quirky aspect of casting that you might want to know, that's all.

Double doubleVal = 1d;
Float floatVal = (float) doubleVal; // Inconvertible types!

Working version:

Double doubleVal = 1d;
Float floatVal = (float) (double) doubleVal;

Can an int be null in Java?

int can't be null, but Integer can. You need to be careful when unboxing null Integers since this can cause a lot of confusion and head scratching!

e.g. this:

int a = object.getA(); // getA returns a null Integer

will give you a NullPointerException, despite object not being null!

To follow up on your question, if you want to indicate the absence of a value, I would investigate java.util.Optional<Integer>

How to convert null to int without casting to integer?

The fundamental issue here is that you are declaring, up front, three arrays without knowing how many values they will need to hold. Sure, you know a maximum number of values they'll hold (10), but you don't know the minimum. This then affects your calls to Arrays.toString.

Using a List would be the preferred method, but if you haven't covered casting in your class then I'm guessing that lists have also not been covered yet. Why not just have your server return a String instead? (I'm purposely leaving out StringBuilders here for simplicity, again as I doubt they've been covered).

Main:

import java.util.Scanner;
public class Main {
    public static void main (String[] args){
        Scanner input = new Scanner(System.in);
        int [] array = new int[10];
        System.out.print("Insert the 10 values of your array.");
        for(int i=0; i<array.length; i++){
            array[i] = input.nextInt();
        }

        System.out.println("The even numbers in the array are...");
        System.out.println(Server.getEven(array));
        System.out.println("The odd numbers in the array are...");
        System.out.println(Server.getOdd(array));
        System.out.println("The negative numbers in the array are...");
        System.out.println(Server.getNeg(array));

        input.close();
    }
}

Server

public class Server {
    public static String getEven(int[] array){
        String result = "";
        for(int i=0; i<array.length; i++){
            if(array[i]%2 ==0){
                if(result.length() != 0)
                {
                    result = result + ", ";
                }
                result = result + array[i];
            }
        }
        return result;
    }

    public static String getOdd(int[] array){
        String result = "";
        for(int i=0; i<array.length; i++){
            if(array[i]%2 !=0){
                if(result.length() != 0)
                {
                    result = result + ", ";
                }
                result = result + array[i];
            }
        }
        return result;
    }

    public static String getNeg(int[] array){
        String result = "";
        for(int i=0; i<array.length; i++){
            if(array[i]<0){
                if(result.length() != 0)
                {
                    result = result + ", ";
                }
                result = result + array[i];
            }
        }
        return result;
    }
}

Cast a Null String into Integer

You cannot cast from String to Integer. However, if you are trying to convert string into integer and if you have to provide an implementation for handling null Strings, take a look at this code snippet:

String str = "...";
// suppose str becomes null after some operation(s).
int number = 0;
try
{
    if(str != null)
      number = Integer.parseInt(str);
}
catch (NumberFormatException e)
{
    number = 0;
}

How to set to int value null? Java Android

In this case, I would avoid using null all together.

Just use -1 as your null

If you need -1 to be an acceptable (not null) value, then use a float instead. Since all your real answers are going to be integers, make your null 0.1

Or, find a value that the x will never be, like Integer.MAX_VALUE or something.

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