Impara NullPointerException

NPE

Let's understand the concept of the null value for a string variable. When we create a string variable but forget to initialize, it acquires a null value. In comparison, if we don't initialize an int variable, it acquires a value of 0.

Let's consider an example of a string variable with a null value:

Main.java


              12345678
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String string = null;
        System.out.println(string);
    }
}

As you can see, the output on the screen is null, and the code seems to be working. But let's now try using any method with a variable with the null value:

Main.java


              123456789
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String string = null;
        string.toLowerCase();
        System.out.println(string);
    }
}

When attempting to call any method on a string variable with a null value, we receive a NullPointerException.

Ways to avoid NPE:

Be attentive. If you create a string variable, double-check if you have initialized it;
Use a null check before using the variable, for example.

Main.java


              123456789101112
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String string = null;
        if (string == null) { // null check
            System.out.println("Can't use methods with string, because value is null");
        } else {
            System.out.println(string);
        }
    }
}

Compito

Swipe to start coding

You're developing a system that processes employee emails.
Some email entries might be missing (null).
Your task is to validate and process only the non-null and properly formatted emails.

You are given an array of employee emails.
Loop through each email in the array.
For each entry:
- Check if the email is not null
- And check if it contains the @ symbol
If both conditions are met, print the email to the console.
Otherwise, print:
"Invalid or missing email"

Soluzione

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Grazie per i tuoi commenti!

Sezione 1. Capitolo 33

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NPE

Let's consider an example of a string variable with a null value:

Main.java


              12345678
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String string = null;
        System.out.println(string);
    }
}

As you can see, the output on the screen is null, and the code seems to be working. But let's now try using any method with a variable with the null value:

Main.java


              123456789
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String string = null;
        string.toLowerCase();
        System.out.println(string);
    }
}

When attempting to call any method on a string variable with a null value, we receive a NullPointerException.

Ways to avoid NPE:

Be attentive. If you create a string variable, double-check if you have initialized it;
Use a null check before using the variable, for example.

Main.java


              123456789101112
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String string = null;
        if (string == null) { // null check
            System.out.println("Can't use methods with string, because value is null");
        } else {
            System.out.println(string);
        }
    }
}

Compito

Swipe to start coding

You are given an array of employee emails.
Loop through each email in the array.
For each entry:
- Check if the email is not null
- And check if it contains the @ symbol
If both conditions are met, print the email to the console.
Otherwise, print:
"Invalid or missing email"

Soluzione

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Tutto è chiaro?

Grazie per i tuoi commenti!

Sezione 1. Capitolo 33

single