Lernen 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);
        }
    }
}

Aufgabe

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"

Lösung

War alles klar?

Danke für Ihr Feedback!

Abschnitt 1. Kapitel 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);
        }
    }
}

Aufgabe

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"

Lösung

Wechseln Sie zum Desktop, um in der realen Welt zu übenFahren Sie dort fort, wo Sie sind, indem Sie eine der folgenden Optionen verwenden

War alles klar?

Danke für Ihr Feedback!

Abschnitt 1. Kapitel 33

single