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Java Basics

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NullPointerException

NPE

NullPointerException or NPE is an error every Java programmer is familiar with. This error holds the record for irritating programmers.

First, 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:

java

Main

copy

              
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:

java

Main

copy

              
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.

You might think this error is not so serious, but later, when you receive values not written by you personally, you may often come across it. Therefore, it is crucial to understand how to resolve this error now.

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.

java

Main

copy

              
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);
        }
    }
}
Task

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.

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

Solution

java

solution

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SectionΒ 5. ChapterΒ 5

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book
NullPointerException

NPE

NullPointerException or NPE is an error every Java programmer is familiar with. This error holds the record for irritating programmers.

First, 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:

java

Main

copy

              
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:

java

Main

copy

              
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.

You might think this error is not so serious, but later, when you receive values not written by you personally, you may often come across it. Therefore, it is crucial to understand how to resolve this error now.

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.

java

Main

copy

              
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);
        }
    }
}
Task

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.

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

Solution

java

solution

Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
Everything was clear?

How can we improve it?

Thanks for your feedback!

SectionΒ 5. ChapterΒ 5
Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
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