Kursinnehåll

Java Basics (unit tests)

1. Getting Started

Backend Development Role of Java in the Backend Java Advantages Memory Types In Java Java's Syntax Your First Program Comments

2. Basic Types and Operations

Primitive Data Types Challenge: Primitive Types Mathematical Operations on char Mathematical Operations in Java If-Else Statement Switch-Case Statement New Task Structure Challenge: Weekday Switch

3. Loops

Introduction to Loops While-Loop Do-While Loop Challenge: Building a Server Retry Mechanism For-Loop Increment and Decrement Infinite Loop Challenge: Filtering Numbers in a Loop

4. Arrays

What is Array?Using Loops with Arrays Index Out Of Bounds Two-Dimentional Array Iteration In Two-Dimensional Array For-Each Loop

5. String

What Is String? Basic Methods In String Practical Applications of String Methods Challenge: Filter Failure Messages NullPointerException StringBuilder String Pool, Method Equals() Summary

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:

Main


              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


              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.

Main


              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);
        }
    }
}

Uppgift

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"

Lösning

solution

Byt till skrivbordet för praktisk övningFortsätt där du är med ett av alternativen nedan

Var allt tydligt?

Tack för dina kommentarer!

Avsnitt 5. Kapitel 5

NullPointerException

NPE

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

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

Main


              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


              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


              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);
        }
    }
}

Uppgift

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"

Lösning

solution

Byt till skrivbordet för praktisk övningFortsätt där du är med ett av alternativen nedan

Var allt tydligt?

Tack för dina kommentarer!

Avsnitt 5. Kapitel 5

Byt till skrivbordet för praktisk övningFortsätt där du är med ett av alternativen nedan