🌟 Java : How to Check If a String is Null or Empty

Описание: ⚠️ "Avoid NullPointerException Like a Pro!"
🧩 What’s the Goal of This Program?
This Java program teaches how to safely check whether a string is null or empty.

It covers:

Recognizing when a string is completely missing (null)

Recognizing when a string exists but has no content ("")

Combining checks to avoid runtime errors like NullPointerException

This is essential when validating form input, reading user data, or processing API responses!

🧠 Step-by-Step Breakdown: What’s Going On?
1️⃣ Declare Three Strings
str1 is set to null (not pointing to any object)

str2 is an empty string ("")

str3 is a valid string containing "Hello"

2️⃣ Call a Utility Method to Check
A method named isNullOrEmpty() is used to evaluate each string

It returns true if the string is either:

null (not initialized)

empty (length = 0)

3️⃣ Print the Results
Each result is printed with a message indicating if that string is null or empty

🔧 Inside the Method: How the Logic Works
The condition used is:

str == null || str.isEmpty()

First checks if the string is null

If not null, then checks if it’s empty

This prevents NullPointerException when calling isEmpty()

🎓 Top 5 Java String Null or Empty Check Interview Questions
1. Q: How do you check if a string is null or empty in Java?
👉 Use: str == null || str.isEmpty()

2. Q: What’s the difference between isEmpty() and isBlank()? (Java 11+)
👉 isEmpty() checks if string length is 0.
👉 isBlank() also returns true for whitespace-only strings like " ".

3. Q: How can you safely compare strings to avoid NullPointerException?
👉 Call .equals() on a constant:
"hello".equals(str) instead of str.equals("hello")

4. Q: How do you check if a string is only whitespace in older Java versions?
👉 Use: str.trim().isEmpty()

5. Q: What happens if you call isEmpty() on a null string?
👉 It throws a NullPointerException because the method is called on a non-existent object.

🧾 Conclusion: Null or Empty? Avoid the Trap!
In Java, checking if a string is null or empty is a basic but critical skill. Whether you’re validating user input, working with data from files, or calling APIs — safely handling string content will protect your program from common runtime crashes.

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