[JAVA] What is null? ]

** 1. Trigger**

The reason I chose this theme was because when I was educating a new engineer, I was asked to explain null and couldn't say anything.

String A = null;
String B = "";

When I set null and "" (empty string) for the String type, I couldn't explain what was different, so I'll dig into that and deepen my understanding. (At that time, I answered that "null" has no value and "" is an empty string. Reflection.)


When does the so-called "nullpo" occur? As an example, it looks like this.

public class Sample {
	public static void main(String[] args) {
		String str = null;

I get a NullPointerException when I try to execute a method with null. (Null cannot use any method), so if there is a possibility that a null value will be set, I think it is common to put a check process called "null check" as shown in the code below.

public class Sample {
	public static void main(String[] args) {
		String str = strMake();
		if (str != null) {
	private static String strMake () {
		return null; //Write something

** 3. Methods provided in the string class **

The String class has various methods. As a defense record, I will post a link to the reference site.

[Official document] https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/StringUtils.html

** 4. String type = reference type **

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