Is the final variable really constant in Java?

My answer is no. Oh, it was a bold answer. Let's see why.

Constant is something that can not be changed. The final variable by definition is a variable that can be initialized once.

The difference is slight. If the variable can be initialized only once, how can we change it?

Let's think about objects. We can initialize a variable to the object. In fact, we initialize the variable to the address of the object in the main memory).

Recall how the objects are stored in Java. See the picture below.

In the variable name1 we store the address of the place in the main memory where the object is stored, which is 42 in the picture).

When we declare a variable as final and initialize it, we can not change the address after that, but we can change the object!

Let's look at an example.

Why does Java use the Unicode?

I like this description, so I quote it. It answers the question why java uses Unicode.

The Java programming language uses the Unicode character set for managing text.

A character set is simply an ordered list of characters, each corresponding to a particular numeric value.

Unicode is an international character set that contains letters, symbols, and ideograms for languages all over the world. Each character is represented as a 16-bit unsigned numeric value. Unicode, therefore, can support over 65,000 unique characters.

Only about half of those values have characters assigned to them at this point. The Unicode character set continues to be refined as characters from various languages are included.

Many programming languages still use the ASCII character set. ASCII stands for the American Standard Code for Information Interchange.

The 8-bit extended ASCII set is quite small, so the developers of Java opted to use Unicode in order to support international users.

However, ASCII is essentially a subset of Unicode, including corresponding numeric values, so programmers used to ASCII should have no problems with Unicode.

John Lewis. Java Software Solutions: Foundations of Program Design.

What data must I check in PHP and how?

I hope you know the golden rule that every and each PHP programmer must know.

The First Rule

Check all data received from user!

All the data that we've got in the global arrays such as $_POST and $_GET must be checked.

Use filter_input() method to delete not needed symbols.

There are some types of filters for this method. Some useful examples:

$name = filter_input(INPUT_GET, 'name', FILTER_SANITIZE_STRING);
$email = filter_input(INPUT_GET, 'email', FILTER_SANITIZE_EMAIL);

There is one more rule.

The Second Rule

How to compare strings in Java

String in Java is not a primitive type, such as integer, boolean etc.
Every string is an object.
We can compare strings using method equals() or the identity operator ==.
Let's separate between the two types of comparing here.

• While using equals() the comparison is done by characters. String name1 = "Hello, world!";
String name2 = "Hello, world!";
if (name1.equals(name2))
    System.out.println("The names are the same.");
// Output "The names are the same."
  That means string "Hello, world!" is equal to the second "Hello, world!", because these strings have the same characters.
• While using == the pointers to strings are compared. if (name1 == name2)
    System.out.println("The names are the same.");
else
    System.out.println("Oh, no!"); // Output "Oh, no!" When we write this code:
  String name1 = "Hello, world!"; we have this picture in the main memory:
  
  The variable name1 holds the address of object in the memory (in our example the address is 42).
  name1 is a pointer to the location in memory where the object is held.
  
  Let's create strings as different objects.

How to compare float numbers safely?

This short article explains why we cannot use "==" to compare floats and how to compare floats right.
You can try to run this code in Java to understand the problem. double a = 1.0f / 3.0f;
double b = a + a + a;

System.out.println(a); // Output 0.3333333432674408
System.out.println(b); // Output 1.0000000298023224
System.out.println(1 == b); // Output false

Why?

If the compared values are the results of computation, they can be not equal.
Binary representation of the number 1/3 cannot be precise, because the place where the number is stored is limited. So the number rounding was made: 1/3 = 0.333.. = 0.(3) = 0.3333333432674408
So we have b = 1.0000000298023224 that is not equal to 1.0;

How to get round that problem?