Secrets of final keyword :

In Java, you use the keyword final to denote a constant.

final double PI=3.14;

The keyword final indicates that you can assign to the variable once, then its value is set
once and for all.

Important on final keyword:

System.out is a final variable . It is declared in the System class as

public final class System
{
. . .
public final static PrintStream out = nullPrintStream();
. . .
}

Since out has been declared as final, you cannot reassign another print stream to it:

out = new PrintStream(. . .);     // ERROR--out is final




If you look at the System class, you will notice a method setOut that lets you set System.out to a different stream. You may wonder how that method can change the value of a final variable. However, the setOut method is a static method which calls native method, not implemented in the Java programming language. 

   public static void setOut(PrintStream out) {
        checkIO();
        setOut0(out); //This is a native method 
    }

private static native void setOut0(PrintStream out);

Native methods can bypass the access control mechanisms of the Java language.