Week 3: Objects, files and exceptions/14a Class StringBuilder

As stated earlier, Java STRINGS are MUTATION-FREE. 
This can cause problems in situations where, for example, a string should be assembled from very small pieces. 
As an example, consider a program that adds chunks to a string one at a time:

String str = "";
for (int i=0; i<100; i++) {
    str += "x";
}
 

Every time two strings are CONCATENATED, Java creates a NEW STRING. 
Thus, during the execution of the previous program, 101 strings are created. 

Although the strings that are discarded are not stored anywhere, they remain in MEMORY to haunt you. 
At some point, as free memory nears exhaustion, Java's automatic garbage collection removes the unnecessary objects from memory. 
However, this may result in a temporary slowdown for the user.

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In situations where it is necessary to change a string frequently, it is usually more sensible to use the StringBuilder class instead of the String class. 
StringBuilder is a MUTABLE version of a string: its contents can therefore be changed after initialization.

Let's consider the previous example implemented with the StringBuilder class:

StringBuilder str = new StringBuilder();
for (int i=0; i<100; i++) {
    str.append("x");
}
 


The StringBuilder class contains a variety of useful methods. 
The following example shows how the class works. 
Note that the class is built into Java (i.e., it is included in the java.lang package), so it does not need to be explicitly introduced by an import statement.

StringBuilder str = new StringBuilder("Hey everyone");
System.out.println(str); //Hey everyone

// add at the end of string
str.append("!!!");
System.out.println(str); //Hey everyone!!!

// replaces between indexes 0 and 3
str.replace(0,3, "Bye");
System.out.println(str); //Bye everyone!!!

// reverse order
str.reverse();
System.out.println(str); //!!!enoyreve eyB
 

Program outputs:
Hey everyone
Hey everyone!!!
Bye everyone!!!
!!!enoyreve eyB