StringBuilder str = new StringBuilder("<urStr>");

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

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+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.
+
+===========================================
+
+
+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
+
+
+