Unit 5 (Prog 1) : Introduction and Basics of String

In this example, you will find the basic structure and concepts of Java.lang.String with code example and description.

/
  Java Library provide three class to handle Strings
 * String, StringBuffer and StringBuilder
 * In this section, the details and working of String class is included, however we will discuss the further parts in upcoming sections.
 *
 * String is an immutable Object.
 * By Immutable, we mean, the Object that can never be changed after once created.
 *
 * In this program, declaration of String and Immutable Property is covered.
 *
 */

public class FirstProgram {

   public FirstProgram() {
      // Methods to declare a String.
      String s1 = "abcdef";
      String s2 = new String();
      s2 = "xyz";
      String s3 = new String("abcd");

      // The String Objects are Immutable, but what about their reference ??
      s1 = s1.concat(s2);
      System.out.println(s1);
      // Output : abcdefxyz
      // In above code, new String Object containing the value "abcdefxyz" will be       created and s1 will reference the new object,
      // note that the old object containing value "abcdef" will not changed, it 
      will be unreferenced, in other terms that is lost !!!

      s2.concat(s3);
      System.out.println(s2);
      // Output : xyz
      // In above line of code, a new Object will be created containing the value       "xyzabcd" but it is not referenced, so it is lost !!!
      // But here note that, the original value of original s2 reference is 
      unchanged.

      s3.substring(1, 2);
      s3.toUpperCase();
      s3.replace("c", "q");
      System.out.println(s3);
      // Output : abcd
      // In all above three cases, same logic is there.
      s3 = s3.replace("c", "q").toUpperCase();
      System.out.println(s3);
      // Output : ABQD
      // Now, s3 will reference to the new object "abqd" and "abcd" Object is lost      now !!!
   }

   public void checkYourSelf() {

      // Try to find what will be printed, and for extra concept how many Objects       and Reference will be created.
      String one = "One";
      String two = "Two";
      String three = one.concat(" half ") + two;
      one += two;
      three.concat(two);
      System.out.println(one + " " + two + " " + three);
   }

   public static void main(String a[]) {
      new FirstProgram();
   }
}