CS173: Intro to Computer Science - Strings

Activity Goals

The goals of this activity are:
  1. To be able to explain that a String is a class that stores text and provides functionality that manipulates that text.
  2. To be able to explain that a String is immutable, and that new memory is allocated for a String when it is modified.
  3. To be able to create and manipulate a String.
  4. To be able to invoke the concat(), indexOf(), substring() and replace() methods of a String.

Supplemental Reading

Feel free to visit these resources for supplemental background reading material.

The Activity

Directions

Consider the activity models and answer the questions provided. First reflect on these questions on your own briefly, before discussing and comparing your thoughts with your group. Appoint one member of your group to take notes for the group, and appoint another member to discuss your findings with the class. After class, think about the questions in the reflective prompt and respond to those individually. Report out on areas of disagreement or items for which you and your group identified alternative approaches. Write down and report out questions you encountered along the way for group discussion.

Model 1: The String

?
1
2
3
4
5
6
7
8
9
10
11
public class Main {
    public static void main(String[] args) {
        // These are equivalent
        // String x = "Test";
        String x = new String("Test");
         
        // So are these:
        x = "Another test";
        x = new String("Another test");
    }
}

Questions

  1. Unlike re-assigning a primitive variable, assigning a variable to an object creates a new object. A String is a class that allocates memory for and stores text. Since String text cannot be reassigned once it is allocated (and must be re-created instead), a String is called an immutable object. Why can’t a String be re-assigned like an int or double can?

Model 2: String Manipulation






Questions

  1. What is the difference between x = x.concat(y) and x = x + y for String objects x and y?
  2. What is the difference between the two indexOf() methods given above? How do you know which version you are calling from a program?
  3. What is the difference between the two replace() methods given above? How do you know which version you are calling from a program?
  4. You can compare two strings using str1.equals(str2). Why do you think str1 == str2 won't work?

Embedded Code Environment

You can try out some code examples in this embedded development environment! To share this with someone else, first have one member of your group make a small change to the file, then click "Open in Repl.it". Log into your Repl.it account (or create one if needed), and click the "Share" button at the top right. Note that some embedded Repl.it projects have multiple source files; you can see those by clicking the file icon on the left navigation bar of the embedded code frame. Share the link that opens up with your group members. Remember only to do this for partner/group activities!

Submission

Submit your answers to the questions using the Collaborative Spaces section of OneNote. You can add a page with your name and your group members' names, and today's date, as the title. Under the appropriate section (i.e., "Class Notes", "Collaborative Spaces", "Reflective Prompts") that you can select on the left side of the screen, you can click "Add Page" on the right side. You can answer any reflective prompt questions in the Reflective Journal section of your OneNote Classroom personal section.

For Additional Practice

Feel free to visit these resources for additional practice exercises.