📓2.7: String Methods

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 2.7

📝 Take notes in a Codespace during class, coding along with the instructor.

  1. Go to GitHub and click on your picture in the TOP RIGHT corner
  2. Select Your repositories
  3. Open CS2-Unit-2-Notes
  4. Now on your repository, click and select the Codespaces tab
  5. Click Create Codespace on main (unless you already have one listed there), wait for the environment to load, then you’re ready to code!

String Methods

A string holds characters in a sequence. Each character is at a position, or index, which starts with 0 as shown below. An index is a number associated with a position in a string. The length of a string is the number of characters in it including any spaces or special characters. The string below has a length of 14:

image

The first character in a String is at index 0 and the last character is at length - 1.

For the AP CSA exam, you only need to know how to use the following String methods. All of the String method descriptions are included in the AP CSA Java Quick Reference Sheet that you get during the exam so you don’t have to memorize these.

Method Output
int length() Returns the number of characters in the string, including spaces and special characters like punctuation.
String substring(int from, int to) Returns a new string with the characters in the current string starting with the character at the from index and ending at the character before the to index (if the to index is specified, and if not specified it will contain the rest of the string).
int indexOf(String str) Searches for the string str in the current string and returns the index of the beginning of str in the current string, or returns -1 if it isn’t found.
int compareTo(String other) Returns a negative value if the current string is less than the other string alphabetically, 0 if they have the same characters in the same order, and a positive value if the current string is greater than the other string alphabetically.
boolean equals(String other) Returns true when the characters in the current string are the same as the ones in the other string. This method is inherited from the Object class, but is overridden which means that the String class has its own version of that method.

length, substring, indexOf

Test the code below to see the output from the String methods length, substring, and indexOf. The length method returns the number of characters in the string, not the last index which is length -1. The str.substring(from,to) method returns the substring from the from index up to (but not including) the to index. The method str.indexOf(substring) searches for the substring in str and returns the index of where it finds substring in str or -1 if it is not there.

This code shows the output from String methods length, substring, and indexOf.

💬 Discuss: How many letters does substring(0,3) return? What does indexOf return when its argument is not found?

String message1 = "This is a test";
String message2 = "Hello Class";

System.out.println(message1.length());
System.out.println(message2.length());

System.out.println(message1.substring(0, 3));
System.out.println(message1.substring(2, 3));
System.out.println(message1.substring(5));

System.out.println(message1.indexOf("is")); // This will match the is in "This"!
System.out.println(message1.indexOf("Hello"));
System.out.println(message2.indexOf("Hello"));

// lowercase and uppercase methods are not on the AP exam, but still useful
System.out.println(message2.toLowerCase());
System.out.println(message2.toUpperCase());

Remember that substring(from,to) does not include the character at the to index! To return a single character at index i, use str.substring(index, index + 1).

CompareTo and Equals

We can compare primitive types like int and double using operators like == and < or >, which you will learn about in the next unit. However, with reference types like String, you must use the methods equals and compareTo, not == or < or >.

The method compareTo compares two strings character by character. If they are equal, it returns 0. If the first string is alphabetically ordered before the second string (which is the argument of compareTo), it returns a negative number. And if the first string is alphabetically ordered after the second string, it returns a positive number. (The actual number that it returns does not matter, but it is the distance in the first letter that is different, e.g. A is 7 letters away from H.)

image

The equals method compares the two strings character by character and returns true or false. Both compareTo and equals are case-sensitive. There are case-insensitive versions of these methods, compareToIgnoreCase and equalsIgnoreCase, which are not on the AP exam.

Test the code below to see the output from compareTo and equals.

String message = "Hello!";

System.out.println(message.compareTo("Hello!"));
System.out.println(message.compareTo("And"));
System.out.println(message.compareTo("Zoo"));

System.out.println(message.equals("Hello!"));
System.out.println(message.equals("hello!"));

Since "Hello!" would be alphabetically ordered after "And", compareTo returns a positive number. Since "Hello!" would be alphabetically ordered before "Zoo", compareTo returns a negative number. Notice that equals is case-sensitive.

There are lots of other methods in the String class. You can look through the **Java documentation **for the String class online. You don’t have to know all of these for the exam, but you can use them if you want to on the exam.

An Application Programming Interface (API) is a library of prewritten classes that simplify complex programming tasks for us. These classes are grouped together in a package like java.lang and we can import these packages (or individual classes) into our programs to make use of them. For instance, we have just discussed the String library built into the default java.lang package - it takes care of the detailed work of manipulating strings for us. There are many other useful library packages as well, both in the java.lang package and in other packages. Documentation for APIs and libraries are essential to understanding how to use these classes.

{.highlight} Strings are immutable which means that they can’t change after creation. Anything that you do to modify a string (like creating a substring or appending strings) returns a new string.

Common Mistakes with Strings

Here is a list of common mistakes made with Strings.

  • Thinking that substrings include the character at the last index when they don’t.

  • Thinking that strings can change when they can’t. They are immutable.

  • Trying to access part of a string that is not between index 0 and length -1. This will throw an IndexOutOfBoundsException.

  • Trying to call a method like indexOf on a string reference that is null. You will get a null pointer exception.

  • Using == to test if two strings are equal. This is actually a test to see if they refer to the same object. Usually you only want to know if they have the same characters in the same order. In that case you should use equals or compareTo instead.
  • Treating upper and lower case characters the same in Java. If s1 = "Hi" and s2 = "hi" then s1.equals(s2) is false.

String Methods Game

Try the game below written by AP CSA teacher Chandan Sarkar. Click on Strings and then on the letters that would be the result of the string method calls. We encourage you to work in pairs and see how high a score you can get.

⭐️ Summary


🛑 When class ends, don’t forget to SAVE YOUR WORK!

  1. Navigate to the Source Control menu on the LEFT sidebar
  2. Click the button on the LEFT menu
  3. Type a brief commit message at the top of the file that opens, for example: updated Main.java
  4. Click the small ✔️ checkmark in the TOP RIGHT corner
  5. Click the button on the LEFT menu
  6. Finally you can close your Codespace!

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.