2.7. 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.
The first character in a string is at index 0 and the last characters is at length -1.
For the AP CS A exam, you only need to know how to use the following String methods. All of the String method descriptions are included in the AP CS A Java Quick Reference Sheet that you get during the exam so you don’t have to memorize these.
int length() method returns the number of characters in the string, including spaces and special characters like punctuation.
String substring(int from, int to) method returns a new string with the characters in the current string starting with the character at the
fromindex and ending at the character before the
toindex (if the
toindex is specified, and if not specified it will contain the rest of the string).
int indexOf(String str) method searches for the string
strin the current string and returns the index of the beginning of
strin the current string or -1 if it isn’t found.
int compareTo(String other) returns a negative value if the current string is less than the
otherstring alphabetically, 0 if they have the same characters in the same order, and a positive value if the current string is greater than the
boolean equals(String other) returns true when the characters in the current string are the same as the ones in the
otherstring. This method is inherited from the Object class, but is overriden which means that the String class has its own version of that method.
2.7.1. String Methods: length, substring, indexOf¶
Run the code below to see the output from the String methods
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. How many letters does substring(0,3) return? What does indexOf return when its argument is not found?
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).
2.7.2. 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 >.
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.)
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,
equalsIgnoreCase, which are not on the AP exam.
Run the example below to see the output from
equals. 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.
Run the code to see how the String methods equals and compareTo work. Is equals case-sensitive? When does compareTo return a negative number?
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.
Strings are immutable which means that they can’t change. Anything that you do to modify a string (like creating a substring or appending strings) returns a new string.
2.7.3. Common Mistakes with Strings¶
The following code shows some common mistakes with strings.
This code contains some common mistakes with strings. Fix the code to use the string methods correctly.
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
indexOfon a string reference that is null. You will get a null pointer exception.
==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
Treating upper and lower case characters the same in Java. If
s1 = "Hi"and
s2 = "hi"then
2.7.4. Programming Challenge : Pig Latin¶
Create a program that takes a word and transforms it to Pig Latin using String methods. You may need the word’s length, a substring that does not include the first letter, and a substring that is just the first letter (you can get the ith letter of a string using substring(i,i+1) so for example the letter at index 3 would be substring(3,4)).
Use the substring method to transform a word into Pig Latin where the first letter is put at the end and “ay” is added. The word pig is igpay in Pig Latin.
index - A number that represents the position of a character in a string. The first character in a string is at index 0.
length - The number of characters in a string.
substring - A new string that contains a copy of part of the original string.
A String object has index values from 0 to length – 1. Attempting to access indices outside this range will result in an IndexOutOfBoundsException.
String objects are immutable, meaning that String methods do not change the String object. Any method that seems to change a string actually creates a new string.
The following String methods and constructors, including what they do and when they are used, are part of the AP CS A Java Quick Reference Sheet that you can use during the exam:
String(String str) : Constructs a new String object that represents the same sequence of characters as str.
int length() : returns the number of characters in a String object.
String substring(int from, int to) : returns the substring beginning at index from and ending at index (to -1).
String substring(int from) : returns substring(from, length()).
int indexOf(String str) : searches for str in the current string and returns the index of the first occurrence of str; returns -1 if not found.
boolean equals(String other) : returns true if this (the calling object) is equal to other; returns false otherwise.
int compareTo(String other) : returns a value < 0 if this is less than other; returns zero if this is equal to other; returns a value > 0 if this is greater than other.
str.substring(index, index + 1)returns a single character at index in string str.
2.7.6. 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.