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.
The String class which is built into the default java.lang library simplifies a lot of complex programming tasks for us. Classes are grouped together into a package like java.lang. Many other useful library packages can be imported in. Programmers provide Application Program Interfaces (APIs) to allow other programmers to use their code. Documentation for APIs and libraries are essential to understanding the attributes and behaviors of an object of a class.
The String class has many useful methods that you can view in the Java String API. This unit explores a few of the methods.
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 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) 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,
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?
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
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 are used in many programs:
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) : 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.