Strings, Revisited

• Core Java: Volume I—Fundamentals

  • Chapter 3.6

• String Documentation

Java Standard Library

The Java standard library is a set of classes that are built into the language. Java programs can load these classes at runtime, and use them during execution.

String

The String class is built into the Java programming language, and you can use Strings and their methods out-of-the-box. You've already learned and read all about Strings, but didn't dive too deeply into the API. There are over 50 different methods (many more if you consider that most methods accept numerous sets of input types), but these are likely the most useful to you in this course.

Methods

• charAt(int index)

The charAt method returns the character at the specified index.

String word = "hello";          // remember, Strings are 0-indexed
char letter = str.charAt(1);    // letter is 'e'

• compareTo(String anotherString)

The compareTo method compares two Strings lexicographically. The method returns a negative value for lesser Strings (i.e., those that would appear closer to the beginning of a lexicographical ordering), a positive value for greater Strings, and 0 for equivalent Strings.

String first = "apple";
String second = "banana";

int a = first.compareTo(second);    // negative, usually -1
int b = second.compareTo(first);    // positive, usually 1
int c = first.compareTo(first);     // 0

There is a similar method called compareToIgnoreCase, which does the same thing while ignoring differences in capitalization.

• concat(String str)

The concat method concatenates two Strings together, returning a new String entirely.

String one = "fire";
String two = "truck";
String three = one.concat(two);    // firetruck

• contains(CharSequence s)

The contains method returns true if the String contains the specified sequence. Don't pay too much attention to CharSequence. This method works with Strings, and that's probably all you'll need to know.

String word = "alphabetical";
boolean doesContain = word.contains("bet");    // true

• endsWith(String suffix)

The endsWith method returns true if the String ends with the specified suffix.

String word = "happily";
String suffix = "ly";
boolean ends = word.endsWith(suffix);    // true

• equals(Object anObject)

The equals method returns true if both objects are Strings, and their contents are equivalent.

String a = "abc";
String b = "xyz";
boolean isEqual = a.equals(b);    // false

There is a similar method called equalsIgnoreCase, which does the same thing while ignoring differences in capitalization.

• indexOf(String str)

The indexOf method returns the index within a String of the first occurrence of the specified substring.

String sentence = "This sentence has five words.";
int index = sentence.indexOf("s");                    // 3 (it finds the
                                                      // first "s" in the
                                                      // sentence)

There are several other variations of the indexOf method. Check out the API for more on these.

• isEmpty()

The isEmpty method returns true if the String has a length of 0.

String str = "";
boolean empty = str.isEmpty();    // true

There is a similar method called isBlank, which returns true if the String is made up entirely of whitespace.

• lastIndexOf(String str)

The lastIndexOf method behaves in much the same way as the indexOf method, expect it finds the last occurrence rather than the first occurrence.

String sentence = "This sentence has five words.";
int index = sentence.lastIndexOf("s");                // 27 (it finds the
                                                      // last "s" in the
                                                      // sentence)

Again, like indexOf, there are other variations of the lastIndexOf method. Check out the API for more on these.

• length()

Returns the number of characters in the String.

String str = "abcd";
int length = str.length();    // 4

• replace(CharSequence target, CharSequence replacement)

The replace method replaces each occurrence of the target substring with the replacement substring.

String first = "ababa";
String second = first.replace("a", "x");    // "xbxbx"

There are other variations of replace, as well as a similar method called replaceAll. Check out the API for more on these.

• startsWith(String prefix)

The startsWith method returns true if the String begins with the specified prefix.

String word = "antithesis";
boolean starts = word.startsWith("anti");    // true

• strip()

The strip method trims the leading and trailing whitespace from a String.

String raw = "  some text  ";
String clean = raw.strip();      // "some text"

There are similar methods called stripLeading and stripTrailing, which trim only the leading or trailing whitespace, respectively. There's also an outdated method called trim. strip is the Unicode-aware upgraded version, which uses a newer definition of whitespace.

• substring(int beginIndex, int endIndex)

The substring method returns a substring composed of characters between specified starting and ending indices. The starting index is inclusive, but the ending index is not.

String original = "The String class is built into Java.";
String substring = original.substring(11, 16);               // "class"

There is another version of substring that uses only a starting index. The implied ending index is the end of the String.

• toLowerCase()

The toLowerCase method converts a String to lowercase characters. Characters that cannot be converted (i.e., numbers) are left unchanged.

String str = "ABC123XYZ";
String lower = str.toLowerCase();    // "abc123xyz"

• toUpperCase()

The toUpperCase method converts a String to uppercase characters. Characters that cannot be converted (i.e., numbers) are left unchanged.

String str = "abc123xyz";
String upper = str.toUpperCase();    // "ABC123XYZ"

• valueOf(int i)

The valueOf method returns a String representation of its input.

int x = 123;
String str = String.valueOf(x);    // "123" (a String)

Pay attention to how valueOf is used. It's a little different than how most of the other String methods are used. There are variations of this method for many data types.

You should take a look at the full documentation for more details on these and all of the available methods in the String class.