String Methods

Introduction to String Methods

As you may recall, a String, which is widely used in Java, is an object that represents a sequence of characters. it is a great way to store information.

Because character strings are so vital to programming, Java dedicated an entire class to them. This is great news for us because the String class has a lot of useful methods to help us perform operations on Strings and data manipulation. We don't have to import anything to use the String class because it's apart of the java.lang package, which is available by default.

In these notes, we are going to go over several String methods:

• length()

• concat()

• equals()

• indexOf()

• charAt()

• substring()

• toUpperCase()/toLowerCase()

length()

In Java, the length() string method returns the length - total number of characters - of a String.

Suppose we have a String called str, str.length() would return its length.

Take a look at this code for example:

String str = "Hello World!";

System.out.println(str.length());

12 would be printed because str has 12 characters:

H e l l o _ W o r l d !

In theory, the length of a String is the same as the Unicode units of the String. For example, escape sequences such as \n count as only one character.

concat()

The concat() method concatenates one string to the end of another string. Concatenation is the operation of joining two strings together.

Suppose we have a String called str1 and another String called str2, using str1.concat(str2) would return str1 with str2 appended to the end of it.

For example:

String name = new String("Nick's ");  

name = name.concat("Docs");

System.out.println(name);

Nick's Docs would be printed.

Strings are immutable objects, which means that String methods, like concat() do not actually change a String object.

Our variable, name holds a reference to the String object, "Nick's ". When we use concat() on name, we changed its value so that it references a new object - "Nick's ", combined with the String literal, "Docs".

Suppose we do something slightly different. We'll use concat() on name without reassigning its value:

String name = new String("Nick's ");  

name.concat("Docs");

System.out.println(name);

Nick's would be printed instead. The value of the String can't change! Instead, we create a new object and need to assign that new object some variable.

equals()

With objects, such as Strings, we can't use the primitive equality operator == to check for equality between two strings. To test equality with strings, we use a built-in method called equals().

For example:

String flavor1 = "Mango";
String flavor2 = "Peach";

System.out.println(flavor1.equals("Mango"));
// prints true
System.out.println(flavor2.equals("Mango"));
// prints false

Side note, there's also an equalsIgnoreCase() method that compares two strings without considering upper/lower case.

We can also compare String values lexicographically (think dictionary order) using the .compareTo() method. When we call the .compareTo() method, each character in the String is converted to Unicode; then the Unicode character from each String is compared.

The method will return an int that represents the difference between the two Strings.

For example:

String flavor1 = "Mango";
String flavor2 = "Peach";

System.out.println(flavor1.compareTo(flavor2));

Our program will output -3.

When using a compareTo() method, we must pay attention to the return value:

• If the method returns 0, the two Strings are equal.

• If the value is less that 0, then the String object is lexicographically less than the String object argument.

• If the value is greater than 0, then the String object is lexicographically greater than the String object argument.

indexOf()

If we want to know the index of the first occurence of a character in a string, we can use the indexOf() method on a string.

Remember that the indices in Java start with 0:

String letters = "ABCDEFGHIJKLMN";

System.out.println(letters.indexOf("C"));

This would output 2.

Although C is the third letter in the English alphabet, it is located in the second index of the string.

Suppose we want to know the index of the first occurrence of an entire substring. The indexOf() instance method can also return where the substring begins (the index of the first character in the substring):

String letters = "ABCDEFGHIJKLMN";

System.out.println(letters.indexOf("EFG"));

This would output 4 because EFG starts at index 4.

If the indexOf() doesn't find what it's looking for, it'll return a -1.

charAt()

The charAt() method returns the character located at a String's specified index.

For example:

String str = "qwer";

System.out.println(str.charAt(2));

It would output e because that is what is located at index 2.

Suppose we try to return the character located at index 4. It would produce an IndexOutOfBoundsException error because index 4 is out of str's range.

substring()

There may be times when we only want a part of a string. In such cases, we may want to extract a substring from a string.

The substring() method does exactly that.

For example:

String line = "The Heav'ns and all the Constellations rung";

System.out.println(line.substring(24));

It would output Constellations rung because that's what begins at index 24 and ends at the end of line. The substring begins with the character at the specified index and extends the end of the string.

But suppose we want a substring from the middle of the string. We can include two arguments with this string method. For example:

String line = "The Heav'ns and all the Constellations rung";

System.out.println(line.substring(24, 38));

It would output Constellations because that's the substring that begins at index 24 and ends at index 38.

We can use this method to return a single-element substring at a specific index by calling substring() with the wanted index value plus one as the second argument.

For example, we can use this method to output just C:

String line = "The Heav'ns and all the Constellations rung";

System.out.println(line.substring(24, 25));

toUpperCase()/toLowerCase()

There will be times when we have a word in a case other than what we need it in. Luckily, Java has a couple String methods to help us out:

• toUpperCase(): returns the string value converted to uppercase.

• toLowerCase(): returns the string value converted to lowercase.

For example:

String input = "Cricket!";

String upper = input.toUpperCase();
// stores "CRICKET!"

String lower = input.toLowerCase();
// stores "cricket!"

A good use of this functionality is to ensure consistency of the data you store in a database. Making sure all of the data you get from a user is lowercase before you store it in your database will make your database easier to search through later.

String Methods Review

String Method	Value
length()	returns the length
concat()	concatenates two strings
equals()	checks for equality between two strings
indexOf()	returns the index of a substring
charAt()	returns a character
substring()	returns a substring
toUpperCase()	returns the upper case version
toLowerCase()	returns the lower case version