Working with strings in Java

A lecture snippet with a mentor as part of the Codegym University course. Sign up for the full course.

1 Comparing strings

One of the most common operations with strings is comparison. The String class has over ten different methods that are used to compare one string with another string. Below we will take a look at 8 of the main ones.

boolean equals(String str)

boolean equalsIgnoreCase(String str)

int compareTo(String str)

public int compareToIgnoreCase(String str)

boolean regionMatches(int toffset, String str, int offset, int len)

boolean startsWith(String prefix)

boolean endsWith(String suffix)

Let's say you want to write a program that asks the user for a path to a file and then checks the file type based on its extension. The code of such a program might look something like this:

Scanner console = new Scanner(System.in);
String path = console.nextLine();

if (path.endsWith(".jpg") || path.endsWith(".jpeg"))
{
   System.out.println("This is a jpeg!");
}
else if (path.endsWith(".htm") || path.endsWith(".html"))
{
   System.out.println("This is an HTML page");
}
else if (path.endsWith(".doc") || path.endsWith(".docx"))
{
   System.out.println("This is a Word document");
}
else
{
   System.out.println("Unknown format");
}

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Task

New Java Syntax, level 9, lesson 5

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URL validation

In this task, you need to perform URL validation. A simple URL scheme looks like this: ://. The checkProtocol(String) method checks the network protocol (http or https) of the URL passed in input parameter and returns the result of this check — the name of the network protocol as a string. And the

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2 Searching for substrings

After comparing strings, the second most popular operation is finding one string inside another. The String class also has a few methods for this:

int indexOf(String str)

int indexOf(String str, int index)

int lastIndexOf(String str)

int lastIndexOf(String str, int index)

boolean matches(String regex)

The indexOf(String) and indexOf(String, index) methods are often used in combination. The first method lets you find the first occurrence of the passed substring in the current string. And the second method lets you find the second, third, etc. occurrences by skipping the first index characters.

Suppose we have a url like "https://domain.com/about/reviews", and we want to replace the domain name with "codegym.cc". Urls can have all sorts of different domain names, but we know the following:

• The domain name is preceded by two forward slashes — "//"
• The domain name is followed by a single foward slash — "/"

Here's what the code for such a program would look like:

Scanner console = new Scanner(System.in);
String path = console.nextLine();

int index = path.indexOf("//");
int index2 = path.indexOf("/", index + 2);

String first = path.substring(0, index + 2);
String last = path.substring(index2);

String result = first + "codegym.cc" + last;
System.out.println(result);

The lastIndexOf(String) and lastIndexOf(String, index) methods work the same way, only the search is performed from the end of the string to the beginning.

9

Task

New Java Syntax, level 9, lesson 5

Locked

Searching in a string

The getIndexOfFirstWord(String, String) method and the getIndexOfLastWord(String, String) method both accept a string and a word. The getIndexOfFirstWord(String, String) method needs to return the index of the first character of the first instance of the word (the second method parameter) in the st

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3 Creating substrings

A lecture snippet with a mentor as part of the Codegym University course. Sign up for the full course.

In addition to comparing strings and finding substrings, there is another very popular action: getting a substring from a string. As it happens, the previous example showed you a substring() method call that returned part of a string.

Here is a list of 8 methods that return substrings from the current string:

String substring(int beginIndex, int endIndex)

String repeat(int count)

String replace(char oldChar, char newChar)

String replaceFirst(String regex, String replacement)

public String replaceAll(String regex, String replacement)

String toLowerCase()

String toUpperCase()

String trim()

Here is a summary of the available methods:

substring(int beginIndex, int endIndex) method

The substring method returns a new string that consists of characters in the current string, starting at the character with index beginIndex and ending at endIndex. As with all intervals in Java, the character with index endIndex is not included in the interval. Examples:

"Hellos".substring(0, 3);

"Hel"

"Hellos".substring(1, 4);

"ell"

"Hellos".substring(1, 6);

"ellos"

"Hellos".substring(1);

"ellos"

If the endIndex parameter is not specified (which is possible), then the substring is taken from character at beginIndex to the end of the string.

repeat(int n) method

The repeat method simply repeats the current string n times. Example:

"Hello".repeat(3);

"HelloHelloHello"

"Hello".repeat(2);

"HelloHello"

"Hello".repeat(1);

"Hello"

"Hello".repeat(0);

""

replace(char oldChar, char newChar) method

The replace() method returns a new string in which all characters oldChar are replaced with the character newChar. This does not change the length of the string. Example:

"Programming".replace('Z', 'z');

"Programming"

"Programming".replace('g', 'd');

"Prodrammind"

"Programming".replace('a', 'e');

"Progremming"

"Programming".replace('m', 'w');

"Prograwwing"

replaceFirst() and replaceAll() methods

The replaceAll() method replaces all occurrences of one substring with another. The replaceFirst() method replaces the first occurrence of the passed substring with the specified substring. The string to be replaced is specified by a regular expression. We will delve into regular expressions in the Java Multithreading quest.

Examples:

"Good news everyone!".replaceAll("e.", "EX");

"Good nEXs EXEXyonEX"

"Good news everyone!".replaceAll("o.", "-o-");

"G-o-d news every-o-e!"

"Good news everyone!".replaceFirst("e.", "EX");

"Good nEXs everyone!"

"Good news everyone!".replaceFirst("o.", "-o-");

"G-o-d news everyone!"

toLowerCase() and toUpperCase() methods

We got to know these methods when we first learned about calling the methods of the String class.

trim() method

The trim() method removes leading and trailing spaces from a string. Does not touch spaces that are inside a string (i.e. not at the beginning or end). Examples:

"     ".trim();

""

"Hello".trim();

"Hello"

" Hello\n how are you?\n   ".trim();

"Hello\n how are you?\n"

"  Password\n   \n ".trim();

"Password\n   \n"

9

Task

New Java Syntax, level 9, lesson 5

Locked

Path update

Implement the changePath(String, String) method so that it replaces the jdk version in the path passed in the first method parameter with the version passed in the second parameter, and returns the new path. The JDK version starts with "jdk" and ends at "/". Example: path - "/usr/java/jdk1.8/bin" J

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