Working with strings in Java

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 seven of the main ones.

boolean equals(String str)

boolean equalsIgnoreCase(String str)

int compareTo(String str)

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");
}

10

Task

New Java Syntax, level 10, lesson 4

Locked

Populate the Friend class

A friend (Friend class) must have three initializers (three initialize methods): - Name - Name, age - Name, age, sex. Note: name is a String, age is an int, and sex is a char.

Open

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 forward 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.

10

Task

New Java Syntax, level 10, lesson 4

Locked

Populate the Dog class

A dog (Dog class) must have three initializers: - Name - Name, height - Name, height, color.

Open

3. Creating substrings

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)

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"

10

Task

New Java Syntax, level 10, lesson 4

Locked

Let's put together a rectangle

The data for the rectangle (Rectangle class) will be top, left, width, and height. Create as many initialize(...) methods as possible Examples: - 4 parameters are specified: left, top, width, height - width/height is not specified (both are 0); - height is not specified (it is equal to the width), w

Open

10

Task

New Java Syntax, level 10, lesson 4

Locked

Initializing objects

A person (Person class) should have a String name and int age. Add the initialize(String name, int age) method where you will initialize the variables name and age. In the main method, create a Person object and store a reference to it in the variable person. Call the initialize method with any valu

Open

10

Task

New Java Syntax, level 10, lesson 4

Locked

Initializing cats

A cat (Cat class) must have five initializers: - Name - Name, weight, age - Name, age (standard weight) - Weight, color (unknown name, address and age, i.e. a homeless cat) - Weight, color, address (someone else's pet). The initializer's job is to make the object valid. For example, if the weight i

Open

10

Task

New Java Syntax, level 10, lesson 4

Locked

Populate the Circle class

The Circle class must have three initializers: - centerX, centerY, radius - centerX, centerY, radius, width - centerX, centerY, radius, width, color.

Open

10

Task

New Java Syntax, level 10, lesson 4

Locked

Initializing objects

Study the Person class carefully. Correct the class so that only one initialize method initializes all of the Person class's instance variables.

Open