Unit 2 — String Objects

• String Objects - Concatenation, Literals, and More
  • Creating Strings:
  • 📝 Three Ways to Create a String: Let’s Break it Down!
    • First Option:
  • Second Option:
  • Third Option:
• 📝 What is concatentation?
• 📝 Backwards and Forwards Slashes
  • Backslashes
• 📝 String methods

String Objects - Concatenation, Literals, and More

Creating Strings:

public class StringObjects
{
    public static void main(String[] args)
    {
        String name1 = "Skibidi";
        String name2 = new String("Sigma");
        String name3 = new String(name1);

        System.out.println(name1);
        System.out.println(name2);
        System.out.println(name3);
    }
}

StringObjects.main(null);

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📝 Three Ways to Create a String: Let’s Break it Down!

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First Option:

String name1 = "Skibidi";

This method of creating a string consists of the following three parts:

1. Class Name — Defines the type of variable (String, Integer, Array, etc.)

2. Variable Name — The name assigned to the variable. This is how the variable will be referenced in the rest of the program.

3. String Value — The actual value you’d like to assign to the variable

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Second Option:

String name2 = new String("Sigma");

This method is similar to option one but consists of a few extra components (indicated in pink coloring below):

1. Class Name — Again, defines the type of the variable (String, Integers, Arrays, etc.)

2. Variable Name — How the variable will be referenced in the rest of the program

3. new — A Java keyword which is used to explicitly instantiate a new object.

4. Class Name (Part 2) — The ‘new’ key word must be followed with the class name of the object being created (in this case, String)

5. String Value — The actual value you’d like to assign to this variable

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Third Option:

String string1 = "YIPEE"
String name2 = new String(string1);

Finally, a string can be created using the value of another, existing string object. The components needed for this declaration are as follows:

1. Class Name — Again, type of variable will it be? (String, Integers, Arrays, etc.)

2. Variable Name — The name of the variable

3. Java Keyword ‘new’ — The new keyword in Java is used to explicitly create a new object.

4. Class Name (Part 2) — As indicated before, the new keyword has to be followed with the class name of the object being created.

5. Variable Name (Part 2) — The name of the variable whose value you want the new variable to take on.

Importantly, regardless of which creation method you choose, String objects are immutable. This means that every time you attempt to change the value of some string variable, under the hood, a new string object is created with the updated value, and your variable name is used to reference this new object.

📝 What is concatentation?

Concatenation allows you to add strings together.

There are two primary ways to combine strings. Assume a, b, and c are previously created string variables.

• a += b : Appends the string value stored in b to the string value stored in a. In the processs, a is redefined to equal this newly appended string.
• c = a + b : Joins the string values of a and b together, but doesn’t redefine either a or b. Instead, the resultant value is assigned to c.

public class Concatentations
{
    public static void main(String[] args)
    {
        String name1 = "Skibidi";
        String name2 = new String("Sigma");
        String name3 = new String(name1);

        int number1 = 1;
        int number2 = 2;
        String combine = name1 + "" + number1;
        name1 += number2; 

        System.out.println(name1); 
        System.out.println(combine);
    }
}

Concatentations.main(null);

Let’s do an exercise for practice! What will the following code segment print?

public class Concatentations
{
    public static void main(String[] args)
    {
        String name1 = "Skibidi";
        String name2 = new String("Sigma");
        String name3 = new String(name1);

        name1 += "!!"
        String mystery = name1 + name2 + name3

        System.out.println(mystery);
    }
}

// Uncomment the following method call to run the code and check your answer!
// Concatentations.main(null);

📝 Backwards and Forwards Slashes

In Java, there are a few characters with pre-assigned purposes. Backwards and forwards slashes are such characters, and they can be easy to mix up, so it’s important to pay close attention to them!

\: Starts escape sequences. In other words, it can allow you to add special characters to your string. /: Usually used as a division operator. Two forward slashes indicate the beginning of a comment.

Backslashes

\" = “Escapes” the quote, allowing for you to have quotes within a string without ending the string

\\ = Indicates a literal backslash in the string.

\n = A newline character

public class SlashDemo {
    public static void main(String[] args) {
        // Using backslashes for escape sequences:
        System.out.println("This is a double quote: \"");  // Prints a double quote
        System.out.println("This is a backslash: \\");     // Prints a backslash
        System.out.println("This prints on a new line:\nSecond line starts here");

        // Using forward slashes for division and comments:
        int a = 10;
        int b = 2;
        int result = a / b; // Division operation

        System.out.println("Result of 10 / 2 is: " + result); // Prints the result of the division
    }
}

SlashDemo.main(null)

📝 String methods

The following are some important methods that can be used on String objects.

Method	Description
String(String str)	Creates a new String object with the same sequence of characters as the specified string str
int length()	The number of characters in the String object
String substring(int from, int to)	Returns the substring beginning at index from and ending at index to - 1. In other words, the start is inclusive and the end is exclusive.
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 the calling string is equal to other; returns false otherwise
int compareTo(String other)	Returns a value < 0 if the calling string is alphanumerically less than other; returns 0 if it is equal to other; returns a value > 0 if it is alphanumerically greater than other.

public class StringMethodDemo {
    public static void main(String[] args) {
        String wordOfDay = new String("Skibidi");

        System.out.print("Characters in the word 'Skibidi' — ");
        System.out.println(wordOfDay.length());

        System.out.println("\nThis should return -1, since there is no n in the string");
        System.out.println(wordOfDay.lastIndexOf("n"));

        System.out.println("\nThis should display the index of d (5)");
        System.out.println(wordOfDay.lastIndexOf("d"));
        
        // NOTE: Start is inclusive, end is exclusive
        System.out.println("\nThis should display the letters between the 2nd and 6th");
        System.out.println(word.substring(2,6));
    }
}

StringMethodDemo.main(null)

Brief Aside: Substring, like python slicing has the first index be inclusive and the last one be exclusive.

Quick, let’s do an exercise for practice! What will the following code segment return?

public class SubstringOfDemo {
    public static void main(String[] args) {
        String word = new String("skibidi");

        System.out.println("\nWhat is printed if we only pass one parameter into the substring method?");
        System.out.println(word.substring(2));
    }
}

// Uncomment the following method call to run the code and check your answer!
// SubstringOfDemo.main(null)

public class CompareToDemo {
    public static void main(String[] args) {
        String word = new String("skibidi");
        String word2 = new String("skibidi1");
        String word3 = new String("skibidi");

        System.out.println("\nIf word is < word2, a negative value will be printed. If they are equal, 0 will be printed, and if word > word2, a positive value is printed");
        System.out.println(word.compareTo(word2));

        System.out.println("\nComparison between word and word3");
        System.out.println(word.compareTo(word3));
    }
}

CompareToDemo.main(null)

public class EqualToDemo {
    public static void main(String[] args) {
        String word = new String("skibidi");
        String word2 = new String("skibidi1");
        String word3 = new String("skibidi");

        System.out.println("\nThis displays if word1 = word2, if false it returns false, if true it returns true");
        System.out.println(word.equals((word2)));

        System.out.println("\nThis displays if word1 = word3, if false it returns false, if true it returns true");
        System.out.println(word.equals((word3)));
    }
}

EqualToDemo.main(null)