Variables

INFO

When we write code in Java, it needs to be written inside something called a class. This won't mean anything to you right now, but it will soon!

For now, it's enough to know that to make Java code run, it has to go inside some boilerplate like this:

public class Main {
  public static void main(String[] args) {
    // Your code goes here
  }
}

Using var

In Java, we can use var to create new variables.

java

public class Main {
  public static void main(String[] args) {
    var bookTitle = "Persepolis";
    System.out.println(bookTitle);
  }
}

output

Persepolis

The order of the lines matters.

java

public class Main {
  public static void main(String[] args) {
    System.out.println(bookTitle);
    var bookTitle = "Persepolis";
  }
}

output

./Main.java:3: error: cannot find symbol
    System.out.println(bookTitle);
                       ^
  symbol:   variable bookTitle
  location: class Main
1 error
error: compilation failed

Specifying type

Rather than using var, it is possible to explicitly give the type of the variable.

java

public class Main {
  public static void main(String[] args) {
    String bookTitle = "Persepolis";
    System.out.println(bookTitle);
  }
}

output

Persepolis

Whether to use var or the explicit type is a matter of opinion.

INFO

A string in programming is a sequence of text characters. There are other types, too, such as integers and booleans. We will learn all about them soon!

Redefining variables

Variables can be reassigned new values.

java

public class Main {
  public static void main(String[] args) {
    String bookTitle = "Persepolis";
    System.out.println(bookTitle);

    bookTitle = "The Kite Runner";
    System.out.println(bookTitle);
  }
}

output

Persepolis
The Kite Runner

Using final

To define a constant in Java, we use the final keyword.

java

public class Main {
  public static void main(String[] args) {
    final var libraryName = "Central Library";
    System.out.println(libraryName);
  }
}

ouput

Central Library

However, because we used final, we cannot change its value.

java

public class Main {
  public static void main(String[] args) {
    final var libraryName = "Central Library";
    System.out.println(libraryName);

    libraryName = "Quantum Codex"; // This will cause an error
  }
}

output

./Main.java:6: error: cannot assign a value to final variable libraryName
    libraryName = "Quantum Codex";
    ^
1 error
error: compilation failed

Referencing between variables

A variable which points to another variable's value does not update its value when the original variable changes.

java

public class Main {
  public static void main(String[] args) {
    String user1 = "BookishBen99";
    String user2 = user1;

    System.out.println("User 1: " + user1);
    System.out.println("User 2: " + user2);

    user1 = "pageturner_mia";

    // Notice that user2 does not change
    System.out.println("User 1: " + user1);
    System.out.println("User 2: " + user2);
  }
}

ouput

User 1: BookishBen99
User 2: BookishBen99

User 1: pageturner_mia
User 2: BookishBen99