Variables & Types

A variable gives a value a name. A declaration says what kind of value the name can hold: int minutes = 25; creates an integer variable and initializes it. String holds text; boolean holds true or false. Java checks these types before running. The spelling and capitalization matter: String begins with a capital S.

Choose names that explain the value, such as plannedMinutes rather than x. Names are case-sensitive: minutes and Minutes are different. A declaration introduces a name once in its block; later assignment uses the existing name without repeating its type. Read = as “store the value on the right in the variable on the left”, rather than as a mathematics claim that two expressions are always equal.

Follow the current value

Evaluate the right-hand side using the values that exist at that moment, then store the result. A later assignment does not go back and update earlier copies. Trace a little table with a row after every assignment. Predict the printed values from the final row, not from the first declaration.

public class Main {
    public static void main(String[] args) {
        String topic = "Java";
        int minutes = 20;
        boolean finished = false;

        minutes = minutes + 5;
        finished = true;

        System.out.println(topic);
        System.out.println(minutes);
        System.out.println(finished);
    }
}

Java
25
true

The integer update reads 20 and stores 25. The boolean assignment changes a fact about this run; it does not print anything by itself. Put a print before the update to observe both old and new values.

public class Main {
    public static void main(String[] args) {
        int planned = 20;
        int saved = planned;

        planned = 30;

        System.out.println(planned);
        System.out.println(saved);
    }
}

30
20

For these integer values, saved receives a copy of the number 20. It is not a live formula linked to planned. Reverse the assignment order and trace again. We will distinguish this from object references later.

public class Main {
    public static void main(String[] args) {
        int minutes = "25";
        System.out.println(minutes);
    }
}

Your turn

public class Main {
    public static void main(String[] args) {
        int budget = 60;
        // Copy the budget, then subtract each reservation.
    }
}

public class Main {
    public static void main(String[] args) {
        int budget = 60;
        int remaining = budget;

        remaining = remaining - 25;
        remaining = remaining - 10;

        System.out.println(remaining);
        System.out.println(budget);
    }
}

25
60

Change it and explain

Try a budget of 35, then 20. A negative result is possible arithmetic; it does not automatically stop your program. Later you will add a decision to reject an impossible plan. For now, explain what each variable means and why copying the budget protects the original number.

Use final int dailyGoal = 30; when a variable should not be assigned another value after initialization. Try changing dailyGoal afterward and read the error. Final is a rule about assignment, not a promise that all future objects are immutable. Use ordinary variables for values you intend to update.