01. Variables, Data Types, Operators

Practice exercises for: Variables & Data Types · Operators

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Exercise 1 — Predict the Output: String Pool · 🟢 Easy

Given the code below, predict the output of each println without running the program:

String a = "hello";
String b = "hello";
String c = new String("hello");

System.out.println(a == b);       // (1)
System.out.println(a == c);       // (2)
System.out.println(a.equals(c));  // (3)
System.out.println(c.equals(b));  // (4)

Hint

• == compares addresses on the Stack, not content
• String literals use the String Pool — same content → same object
• new String(...) creates a new object on the Heap, bypassing the Pool
• .equals() compares character content

Solution

Line	Output	Explanation
(1)	true	a and b point to the same object in the String Pool
(2)	false	c is a separate Heap object — different address from a
(3)	true	.equals() compares content — both are "hello"
(4)	true	.equals() compares content — both are "hello"

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Exercise 2 — Fix the Bug · 🟢 Easy

The code below has 3 compile errors. Find and fix all of them:

public class Main {
    public static void main(String[] args) {
        var total;                       // (A)
        total = 100.5;

        long population = 8_000_000_000; // (B)

        float price = 19.99;             // (C)

        System.out.println(total + " / " + population + " / " + price);
    }
}

Hint

• What does var require on the right-hand side?
• What suffix does an integer literal larger than int need?
• What is the default type of a floating-point literal in Java?

Solution

Location	Error	Fix
(A)	var cannot be declared without an initializer	Replace with var total = 100.5; or double total;
(B)	8_000_000_000 exceeds int range	Add suffix L: 8_000_000_000L
(C)	19.99 is a double literal — assigning to float loses precision	Add suffix f: 19.99f, or change type to double

public class Main {
    public static void main(String[] args) {
        var total = 100.5;
        long population = 8_000_000_000L;
        float price = 19.99f;
        System.out.println(total + " / " + population + " / " + price);
    }
}

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Exercise 3 — Write Code: Student Card · 🟡 Medium

Write a class StudentCard with a main method that:

1. Declares variables: full name (String), age (int), GPA (double), enrolled (boolean), student ID (long)
2. Assigns sample data.

3. Prints in this exact format:

   ╔══════════════════════════╗
   │      STUDENT CARD        │
   ╠══════════════════════════╣
   │ Name   : Nguyen Van An   │
   │ Age    : 20              │
   │ GPA    : 3.75            │
   │ Status : Enrolled        │
   │ ID     : 20240001        │
   ╚══════════════════════════╝
   

4. Uses var for at least 2 variables.

Hint

• System.out.printf("%-16s", value) — left-align, pad to 16 characters
• Ternary for boolean: isEnrolled ? "Enrolled" : "Not Enrolled"

Solution

public class StudentCard {
    public static void main(String[] args) {
        var name           = "Nguyen Van An";
        int age            = 20;
        var gpa            = 3.75;
        boolean isEnrolled = true;
        long studentId     = 20240001L;

        String status = isEnrolled ? "Enrolled" : "Not Enrolled";

        System.out.println("╔══════════════════════════╗");
        System.out.println("│      STUDENT CARD        │");
        System.out.println("╠══════════════════════════╣");
        System.out.printf( "│ Name   : %-16s│%n", name);
        System.out.printf( "│ Age    : %-16d│%n", age);
        System.out.printf( "│ GPA    : %-16.2f│%n", gpa);
        System.out.printf( "│ Status : %-16s│%n", status);
        System.out.printf( "│ ID     : %-16d│%n", studentId);
        System.out.println("╚══════════════════════════╝");
    }
}

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