Static vs non-static behavior in Java - Hands-On Comparison

Static vs Non-Static Behavior in Java

📖 Scenario: Imagine you are creating a simple program to track the number of books created in a library system. Each book has its own title, but the total number of books is shared across all books.

🎯 Goal: You will build a Java program that shows the difference between static and non-static behavior by counting how many book objects are created and displaying each book's title.

📋 What You'll Learn

Create a Book class with a non-static title variable

Add a static variable bookCount to count all books created

Create a constructor that sets the title and increments bookCount

Write code to create multiple Book objects

Print each book's title and the total number of books using static and non-static variables

💡 Why This Matters

🌍 Real World

Tracking shared data like total users, total sales, or total items created is common in many software systems.

💼 Career

Understanding static vs non-static behavior is essential for writing efficient and correct Java programs in professional development.

1

Create the Book class with a non-static title

Create a class called Book with a non-static String variable called title. Add a constructor that takes a String parameter called title and sets the class variable this.title to it.

Java

public class Book {
    // Your code here
}

Hint

Remember, non-static variables belong to each object. Use this.title to set the instance variable.

2

Add a static variable to count books

Inside the Book class, add a static integer variable called bookCount and initialize it to 0. Then, inside the constructor, add code to increase bookCount by 1 each time a new Book object is created.

Java

public class Book {
    String title;
    // Your code here

    public Book(String title) {
        this.title = title;
        // Your code here
    }
}

Hint

Static variables belong to the class, not to individual objects. Increment bookCount inside the constructor.

3

Create multiple Book objects in main method

Create a public static void main(String[] args) method inside a class called Library. Inside main, create three Book objects with titles "Java Basics", "Python Guide", and "Web Development".

Java

public class Book {
    String title;
    static int bookCount = 0;

    public Book(String title) {
        this.title = title;
        bookCount++;
    }
}

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

Hint

Remember to create the Library class and write the main method inside it. Use the new keyword to create objects.

4

Print each book's title and total book count

Inside the main method, add code to print each book's title using System.out.println and the non-static title variable. Then print the total number of books created using the static variable Book.bookCount.

Java

public class Book {
    String title;
    static int bookCount = 0;

    public Book(String title) {
        this.title = title;
        bookCount++;
    }
}

public class Library {
    public static void main(String[] args) {
        Book book1 = new Book("Java Basics");
        Book book2 = new Book("Python Guide");
        Book book3 = new Book("Web Development");

        // Your code here
    }
}

Hint

Use System.out.println to print each book's title and the static bookCount with the class name Book.bookCount.

Practice

1. Which statement correctly describes a static method in Java?

easy

A. It belongs to the class and can be called without creating an object.

B. It belongs to an object and requires an instance to be called.

C. It can only access non-static variables directly.

D. It cannot be called from the main method.

Solution

1. Step 1: Understand static method ownership

   A static method belongs to the class itself, not to any object instance.

2. Step 2: Check how static methods are called

   Static methods can be called directly using the class name without creating an object.

3. Final Answer:

   It belongs to the class and can be called without creating an object. -> Option A

4. Quick Check:

   Static method = class-level method [OK]

Hint: Static methods belong to class, call without object [OK]

Common Mistakes:

• Thinking static methods need an object to call
• Confusing static with instance methods
• Assuming static methods access instance variables directly

2. Which of the following is the correct way to declare a static variable in Java?

easy

A. int count static = 0;

B. int static count = 0;

C. count static int = 0;

D. static int count = 0;

Solution

1. Step 1: Recall Java syntax for static variables

   The keyword static comes before the data type in variable declaration.

2. Step 2: Match the correct syntax

   static int count = 0; uses static int count = 0; which is the correct order.

3. Final Answer:

   static int count = 0; -> Option D

4. Quick Check:

   static before type = correct syntax [OK]

Hint: Put static before the type when declaring variables [OK]

Common Mistakes:

• Placing static after the variable name
• Using invalid order of keywords
• Confusing static with final keyword

3. What will be the output of the following Java code?

class Test {
    static int count = 0;
    int id;

    Test() {
        count++;
        id = count;
    }

    void display() {
        System.out.println("ID: " + id + ", Count: " + count);
    }
}

public class Main {
    public static void main(String[] args) {
        Test a = new Test();
        Test b = new Test();
        a.display();
        b.display();
    }
}

medium

A. ID: 0, Count: 2
ID: 0, Count: 2

B. ID: 1, Count: 1
ID: 2, Count: 2

C. ID: 1, Count: 2
ID: 2, Count: 2

D. ID: 1, Count: 0
ID: 2, Count: 0

Solution

1. Step 1: Understand static and instance variables in constructor

   Each time a Test object is created, count (static) increments and id (instance) is set to current count.

2. Step 2: Trace object creation and display calls

   First object: count=1, id=1; Second object: count=2, id=2. Both display show current count=2.

3. Final Answer:

   ID: 1, Count: 2 ID: 2, Count: 2 -> Option C

4. Quick Check:

   Static count shared, id unique per object [OK]

Hint: Static variables shared, instance variables unique per object [OK]

Common Mistakes:

• Assuming static variable resets per object
• Confusing instance variable values
• Ignoring static variable's shared nature

4. Identify the error in the following Java code snippet:

public class Example {
    static int number;

    public void setNumber(int number) {
        this.number = number;
    }

    public static void main(String[] args) {
        Example ex = new Example();
        ex.setNumber(5);
        System.out.println(Example.number);
    }
}

medium

A. Cannot use 'this' to access static variable inside non-static method.

B. Static variable 'number' cannot be accessed in main method.

C. 'this' cannot be used in static context like main method.

D. No error; code runs and prints 5.

Solution

1. Step 1: Analyze use of 'this' keyword in setNumber method

   'this' refers to the current object instance and cannot be used to access static variables.

2. Step 2: Understand static variable access rules

   Static variables belong to the class, so they should be accessed using the class name or directly without 'this'.

3. Final Answer:

   Cannot use 'this' to access static variable inside non-static method. -> Option A

4. Quick Check:

   'this' cannot access static variables [OK]

Hint: Use class name, not 'this', to access static variables [OK]

Common Mistakes:

• Thinking 'this' can access static variables
• Confusing static and instance variable access
• Assuming no error in code

5. You want to count how many objects of a class Book have been created. Which approach correctly uses static and non-static members to achieve this?

class Book {
    // Fill in the blanks
}

hard

A. Use a non-static int counter and static title; increment counter in main method.

B. Use a static int counter incremented in the constructor; each object has a non-static title.

C. Use a static int counter incremented in a static method; title is static too.

D. Use a non-static int counter incremented in the constructor; title is static.

Solution

1. Step 1: Identify what should be shared and what is unique

   The count of objects should be shared by all objects, so it must be static. The title is unique per book, so non-static.

2. Step 2: Implement counting in constructor

   Increment the static counter inside the constructor to count each new object created.

3. Final Answer:

   Use a static int counter incremented in the constructor; each object has a non-static title. -> Option B

4. Quick Check:

   Static counter for all objects, non-static title per object [OK]

Hint: Count objects with static variable incremented in constructor [OK]

Common Mistakes:

• Making counter non-static so it resets per object
• Making title static so all objects share same title
• Incrementing counter outside constructor