Static vs non-static behavior in Java

Introduction

Static and non-static help decide if something belongs to the whole class or just one object. This helps organize code and save memory.

When you want a value or method shared by all objects, like a counter for how many objects exist.

When you want each object to have its own separate data, like a name or age.

When you want to call a method without creating an object, like a utility method to add numbers.

When you want to keep track of something common to all objects, like a setting or configuration.

Syntax

Java

class ClassName {
    static int sharedValue; // static variable
    int individualValue;    // non-static variable

    static void staticMethod() {
        // code here
    }

    void nonStaticMethod() {
        // code here
    }
}

Static means the item belongs to the class itself, not any object.

Non-static means the item belongs to each object separately.

Examples

This class counts how many objects are created using a static variable and shows each object's id using a non-static variable.

Java

class Example {
    static int count = 0; // shared by all objects
    int id;               // unique for each object

    Example(int id) {
        this.id = id;
        count++;
    }

    static void showCount() {
        System.out.println("Total objects: " + count);
    }

    void showId() {
        System.out.println("Object id: " + id);
    }
}

Static method is called using the class name. Non-static methods are called on objects.

Java

public class Test {
    public static void main(String[] args) {
        Example e1 = new Example(1);
        Example e2 = new Example(2);

        Example.showCount(); // call static method
        e1.showId();         // call non-static method
        e2.showId();
    }
}

Sample Program

This program creates two Car objects. It uses a static variable to count all cars made and non-static variables to store each car's model.

Java

class Car {
    static int totalCars = 0; // shared by all cars
    String model;             // unique for each car

    Car(String model) {
        this.model = model;
        totalCars++;
    }

    static void showTotalCars() {
        System.out.println("Total cars made: " + totalCars);
    }

    void showModel() {
        System.out.println("Car model: " + model);
    }
}

public class Main {
    public static void main(String[] args) {
        Car c1 = new Car("Toyota");
        Car c2 = new Car("Honda");

        Car.showTotalCars();
        c1.showModel();
        c2.showModel();
    }
}

Important Notes

Static methods cannot use non-static variables directly because non-static variables belong to objects, and static methods belong to the class.

You can call static methods and access static variables without creating an object.

Non-static methods can access both static and non-static variables.

Summary

Static means shared by the whole class, non-static means unique to each object.

Use static for common data or behavior, non-static for individual object data or behavior.

Static methods can be called without objects; non-static methods need objects.

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