Static variables in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a static variable in Java?

A static variable is a variable that belongs to the class, not to any specific object. It is shared by all instances of the class.

beginner

How many copies of a static variable exist in a Java program?

Only one copy of a static variable exists, regardless of how many objects of the class are created.

beginner

How do you declare a static variable in Java?

Use the keyword static before the variable type, for example: static int count;

beginner

Can static variables be accessed without creating an object?

Yes, static variables can be accessed using the class name directly, like <code>ClassName.variableName</code>.

beginner

What happens to a static variable when you change it through one object?

Changing a static variable through one object changes it for all objects because there is only one shared copy.

What keyword is used to declare a static variable in Java?

A volatile

B final

C static

D const

The keyword static declares a variable that belongs to the class.

How many copies of a static variable exist across all instances of a class?

A One

B One per object

C None

D Depends on the number of threads

Static variables have only one copy shared by all objects.

How do you access a static variable without creating an object?

A Using the class name

B Using the object reference

C Using the super keyword

D You cannot access it without an object

Static variables can be accessed directly by the class name.

If you change a static variable through one object, what happens to other objects?

A They get an error

B They keep their own copy

C They reset the value

D They see the updated value

All objects share the same static variable, so changes are visible to all.

Which of these is NOT true about static variables?

A They are shared by all instances

B Each object has its own copy

C They belong to the class

D They can be accessed without creating an object

Static variables have only one copy shared by all objects, not one per object.

Explain what a static variable is and how it differs from instance variables in Java.

Think about who owns the variable: the class or the object?

Describe how you would use a static variable to count how many objects of a class have been created.

Counting objects means sharing a number that all objects can see and update.

Practice

1. What is a key characteristic of a static variable in Java?

easy

A. It belongs to the class and is shared by all instances.

B. It is unique to each object created from the class.

C. It can only be accessed inside instance methods.

D. It is automatically deleted when an object is destroyed.

Solution

1. Step 1: Understand static variable ownership

   A static variable belongs to the class itself, not to any individual object.

2. Step 2: Recognize sharing behavior

   Because it belongs to the class, all objects share the same static variable value.

3. Final Answer:

   It belongs to the class and is shared by all instances. -> Option A

4. Quick Check:

   Static variable = shared by all objects [OK]

Hint: Static means shared by class, not per object [OK]

Common Mistakes:

• Thinking static variables are unique per object
• Confusing static with instance variables
• Assuming static variables are deleted with objects

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

easy

A. static int count;

B. int static count;

C. int count static;

D. count static int;

Solution

1. Step 1: Recall Java syntax for static variables

   The keyword static comes before the type when declaring a static variable.

2. Step 2: Check each option

   Only static int count; follows the correct syntax.

3. Final Answer:

   static int count; -> Option A

4. Quick Check:

   static keyword before type = correct syntax [OK]

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

Common Mistakes:

• Placing static after the variable name
• Mixing order of keywords
• Using invalid syntax order

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

class Test {
    static int count = 0;
    Test() {
        count++;
    }
    public static void main(String[] args) {
        new Test();
        new Test();
        System.out.println(Test.count);
    }
}

medium

A. Compilation error

B. 1

C. 0

D. 2

Solution

1. Step 1: Understand static variable incrementation

   Each time a new Test object is created, the constructor increments count by 1.

2. Step 2: Count the number of objects created

   Two objects are created, so count becomes 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Two objects increment count twice = 2 [OK]

Hint: Static count increments with each new object [OK]

Common Mistakes:

• Assuming count resets for each object
• Confusing instance and static variables
• Expecting compilation error due to static usage

4. Identify the error in the following code snippet:

class Counter {
    static int count;
    void increment() {
        count++;
    }
    public static void main(String[] args) {
        Counter c = new Counter();
        c.increment();
        System.out.println(count);
    }
}

medium

A. Cannot access static variable from instance method.

B. No error; code runs and prints 1.

C. Cannot access static variable directly in static method without class name.

D. Static variable count is not initialized.

Solution

1. Step 1: Check static variable initialization

   Static int variables default to 0 if not explicitly initialized.

2. Step 2: Verify method calls and access

   The instance method increment() correctly increments the static variable. The static method main accesses count directly, which is allowed.

3. Final Answer:

   No error; code runs and prints 1. -> Option B

4. Quick Check:

   Static variable accessible and increments correctly [OK]

Hint: Static vars default to 0; accessible in static main [OK]

Common Mistakes:

• Thinking static vars must be explicitly initialized
• Assuming static vars can't be accessed without class name inside same class
• Confusing instance and static method access rules

5. Consider this Java class:

class Employee {
    static int employeeCount = 0;
    String name;
    Employee(String name) {
        this.name = name;
        employeeCount++;
    }
    static void printCount() {
        System.out.println("Total employees: " + employeeCount);
    }
}

public class Main {
    public static void main(String[] args) {
        Employee e1 = new Employee("Alice");
        Employee e2 = new Employee("Bob");
        e1.printCount();
        Employee.printCount();
    }
}

What will be the output when running Main?

hard

A. Total employees: 1
Total employees: 2

B. Total employees: 2
Total employees: 1

C. Total employees: 2
Total employees: 2

D. Compilation error due to calling static method on instance

Solution

1. Step 1: Count employees created

   Two Employee objects are created, so employeeCount becomes 2.

2. Step 2: Understand static method calls

   Calling printCount() on instance e1 or class Employee both print the same static variable value.

3. Final Answer:

   Total employees: 2 Total employees: 2 -> Option C

4. Quick Check:

   Static method prints shared count = 2 [OK]

Hint: Static method prints shared count; instance call allowed [OK]

Common Mistakes:

• Thinking static method can't be called on instance
• Assuming employeeCount resets per object
• Confusing output order or values