Static variables in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of static variable increment

What is the output of the following Java code?

Java

public class Test {
    static int count = 0;
    public Test() {
        count++;
    }
    public static void main(String[] args) {
        Test t1 = new Test();
        Test t2 = new Test();
        Test t3 = new Test();
        System.out.println(Test.count);
    }
}

A Compilation error

B 0

C 1

D 3

💡 Hint

Static variables are shared among all instances of a class.

The static variable 'count' is incremented each time a new Test object is created. Since three objects are created, the final value is 3.

❓ Predict Output

intermediate

Static variable behavior with multiple classes

What will be printed when running this Java code?

Java

class A {
    static int x = 5;
}
class B extends A {
    static int x = 10;
}
public class Main {
    public static void main(String[] args) {
        System.out.println(A.x);
        System.out.println(B.x);
    }
}

A 10\n10

B 5\n5

C 5\n10

D Compilation error

💡 Hint

Static variables are not overridden but hidden in subclasses.

Class B declares its own static variable 'x' which hides A's 'x'. Accessing A.x prints 5, B.x prints 10.

❓ Predict Output

advanced

Static variable initialization order

What is the output of this Java code?

Java

public class Counter {
    static int count;
    static {
        count = count + 1;
    }
    public static void main(String[] args) {
        System.out.println(count);
    }
}

A 1

B Compilation error: variable count might not have been initialized

C Runtime error: NullPointerException

D 0

💡 Hint

Static variables of type int are initialized to 0 by default before static blocks execute.

Static int variables are initialized to 0 by default before the static initializer block runs. The block sets count = 0 + 1 = 1, so it prints 1.

❓ Predict Output

advanced

Static variable shared across threads

What is the output of this Java program?

Java

public class SharedCounter implements Runnable {
    static int counter = 0;
    public void run() {
        for(int i = 0; i < 1000; i++) {
            counter++;
        }
    }
    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new SharedCounter());
        Thread t2 = new Thread(new SharedCounter());
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println(counter);
    }
}

A 2000

B Less than 2000

C 1000

D Compilation error

💡 Hint

Incrementing a shared static variable without synchronization can cause race conditions.

Both threads increment the static counter 1000 times each. Without synchronization, increments can overlap causing lost updates, so the final value is less than 2000.

🧠 Conceptual

expert

Static variable memory allocation

Which statement about static variables in Java is correct?

A Static variables are stored in the method area and shared among all instances.

B Static variables are stored in the stack and unique to each thread.

C Static variables are stored in the heap and shared among all instances of the class.

D Static variables are stored inside each object instance separately.

💡 Hint

Think about where class-level data is stored in Java memory model.

Static variables belong to the class and are stored in the method area (part of JVM memory), shared by all instances. They are not stored in heap per instance or stack.

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