Static vs non-static behavior in Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a static variable.

Java

public class Example {
    static int [1] = 10;
}

Drag options to blanks, or click blank then click option'

Afinal

Bthis

Ccount

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using keywords like 'this' or 'void' as variable names.

Forgetting to provide a variable name.

The variable count is declared as static, meaning it belongs to the class, not instances.

2fill in blank

medium

Complete the code to call a static method from the class.

Java

public class Calculator {
    public static int add(int a, int b) {
        return a + b;
    }

    public static void main(String[] args) {
        int result = Calculator.[1](5, 3);
        System.out.println(result);
    }
}

Drag options to blanks, or click blank then click option'

Amultiply

Badd

Csubtract

Dsum

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a method name that does not exist.

Trying to call a non-static method without an instance.

The static method add is called using the class name Calculator.

3fill in blank

hard

Fix the error in accessing a non-static variable from a static method.

Java

public class Counter {
    int count = 0;

    public static void main(String[] args) {
        Counter c = new Counter();
        System.out.println(c.[1]);
    }
}

Drag options to blanks, or click blank then click option'

Acount

Bthis.count

Cstatic count

DCounter.count

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to access non-static variables directly inside static methods.

Using 'this' keyword inside static methods.

To access a non-static variable inside a static method, you need an instance. Here, c.count is correct.

4fill in blank

hard

Fill both blanks to create a static method that returns the static variable.

Java

public class Data {
    static int value = 100;

    public static [1] getValue() {
        return [2];
    }
}

Drag options to blanks, or click blank then click option'

Aint

Bvalue

Cvoid

Dthis.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as return type when a value is returned.

Using 'this.value' inside a static method.

The method returns an int and returns the static variable value.

5fill in blank

hard

Fill both blanks to create a class with a static variable, a non-static variable, and a method that prints both.

Java

public class Info {
    static String [1] = "StaticVar";
    String [2] = "InstanceVar";

    public void printInfo() {
        System.out.println([1] + " and " + [2]);
    }
}

Drag options to blanks, or click blank then click option'

AstaticVar

BinstanceVar

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for both static and instance variables.

Trying to access instance variables without an object.

The static variable is named staticVar, the instance variable instanceVar, and the method prints both correctly.

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