Default values in Java - Mini Project: Build & Apply

Default values

📖 Scenario: Imagine you are creating a simple Java program to understand how default values work for different types of variables in Java.

🎯 Goal: You will create a Java class with different types of variables and observe their default values when they are not explicitly initialized.

📋 What You'll Learn

Create a Java class named DefaultValuesDemo

Declare instance variables of types int, double, boolean, and String without initializing them

Create a method named printDefaults that prints the default values of these variables

Create a main method to create an object of DefaultValuesDemo and call printDefaults

💡 Why This Matters

🌍 Real World

Understanding default values helps avoid bugs caused by uninitialized variables in Java programs.

💼 Career

Java developers must know default values to write safe and predictable code, especially when working with classes and objects.

1

Create the class and declare variables

Create a public class named DefaultValuesDemo and declare four instance variables without initializing them: int number, double price, boolean available, and String name.

Java

public class DefaultValuesDemo {
    // Declare instance variables here
    // Your code here
}

Hint

Instance variables in Java have default values if not initialized.

2

Add the printDefaults method

Inside the DefaultValuesDemo class, create a public method named printDefaults that prints the values of number, price, available, and name using System.out.println.

Java

public class DefaultValuesDemo {
    int number;
    double price;
    boolean available;
    String name;

    // Create printDefaults method here
    // Your code here
}

Hint

Use System.out.println to print each variable with a label.

3

Add the main method

Inside the DefaultValuesDemo class, add a public static main method. Inside it, create an object of DefaultValuesDemo named demo and call the printDefaults method on it.

Java

public class DefaultValuesDemo {
    int number;
    double price;
    boolean available;
    String name;

    public void printDefaults() {
        System.out.println("number = " + number);
        System.out.println("price = " + price);
        System.out.println("available = " + available);
        System.out.println("name = " + name);
    }

    // Create main method here
    // Your code here
}

Hint

The main method is the program entry point. Create an object and call the method.

4

Run and observe the output

Run the program and observe the output printed by the printDefaults method. The output should show the default values of the variables.

Java

public class DefaultValuesDemo {
    int number;
    double price;
    boolean available;
    String name;

    public void printDefaults() {
        System.out.println("number = " + number);
        System.out.println("price = " + price);
        System.out.println("available = " + available);
        System.out.println("name = " + name);
    }

    public static void main(String[] args) {
        DefaultValuesDemo demo = new DefaultValuesDemo();
        demo.printDefaults();
    }
}

// Run the program to see the output

Hint

Default values for instance variables are 0 for int, 0.0 for double, false for boolean, and null for String.

Practice

1. In Java, what is the default value of an uninitialized int variable inside a class?

easy

A. 0

B. null

C. false

D. 1

Solution

1. Step 1: Understand default values for primitive types

   In Java, primitive numeric types like int default to 0 if not initialized.

2. Step 2: Apply to int variable

   An uninitialized int inside a class will have the default value 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   int default = 0 [OK]

Hint: Remember: int defaults to zero, not null or false [OK]

Common Mistakes:

• Confusing int default with null (which is for objects)
• Thinking boolean default applies to int
• Assuming uninitialized variables cause compile error

2. Which of the following is the correct way to declare a boolean instance variable with default value in Java?

easy

A. boolean flag = true;

B. boolean flag = null;

C. boolean flag = 0;

D. boolean flag;

Solution

1. Step 1: Check valid boolean declarations

   In Java, a boolean can be declared without initialization; it defaults to false.

2. Step 2: Identify invalid initializations

   Assigning null or 0 to boolean is invalid and causes errors.

3. Final Answer:

   boolean flag; -> Option D

4. Quick Check:

   boolean defaults to false if uninitialized [OK]

Hint: Declare boolean without value to get default false [OK]

Common Mistakes:

• Assigning null to boolean (not allowed)
• Using 0 or 1 as boolean values
• Assuming default boolean is true

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

class Test {
    int x;
    boolean b;
    String s;
    void printValues() {
        System.out.println(x + "," + b + "," + s);
    }
}
public class Main {
    public static void main(String[] args) {
        Test t = new Test();
        t.printValues();
    }
}

medium

A. 0,false,null

B. 0,true,null

C. 0,false,

D. Compilation error

Solution

1. Step 1: Identify default values for fields

   Instance variables: int defaults to 0, boolean to false, and String (object) to null.

2. Step 2: Understand print output

   Printing concatenates values with commas, so output is "0,false,null".

3. Final Answer:

   0,false,null -> Option A

4. Quick Check:

   int=0, boolean=false, String=null [OK]

Hint: Remember objects default to null, primitives to zero or false [OK]

Common Mistakes:

• Assuming String defaults to empty string instead of null
• Thinking boolean defaults to true
• Expecting compilation error due to uninitialized variables

4. Consider this Java code snippet:

public class Sample {
    static int count;
    public static void main(String[] args) {
        int number;
        System.out.println(count);
        System.out.println(number);
    }
}

What is the error in this code?

medium

A. Static variable 'count' cannot be accessed in static method

B. Local variable 'number' might not have been initialized

C. Syntax error in class declaration

D. No error, code runs fine

Solution

1. Step 1: Check variable initialization rules

   Static variable 'count' is initialized to 0 by default and can be printed.

2. Step 2: Analyze local variable 'number'

   Local variables must be explicitly initialized before use; 'number' is not initialized.

3. Final Answer:

   Local variable 'number' might not have been initialized -> Option B

4. Quick Check:

   Local vars need explicit init [OK]

Hint: Local variables need explicit initialization before use [OK]

Common Mistakes:

• Assuming all variables get default values
• Thinking static variables can't be accessed in static methods
• Ignoring compiler error for uninitialized local variables

5. You want to create a Java class with an array of double values representing temperatures. You declare:

double[] temps = new double[5];

What are the default values stored in temps immediately after this declaration?

hard

A. All elements are null

B. All elements are NaN

C. All elements are 0.0

D. Array elements are uninitialized and cause error if accessed

Solution

1. Step 1: Understand default values for array elements

   In Java, arrays of primitive types get default values for each element; for double, it's 0.0.

2. Step 2: Apply to the given array

   After new double[5], all 5 elements are initialized to 0.0 automatically.

3. Final Answer:

   All elements are 0.0 -> Option C

4. Quick Check:

   double array defaults to 0.0 [OK]

Hint: Primitive arrays auto-fill with zero values for their type [OK]

Common Mistakes:

• Thinking array elements default to null (only for object arrays)
• Assuming elements are NaN by default
• Believing uninitialized array elements cause runtime error