Default values in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an integer variable with its default value.

Java

int number = [1];

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

A"0"

Bnull

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using null for int variables causes errors.

Using quotes around 0 makes it a string, not an int.

In Java, the default value for an int variable is 0.

2fill in blank

medium

Complete the code to declare a boolean variable with its default value.

Java

boolean isActive = [1];

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

Anull

Bfalse

Ctrue

D"false"

Attempts:

3 left

💡 Hint

Common Mistakes

Using null for boolean causes errors.

Using quotes makes it a string, not a boolean.

The default value for a boolean in Java is false.

3fill in blank

hard

Fix the error in the code by completing the declaration of a String variable with its default value.

Java

String name = [1];

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

Anull

B0

C""

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty quotes instead of null for default String value.

Assigning 0 or false to a String causes errors.

In Java, the default value for a String (which is an object) is null.

4fill in blank

hard

Fill both blanks to declare a double and a char variable with their default values.

Java

double price = [1];
char grade = [2];

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

A0.0

B'\u0000'

C0

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 for char instead of '\u0000'.

Using null for primitive types causes errors.

The default value for double is 0.0 and for char is the null character '\u0000'.

5fill in blank

hard

Fill all three blanks to declare variables of types long, float, and boolean with their default values.

Java

long count = [1];
float rate = [2];
boolean flag = [3];

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

A0

B0.0f

Cfalse

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using null for primitives causes errors.

Forgetting the 'f' suffix for float literals.

Default values: long is 0, float is 0.0f, boolean is false.

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