Default values in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of uninitialized instance variables

What is the output of this Java program?

Java

public class Test {
    int number;
    boolean flag;
    String text;

    public static void main(String[] args) {
        Test obj = new Test();
        System.out.println(obj.number + "," + obj.flag + "," + obj.text);
    }
}

A 0,false,empty string

B 0,true,null

C Compilation error

D 0,false,null

💡 Hint

Instance variables have default values if not initialized.

In Java, instance variables of type int default to 0, boolean to false, and object references like String to null.

❓ Predict Output

intermediate

Default values of local variables

What happens when you try to compile and run this Java code?

Java

public class Test {
    public static void main(String[] args) {
        int x;
        System.out.println(x);
    }
}

A Compilation error: variable x might not have been initialized

B Prints 0

C Runtime error

D Prints a random number

💡 Hint

Local variables must be initialized before use.

Local variables in Java do not have default values and must be explicitly initialized before use, otherwise compilation fails.

❓ Predict Output

advanced

Default values in arrays

What is the output of this Java program?

Java

public class Test {
    public static void main(String[] args) {
        boolean[] flags = new boolean[3];
        System.out.println(flags[1]);
    }
}

A null

B true

C false

D Compilation error

💡 Hint

Array elements have default values based on their type.

Boolean array elements default to false in Java.

❓ Predict Output

advanced

Default values of object fields in inheritance

What is the output of this Java program?

Java

class Parent {
    int value;
}

class Child extends Parent {
    boolean flag;
}

public class Test {
    public static void main(String[] args) {
        Child c = new Child();
        System.out.println(c.value + "," + c.flag);
    }
}

A 0,true

B 0,false

C Compilation error

D null,false

💡 Hint

Inherited fields also get default values.

Both inherited int and boolean fields get default values 0 and false respectively.

🧠 Conceptual

expert

Default values of static variables

Consider this Java class with static variables. What is the value of staticVar after the program runs?

Java

public class Test {
    static int staticVar;

    public static void main(String[] args) {
        System.out.println(staticVar);
    }
}

A 0

B Undefined behavior

C Compilation error

D null

💡 Hint

Static variables also get default values if not initialized.

Static variables of primitive types get default values like instance variables. int defaults to 0.

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