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.