Variable declaration and initialization in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of variable initialization with expressions

What is the output of the following Java code?

Java

public class Main {
    public static void main(String[] args) {
        int a = 5;
        int b = a * 2;
        int c = b - a / 5;
        System.out.println(c);
    }
}

A 9

B 10

C 8

D 7

💡 Hint

Remember the order of operations: division before subtraction.

a = 5, b = 10 (5*2), c = 10 - (5/5) = 10 - 1 = 9

❓ Predict Output

intermediate

Value of variable after multiple declarations

What is the value of variable num after running this code?

Java

public class Main {
    public static void main(String[] args) {
        int num = 10;
        num = num + 5;
        num = num * 2;
        System.out.println(num);
    }
}

A 15

B 20

C 30

D 25

💡 Hint

Follow the operations step by step.

num starts at 10, then 10 + 5 = 15, then 15 * 2 = 30.

❓ Predict Output

advanced

Output with uninitialized local variable

What error or output does this code produce?

Java

public class Main {
    public static void main(String[] args) {
        int x;
        // System.out.println(x); // Uncommenting this line causes error
        x = 5;
        System.out.println(x);
    }
}

A 0

B 5

C Compilation error: variable x might not have been initialized

D Runtime error

💡 Hint

The variable is assigned before printing.

Variable x is declared but not initialized at first. It is assigned 5 before printing, so output is 5.

❓ Predict Output

advanced

Output of variable shadowing in nested blocks

What is the output of this Java program?

Java

public class Main {
    public static void main(String[] args) {
        int val = 10;
        {
            int val = 20;
            System.out.println(val);
        }
        System.out.println(val);
    }
}

A Compilation error due to duplicate variable declaration

B 10\n20

C 20\n10

D 20\n20

💡 Hint

Java allows redeclaring a variable in an inner block (shadowing).

Java does not allow redeclaring a local variable with the same name in an inner block. This causes a compilation error.

🧠 Conceptual

expert

Understanding default values of instance variables

Consider the following Java class. What is the output when new Test().printValues(); is called?

Java

public class Test {
    int a;
    boolean b;
    double c;
    String d;

    void printValues() {
        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
        System.out.println(d);
    }
}

A Compilation error due to uninitialized variables

B Garbage values printed

C 0\ntrue\n0.0\n""

D 0\nfalse\n0.0\nnull

💡 Hint

Instance variables have default values if not initialized.

Instance variables get default values: int=0, boolean=false, double=0.0, String=null.