Output formatting basics in Java - Mini Project: Build & Apply

Output formatting basics

📖 Scenario: You are creating a simple Java program to display a person's name and age in a clear and friendly way. This is like writing a small introduction card.

🎯 Goal: Build a Java program that stores a person's name and age, then prints a formatted sentence introducing the person.

📋 What You'll Learn

Create a variable for the person's name

Create a variable for the person's age

Use System.out.printf to print the formatted output

💡 Why This Matters

🌍 Real World

Output formatting is used in many programs to show information clearly, like printing reports, messages, or user data.

💼 Career

Knowing how to format output is a basic skill for software developers, especially when creating user interfaces or logs.

1

Create variables for name and age

Create a String variable called name and set it to "Alice". Create an int variable called age and set it to 30.

Java

// Your code here

Hint

Use String name = "Alice"; and int age = 30; inside the main method.

2

Add a format string

Create a String variable called format and set it to "My name is %s and I am %d years old.".

Java

public class Main {
    public static void main(String[] args) {
        String name = "Alice";
        int age = 30;
        // Your code here
    }
}

Hint

Use String format = "My name is %s and I am %d years old."; to prepare the format.

3

Print formatted output using printf

Use System.out.printf with the format string, passing name and age as arguments.

Java

public class Main {
    public static void main(String[] args) {
        String name = "Alice";
        int age = 30;
        String format = "My name is %s and I am %d years old.";
        // Your code here
    }
}

Hint

Use System.out.printf(format, name, age); to print the formatted sentence.

4

Print a newline after the formatted output

Add System.out.println() after the printf line to move to the next line.

Java

public class Main {
    public static void main(String[] args) {
        String name = "Alice";
        int age = 30;
        String format = "My name is %s and I am %d years old.";
        System.out.printf(format, name, age);
        // Your code here
    }
}

Hint

Use System.out.println(); to print a newline after the formatted text.

Practice

1. Which Java method is commonly used to format output with placeholders like %d or %s?

easy

A. System.out.read

B. System.out.printf

C. System.out.println

D. System.out.print

Solution

1. Step 1: Identify formatting methods in Java

   System.out.printf and System.out.format are used for formatted output, unlike println or print which just print raw text.

2. Step 2: Recognize the method that uses placeholders

   System.out.printf uses format specifiers like %d for integers and %s for strings to format output.

3. Final Answer:

   System.out.printf -> Option B

4. Quick Check:

   Formatted output uses printf [OK]

Hint: Look for method with % placeholders for formatting [OK]

Common Mistakes:

• Confusing println with printf
• Using print which lacks formatting
• Thinking read formats output

2. Which of the following is the correct syntax to print an integer variable num with two digits padded with zeros using printf?

easy

A. System.out.printf("%02d", num);

B. System.out.printf("%2d", num);

C. System.out.printf("%d2", num);

D. System.out.printf("%d02", num);

Solution

1. Step 1: Understand zero-padding format specifier

   In printf, %02d means print an integer padded with zeros to width 2.

2. Step 2: Check each option's correctness

   %2d pads with spaces, %02d pads with zeros, %d2 and %d02 are invalid formats.

3. Final Answer:

   System.out.printf("%02d", num); -> Option A

4. Quick Check:

   Zero padding uses %0Nd [OK]

Hint: Zero pad with %0 before width number [OK]

Common Mistakes:

• Using %2d pads with spaces, not zeros
• Placing digits after %d incorrectly
• Confusing padding syntax

3. What is the output of the following Java code?

int x = 7;
System.out.printf("Value: %4d!", x);

medium

A. Value: 7!

B. Value: 7 !

C. Value: 07!

D. Value:7!

Solution

1. Step 1: Understand %4d formatting

   %4d means print integer right-aligned in a field width of 4 characters, padding with spaces on the left.

2. Step 2: Apply formatting to value 7

   7 is one digit, so 3 spaces are added before it, resulting in " 7".

3. Final Answer:

   Value: 7! -> Option A

4. Quick Check:

   Right-align integer in 4 spaces [OK]

Hint: Width number pads spaces on left for integers [OK]

Common Mistakes:

• Assuming padding is on the right
• Confusing zero padding with space padding
• Ignoring field width

4. The following code is intended to print a floating-point number with 2 decimal places:

double pi = 3.14159;
System.out.printf("%.2f", pi);

What is the error in this code?

medium

A. Missing newline character causes error.

B. %.2f is invalid format specifier for double.

C. No error; it prints 3.14 correctly.

D. Must cast pi to float before printing.

Solution

1. Step 1: Check format specifier for double

   %.2f formats floating-point numbers with 2 decimal places, valid for double type.

2. Step 2: Verify code correctness

   Code prints 3.14 without error; newline is optional and casting is unnecessary.

3. Final Answer:

   No error; it prints 3.14 correctly. -> Option C

4. Quick Check:

   %.2f formats double with 2 decimals [OK]

Hint: %.Nf formats float/double to N decimals [OK]

Common Mistakes:

• Thinking casting to float is needed
• Assuming newline is mandatory
• Believing %.2f is invalid

5. You want to print a table of names and ages aligned in columns using printf. Which format string correctly aligns the name left in 10 spaces and age right in 3 spaces?

String name = "Anna";
int age = 9;
System.out.printf("???", name, age);

hard

A. "%-3s %-10d"

B. "%10s %-3d"

C. "%10s %3d"

D. "%-10s %3d"

Solution

1. Step 1: Understand alignment flags in printf

   %-10s means left-align string in 10 spaces; %3d means right-align integer in 3 spaces.

2. Step 2: Match format to requirement

   "%-10s %3d" matches left-align name and right-align age correctly; others swap or misalign.

3. Final Answer:

   "%-10s %3d" -> Option D

4. Quick Check:

   Left-align string with - flag, right-align number default [OK]

Hint: Use %- for left-align strings, default right-align numbers [OK]

Common Mistakes:

• Swapping alignment flags
• Using wrong width numbers
• Misplacing % signs