String Formatting in Java

The String.format() method allows creating formatted strings using specified format specifiers and arguments.

• format(String format, Object... args): Uses the default locale.
• format(Locale l, String format, Object... args): Applies a specific locale during formatting.

Date and Time Formatting

Date and time representations can be customized using specific conversion characters prefixed with %t.

General Data Type Formatting

These specifiers apply to any argument type.

import java.util.Date;

public class FormatDemo {
    public static void main(String[] args) {
        Date now = new Date();
        String yr = String.format("%tY", now);
        String mo = String.format("%tm", now);
        String dy = String.format("%td", now);
        System.out.printf("Date: %s-%s-%s%n", yr, mo, dy);

        String hr = String.format("%tH", now);
        String mn = String.format("%tM", now);
        String sc = String.format("%tS", now);
        System.out.printf("Time: %s:%s:%s%n", hr, mn, sc);

        String fullIso = String.format("%tF", now);
        String fullTime = String.format("%tT", now);
        System.out.println("ISO Date: " + fullIso);
        System.out.println("24-Hour Time: " + fullTime);

        String boolVal = String.format("%b", 10 > 5);
        String decVal = String.format("%d", 255);
        String hexVal = String.format("%x", 255);
        System.out.println("Comparison result: " + boolVal);
        System.out.println("Decimal 255 -> Hex: " + hexVal);
    }
}

Regular Expressions

Regular expressions define search patterns for string matching and validation. They rely heavily on metacharacters and quantifiers.

Common Metacharacters

Character classes use brackets to define sets of valid characters, such as [a-z] for lowercase letters or [^0-9] for non-digits. Quantifiers specify occurrence counts:

public class RegexValidator {
    public static void main(String[] args) {
        String emailPattern = "\\w+@\\w+(\\.\\w{2,3})*\\.\\w{2,3}";
        String[] testEmails = {"user@", "invalid", "admin@example.org"};

        for (String address : testEmails) {
            if (address.matches(emailPattern)) {
                System.out.println(address + " is a valid email format.");
            } else {
                System.out.println(address + " is NOT a valid email format.");
            }
        }
    }
}

StringBuilder for Efficient String Manipulation

String objects in Java are immutable; operations like concatenation create new instances, which degrades performance in loops. The StringBuilder class provides a mutable sequence of characters, starting with a default capacity of 16. It automatically expands when needed.

Core methods include:

• append(Object obj): Appends the string representation of the argument.
• insert(int offset, Object obj): Inserts the argument at the specified position.
• delete(int start, int end): Removes characters in the specified substring.

public class StringBuilderPerf {
    public static void main(String[] args) {
        int iterations = 10000;

        long startMs = System.currentTimeMillis();
        String immutableStr = "";
        for (int i = 0; i < iterations; i++) {
            immutableStr += i;
        }
        long endMs = System.currentTimeMillis();
        System.out.println("String concatenation took: " + (endMs - startMs) + " ms");

        startMs = System.currentTimeMillis();
        StringBuilder mutableBuffer = new StringBuilder("");
        for (int i = 0; i < iterations; i++) {
            mutableBuffer.append(i);
        }
        endMs = System.currentTimeMillis();
        System.out.println("StringBuilder took: " + (endMs - startMs) + " ms");
    }
}

Practical String Operations

public class StringExercises {
    public static void main(String[] args) {
        // Case conversion
        String mixedCase = "AbCdEfG";
        System.out.println("Upper: " + mixedCase.toUpperCase());
        System.out.println("Lower: " + mixedCase.toLowerCase());

        // Substring comparison
        String phrase1 = "programming";
        String phrase2 = "programmatic";
        String sub1 = phrase1.substring(0, 7);
        String sub2 = phrase2.substring(0, 7);
        System.out.println("Substrings equal? " + sub1.equals(sub2));

        // Mobile number regex validation
        String phoneRegex = "^1[3-9]\\d{9}$";
        String mobile = "1381234567";
        System.out.println(mobile + " valid? " + mobile.matches(phoneRegex));

        // StringBuilder number appending
        StringBuilder sequence = new StringBuilder("Numbers:");
        for (int k = 1; k <= 10; k++) {
            sequence.append(k);
        }
        System.out.println(sequence.toString());
    }
}