1. Libraries

The Java standard library is large. Prefer standard library APIs first, and add third-party dependencies only when the standard library does not cover the problem well.

1.1. Math

for (int i = 0; i < 5; i++) {
  var num = Math.random();
  var s = String.format("%.5f", num);
  System.out.println(s);
}

1.2. StringBuilder

var entity = "One-Off Coder";
var address = "7526 Old Linton Hall Road";
var city = "Gainesville";
var state = "VA";
var zip = 20155;
var www = "https://www.oneoffcoder.com";
var email = "info@oneoffcoder.com";

var label = new StringBuilder()
    .append(entity).append('\n')
    .append(address).append('\n')
    .append(city).append(' ')
    .append(state).append(' ')
    .append(zip).append('\n')
    .append(www).append('\n')
    .append(email)
    .toString();

System.out.println(label);

1.3. HTTP

Use java.net.http.HttpClient for new HTTP code. JDK 26 adds HTTP/3 support to this API, but ordinary beginner code still uses the same HttpClient, HttpRequest, and HttpResponse types.

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

String uri = "http://www.oneoffcoder.com";

HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create(uri))
    .build();

HttpResponse<String> response =
    client.send(request, HttpResponse.BodyHandlers.ofString());

String body = response.body();

System.out.println(body.length());

1.4. CSV

1.4.1. Writing data to a CSV

import com.opencsv.CSVWriterBuilder;
import com.opencsv.ICSVParser;
import com.opencsv.ICSVWriter;
import java.io.FileWriter;

var writer = new CSVWriterBuilder(new FileWriter("demo.csv"))
    .withSeparator(ICSVParser.DEFAULT_SEPARATOR)
    .withQuoteChar(ICSVParser.DEFAULT_QUOTE_CHARACTER)
    .withEscapeChar(ICSVParser.DEFAULT_ESCAPE_CHARACTER)
    .withLineEnd(ICSVWriter.DEFAULT_LINE_END)
    .build();

var entries = new String[][] {
    { "first_name", "last_name" },
    { "John", "Doe" },
    { "Jane", "Smith" }
};

for (String[] row : entries) {
  writer.writeNext(row);
}

writer.close();

1.4.2. Reading data from a CSV

import com.opencsv.CSVParserBuilder;
import com.opencsv.CSVReaderBuilder;
import com.opencsv.ICSVParser;
import java.io.FileReader;

final var parser = new CSVParserBuilder()
    .withSeparator(ICSVParser.DEFAULT_SEPARATOR)
    .withQuoteChar(ICSVParser.DEFAULT_QUOTE_CHARACTER)
    .withEscapeChar(ICSVParser.DEFAULT_ESCAPE_CHARACTER)
    .build();

final var reader = new CSVReaderBuilder(new FileReader("demo.csv"))
    .withSkipLines(1)
    .withCSVParser(parser)
    .build();

String[] line;
while ((line = reader.readNext()) != null) {
  for (int i = 0; i < line.length; i++) {
    System.out.print(line[i]);
    if (i < line.length - 1) {
      System.out.print(", ");
    } else if (i == line.length - 1) {
      System.out.println();
    }
  }
}

reader.close();

1.5. Static imports

import static java.lang.Math.pow;
import static java.lang.Math.sqrt;

var num = 100.0d;
var s = sqrt(num);
var p = pow(num, 2.0d);

1.6. String tokenization

import java.util.StringTokenizer;

String s = "firstName=John; lastName=John; age=18";

StringTokenizer tokenizer = new StringTokenizer(s, "=;");

while (tokenizer.hasMoreTokens()) {
  String key = tokenizer.nextToken().strip();
  String val = tokenizer.nextToken().strip();

  System.out.println(key + " is " + val);
}

1.7. Optional

Optional is most useful as a return type when a method may not have a result. Avoid using it as a field type or method parameter in beginner code; plain values and validation are usually clearer there.

1.7.1. Basic use of optional

import java.util.Optional;

var x = Optional.of("John");
// check if empty
System.out.println(x.isEmpty()); // false

// get value
System.out.println(x.get()); // John

1.7.2. Nullable optional

import java.util.Optional;

var x = Optional.ofNullable(null);
// check if empty and presentOptionalExample
System.out.println(x.isEmpty()); // true
System.out.println(x.isPresent()); // false

System.out.println(x.orElse("No name!"));

1.8. Date

Date and Calendar are legacy APIs. They appear in older code, but new code should usually use the java.time package covered in the next chapter.

1.8.1. Basic date

import java.util.Date;

Date date = new Date();

System.out.println(date.getTime());

1.8.2. Basic calendar

import java.text.SimpleDateFormat;
import java.util.Calendar;
import static java.lang.String.valueOf;
import static java.lang.String.format;

Calendar calendar = Calendar.getInstance();

var month = calendar.get(Calendar.MONTH) + 1;
var day = calendar.get(Calendar.DATE);
var year = calendar.get(Calendar.YEAR);
var hour = calendar.get(Calendar.HOUR);
var minute = calendar.get(Calendar.MINUTE);
var second = calendar.get(Calendar.SECOND);

String date = String.join("-",
    valueOf(year),
    format("%02d", month),
    format("%02d", day));

String time = String.join(":",
    format("%02d", hour),
    format("%02d", minute),
    format("%02d", second));

System.out.println(date + " " + time);

// or use SimpleDateFormat
SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
System.out.println(formatter.format(calendar.getTime()));

1.8.3. Calendar creation

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;

Calendar calendar = new GregorianCalendar(2019, 0, 31);

SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
System.out.println(formatter.format(calendar.getTime()));

1.8.4. Calendar manipulation

import java.text.SimpleDateFormat;
import java.util.Calendar;

SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");

Calendar calendar = Calendar.getInstance();

// add 1 year
calendar.add(Calendar.YEAR, 1);
System.out.println(formatter.format(calendar.getTime()));

// subtract 2 years
calendar.add(Calendar.YEAR,-2);
System.out.println(formatter.format(calendar.getTime()));

1.9. Random number generation

import java.util.Random;

Random random = new Random(37L);

var a = random.nextBoolean();
var b = random.nextDouble();
var c = random.nextInt();
var d = random.nextInt(100);
var e = random.nextFloat();
var f = random.nextGaussian();
var g = random.nextLong();