How do I use atTime() method of Java Date-Time API?

By Wayan in Java Date Time API 0 Comment

The atTime() method belongs to the LocalDate class in the Java Date-Time API, not the Date class. This method combines this date with a time to create a LocalDateTime.

Here’s an example:

package org.kodejava.datetime;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;

public class AtTimeExample {
    public static void main(String[] args) {
        // Create a LocalDate instance
        LocalDate date = LocalDate.of(2023, 1, 23);

        // Create a LocalTime instance
        // 24-hour clocks
        LocalTime time = LocalTime.of(13, 45);

        // Use atTime() to combine date and time into a LocalDateTime
        LocalDateTime dateTime = date.atTime(time);

        System.out.println(dateTime);
    }
}

Output:

2023-01-23T13:45

In this example, a LocalDate and a LocalTime are combined into a LocalDateTime using the atTime() method.

The LocalDate class also has an overloaded atTime method that takes the hour and minute directly, instead of a LocalTime instance.

Here’s an example where we set 14 hours and 30 minutes directly.

package org.kodejava.datetime;

import java.time.LocalDate;
import java.time.LocalDateTime;

public class AtTimeOtherExample {
    public static void main(String[] args) {
        // Create a LocalDate instance
        LocalDate date = LocalDate.of(2023, 1, 23);

        // Use atTime() to combine date and hour and minute into a
        // LocalDateTime
        LocalDateTime dateTime = date.atTime(14, 30);

        System.out.println(dateTime);
    }
}

Output:

2023-01-23T14:30

In this example, 14:30 (in 24-hour clock) is directly passed into atTime. There is another version of atTime() that takes hrs, min, and sec. That would look like this:

// include seconds
LocalDateTime dateTime = date.atTime(14, 30, 20);

// include nano seconds
LocalDateTime dateTime = date.atTime(14, 30, 20, 200);

These are all the overloaded versions of atTime() in LocalDate.

Remember, LocalDate, LocalTime, LocalDateTime and others from Java Date-Time API are designed to replace the old Date and Calendar classes from java.util package. They are more consistent, easier to understand and use.

How do I use map() method of Optional object?

By Wayan in Java 8, Util Package 0 Comment

The map method of the Optional class in Java is used to transform the value contained in the Optional. map allows you to apply a function on the value inside the Optional and returns an Optional that contains the result of the function.

Here is an example of how to use it:

package org.kodejava.util;

import java.util.Optional;

public class OptionalMapExample {
    public static void main(String[] args) {

        // Create an Optional<String>
        Optional<String> optional = Optional.of("Hello");

        // Use map method to transform the contained value
        Optional<Integer> transformedOptional = optional.map(String::length);

        // Use ifPresent to print the result if the Optional is not empty
        transformedOptional.ifPresent(System.out::println);
    }
}

In this example, we start with an Optional<String> that contains the string “Hello”. We then use map to apply the String::length method on the contained string. This transforms the Optional<String> into an Optional<Integer>, where the integer is the length of the string.

Lastly, we use ifPresent to print the result. In this case, the integer 5 will be printed.

Here is another example, where map helps us to handle null values:

Optional<String> optional = Optional.ofNullable(null);

// If optional is not present, it will print "0"
System.out.println(optional.map(String::length).orElse(0));

In this case, trying to apply String::length on a null value would result in a NullPointerException. However, using map in combination with Optional, allows us to safely transform the value and even provide a default result (“0” in this case) if the Optional is empty. This makes handling null values more reliable and your code less error-prone.

How do I use flatMap() method of Optional object?

By Wayan in Java 8, Util Package 0 Comment

The flatMap method is a special method in the Optional class in Java, if a method returns an Optional, you can use flatMap to avoid nested Optional<Optional<T>> situations.

Here is an example:

package org.kodejava.util;

import java.util.Optional;

public class OptionalFlatMap {
    public static void main(String[] args) {
        Optional<String> nonEmptyGender = Optional.of("male");
        Optional<String> emptyGender = Optional.empty();

        System.out.println("Non-Empty Optional:: " + nonEmptyGender.flatMap(OptionalFlatMap::getGender));
        System.out.println("Empty Optional:: " + emptyGender.flatMap(OptionalFlatMap::getGender));
    }

    static Optional<String> getGender(String gender) {
        if (gender.equals("male")) {
            return Optional.of("Gender is male");
        } else if (gender.equals("female")) {
            return Optional.of("Gender is female");
        } else {
            return Optional.empty();
        }
    }
}

In this example, two Optional<String> objects are created: one with a value (nonEmptyGender) and one without a value (emptyGender).

The flatMap method is used to apply the method getGender to the value of each Optional<String> (if it exists). Since getGender returns an Optional<String>, using flatMap avoids creating Optional<Optional<String>> objects, and instead directly returns an Optional<String>, that we can easily consume.

The getGender method returns an Optional object, that describes the gender if it is “male” or “female”, or an empty Optional if the gender is neither “male” nor “female”.

The result of calling flatMap will hence be an Optional<String> describing the gender if the gender is “male” or “female”, or an empty Optional in all other cases. This applies to both the non-empty and the empty Optional<String> in the example.

The final output will be:

Non-Empty Optional:: Optional[Gender is male]
Empty Optional:: Optional.empty

In both cases, note that flatMap directly returns the result of getGender, which itself is an Optional. This is different from if map was used, which would have resulted in a nested Optional.

How do I use filter() method of Optional object?

By Wayan in Java 8, Util Package 0 Comment

The java.util.Optional class in Java provides a filter method. It’s used to apply a condition on the value held by this Optional.

Here is an example of how to use Optional‘s filter method:

package org.kodejava.util;

import java.util.Optional;

public class OptionalFilter {
    public static void main(String[] args) {

        // Creating Optional object and assigning a value
        Optional<String> myOptional = Optional.of("Hello");

        // Applying filter method on Optional
        Optional<String> result = myOptional.filter(value -> value.length() > 5);

        // Print the result
        // This will not print anything because the length of "Hello" 
        // is not greater than 5.
        result.ifPresent(System.out::println);
    }
}

In this example, the filter method is used to apply a condition on the value held by this myOptional object. The condition is that the length of the value should be greater than 5. If the value satisfies the condition, it is returned. Otherwise, an empty Optional object is returned.

The ifPresent method is used to print the value held by this Optional, if it is non-empty. This particular use of filter will not print anything because the string “Hello” length is not greater than 5.

You can use isEmpty method to check whether Optional is empty.

if (result.isEmpty()) {
   System.out.println("The Optional is empty");
}

In this case, it would print “The Optional is empty”.

How do I use String.join() method in Java?

By Wayan in Java 8, Lang Package 0 Comment

The String.join() method in Java is a static method added in Java 8 to the java.lang.String class. The String.join() is a static utility method used to concatenate multiple strings, arrays or collections (like lists and sets) of strings. This method makes it easier to join multiple strings with a specific delimiter. A delimiter is a sequence of characters used to separate strings.

This method returns a new String composed of copies of the CharSequence elements joined together with a copy of the specified delimiter. This method saves us from writing boilerplate loop code just for concatenating strings with a delimiter.

Here is an example of how you can use it:

package org.kodejava.lang;

import java.util.Arrays;
import java.util.List;

public class StringJoinList {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("Java", "is", "cool");
        String result = String.join(" ", list);
        System.out.println(result);
    }
}

Output:

Java is cool

In this example, String.join() takes two parameters:

  1. A delimiter that is a CharSequence (like a String) that is placed between each joined String.
  2. An Iterable object like a List or a Set, over which the method iterates and joins all elements into a single String.

You can also use String.join() with an array of elements:

package org.kodejava.lang;

public class StringJoinArray {
    public static void main(String[] args) {
        String[] array = new String[]{"Java", "is", "cool"};
        String result = String.join(" ", array);
        System.out.println(result);
    }
}

Output:

Java is cool

In this case, String.join() still takes a delimiter as the first argument, but the second argument is an Array of elements to be joined.