Java | Exercises in .NET with Andras Nemes

Insert a non-existent value into a Map in Java

October 4, 2017 Leave a comment

Consider the following Employee class:

public class Employee
{
    private UUID id;
    private String name;
    private int age;

    public Employee(UUID id, String name, int age)
    {
        this.id = id;
        this.name = name;
        this.age = age;
    }
        
    public UUID getId()
    {
        return id;
    }

    public void setId(UUID id)
    {
        this.id = id;
    }

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }    
    
    public int getAge()
    {
        return age;
    }

    public void setAge(int age)
    {
        this.age = age;
    }
}

Let’s put some Employee objects into a hash map:

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Check available number of bytes in an input stream in Java

October 3, 2017 Leave a comment

In this post we saw how to read the bytes contained in an input stream. The most common way to achieve it is by way of one of the read methods. The overloaded version where we provide a target byte array, an offset and a total byte count to be read is probably used most often.

It can happen in real-life situations that we provide the total number of bytes to be extracted but those bytes have not yet “arrived”, i.e. are not yet available in the input stream. This can occur when reading the bytes from a slow network connection. The bytes will eventually be available. The read method will block the thread it’s running in while it is waiting for the bytes to be loaded.

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Various ways to read bytes from an input stream in Java

October 2, 2017 Leave a comment

Say you want to fill a byte array from a source, e.g. a from a randomly simulated data of 50 bytes like the following:

byte[] simulatedSource = new byte[50];
Random random = new Random();
random.nextBytes(simulatedSource);
InputStream inputStream = new ByteArrayInputStream(simulatedSource);

At a basic level we can read each byte one by one from the input stream as follows:

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Time zones in the Java Date and Time API

October 1, 2017 Leave a comment

Introduction

I know for a fact that all programmers love working with time zones. Chances are high that you, as a reader of this blog, are also a programmer so I bet you also just love time zones. Let’s see what Java 8 offers as far as time zones are concerned.

Time zones

So far in this series on the date and time in Java 8 we always worked with the local time zone found on your computer. All date-related classes, such as LocalTime or LocalDateTime allow you to easily set the time zone. Here’s an example with the LocalDateTime class:

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Formatting dates in Java using DateTimeFormatter

September 30, 2017 Leave a comment

Introduction

Formatting dates – and numbers for that matter – can be a complex matter. The DateTimeFormatter class provides pre-defined formats that adhere to ISO and RCF specifications.

DateTimeFormatter

The following date related classes we’ve seen on this blog, i.e.

…have a method called “format” which accepts a DateTimeFormatter class. This class has a number of pre-defined formats that you can readily use in your application. Note that not all such formats will be available. The availability depends on the exact object type of the date class. E.g. ISO_ZONED_DATE_TIME won’t work with LocalDateTime as it is meant to format zoned dates. Also, ISO_DATE_TIME won’t be available for the LocalDate class as it has no concept of time units below the level of days.

Let’s test all the predefined values with the above date types.

LocalDate

We run the following code and check the output:

System.out.println("Running example with LocalDate class.");
LocalDate now = LocalDate.now();
        try
        {
            System.out.println("ISO_DATE: " + now.format(DateTimeFormatter.ISO_DATE));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_DATE is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("BASIC_ISO_DATE: " + now.format(DateTimeFormatter.BASIC_ISO_DATE));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("BASIC_ISO_DATE is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_DATE_TIME: " + now.format(DateTimeFormatter.ISO_DATE_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_DATE_TIME is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_INSTANT: " + now.format(DateTimeFormatter.ISO_INSTANT));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_INSTANT is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_LOCAL_DATE: " + now.format(DateTimeFormatter.ISO_LOCAL_DATE));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_LOCAL_DATE is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_LOCAL_DATE_TIME: " + now.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_LOCAL_DATE_TIME is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_LOCAL_TIME: " + now.format(DateTimeFormatter.ISO_LOCAL_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_LOCAL_TIME is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_OFFSET_DATE: " + now.format(DateTimeFormatter.ISO_OFFSET_DATE));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_OFFSET_DATE is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_OFFSET_DATE_TIME: " + now.format(DateTimeFormatter.ISO_OFFSET_DATE_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_OFFSET_DATE_TIME is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_OFFSET_TIME: " + now.format(DateTimeFormatter.ISO_OFFSET_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_OFFSET_TIME is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_ORDINAL_DATE: " + now.format(DateTimeFormatter.ISO_ORDINAL_DATE));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_ORDINAL_DATE is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_TIME: " + now.format(DateTimeFormatter.ISO_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_TIME is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_WEEK_DATE: " + now.format(DateTimeFormatter.ISO_WEEK_DATE));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_WEEK_DATE is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("ISO_ZONED_DATE_TIME: " + now.format(DateTimeFormatter.ISO_ZONED_DATE_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("ISO_ZONED_DATE_TIME is not supported: " + e.getMessage());
        }
        try
        {
            System.out.println("RFC_1123_DATE_TIME: " + now.format(DateTimeFormatter.RFC_1123_DATE_TIME));
        } catch (UnsupportedTemporalTypeException e)
        {
            System.out.println("RFC_1123_DATE_TIME is not supported: " + e.getMessage());
        }

We get the following output:

Running example with LocalDate class.
ISO_DATE: 2014-11-01
BASIC_ISO_DATE: 20141101
ISO_DATE_TIME is not supported: Unsupported field: HourOfDay
ISO_INSTANT is not supported: Unsupported field: InstantSeconds
ISO_LOCAL_DATE: 2014-11-01
ISO_LOCAL_DATE_TIME is not supported: Unsupported field: HourOfDay
ISO_LOCAL_TIME is not supported: Unsupported field: HourOfDay
ISO_OFFSET_DATE is not supported: Unsupported field: OffsetSeconds
ISO_OFFSET_DATE_TIME is not supported: Unsupported field: HourOfDay
ISO_OFFSET_TIME is not supported: Unsupported field: HourOfDay
ISO_ORDINAL_DATE: 2014-305
ISO_TIME is not supported: Unsupported field: HourOfDay
ISO_WEEK_DATE: 2014-W44-6
ISO_ZONED_DATE_TIME is not supported: Unsupported field: HourOfDay
RFC_1123_DATE_TIME is not supported: Unsupported field: HourOfDay

LocalTime

We run the same code as above but change the class type:

System.out.println("Running example with LocalTime class.");
LocalTime now = LocalTime.now();

…and here’s the output, again with Locale set to Sweden:

Running example with LocalTime class.
ISO_DATE is not supported: Unsupported field: Year
BASIC_ISO_DATE is not supported: Unsupported field: Year
ISO_DATE_TIME is not supported: Unsupported field: Year
ISO_INSTANT is not supported: Unsupported field: InstantSeconds
ISO_LOCAL_DATE is not supported: Unsupported field: Year
ISO_LOCAL_DATE_TIME is not supported: Unsupported field: Year
ISO_LOCAL_TIME: 22:02:52.932
ISO_OFFSET_DATE is not supported: Unsupported field: Year
ISO_OFFSET_DATE_TIME is not supported: Unsupported field: Year
ISO_OFFSET_TIME is not supported: Unsupported field: OffsetSeconds
ISO_ORDINAL_DATE is not supported: Unsupported field: Year
ISO_TIME: 22:02:52.932
ISO_WEEK_DATE is not supported: Unsupported field: WeekBasedYear
ISO_ZONED_DATE_TIME is not supported: Unsupported field: Year
RFC_1123_DATE_TIME is not supported: Unsupported field: DayOfMonth

Most formats are not supported by LocalTime as there’s no notion of days, months etc. in that object.

LocalDateTime

Run the same code with “now” set as follows:

System.out.println("Running example with LocalDateTime class.");
LocalDateTime now = LocalDateTime.now();

…and I got the following output:

Running example with LocalTime class.
ISO_DATE: 2014-11-01
BASIC_ISO_DATE: 20141101
ISO_DATE_TIME: 2014-11-01T22:07:24.329
ISO_INSTANT is not supported: Unsupported field: InstantSeconds
ISO_LOCAL_DATE: 2014-11-01
ISO_LOCAL_DATE_TIME: 2014-11-01T22:07:24.329
ISO_LOCAL_TIME: 22:07:24.329
ISO_OFFSET_DATE is not supported: Unsupported field: OffsetSeconds
ISO_OFFSET_DATE_TIME is not supported: Unsupported field: OffsetSeconds
ISO_OFFSET_TIME is not supported: Unsupported field: OffsetSeconds
ISO_ORDINAL_DATE: 2014-305
ISO_TIME: 22:07:24.329
ISO_WEEK_DATE: 2014-W44-6
ISO_ZONED_DATE_TIME is not supported: Unsupported field: OffsetSeconds
RFC_1123_DATE_TIME is not supported: Unsupported field: OffsetSeconds

ZonedDateTime

This is probably the most interesting case as it supports all the predefined formats in DateTimeFormatter. Let’s look at an example with Australia/Adelaide:

System.out.println("Running example with ZonedDateTime class.");
ZoneId brisbane = ZoneId.of("Australia/Adelaide");
ZonedDateTime now = ZonedDateTime.of(LocalDateTime.now(), brisbane);

Running example with ZonedDateTime class.
ISO_DATE: 2014-11-01+10:30
BASIC_ISO_DATE: 20141101+1030
ISO_DATE_TIME: 2014-11-01T22:13:48.87+10:30[Australia/Adelaide]
ISO_INSTANT: 2014-11-01T11:43:48.870Z
ISO_LOCAL_DATE: 2014-11-01
ISO_LOCAL_DATE_TIME: 2014-11-01T22:13:48.87
ISO_LOCAL_TIME: 22:13:48.87
ISO_OFFSET_DATE: 2014-11-01+10:30
ISO_OFFSET_DATE_TIME: 2014-11-01T22:13:48.87+10:30
ISO_OFFSET_TIME: 22:13:48.87+10:30
ISO_ORDINAL_DATE: 2014-305+10:30
ISO_TIME: 22:13:48.87+10:30
ISO_WEEK_DATE: 2014-W44-6+10:30
ISO_ZONED_DATE_TIME: 2014-11-01T22:13:48.87+10:30[Australia/Adelaide]
RFC_1123_DATE_TIME: Sat, 1 Nov 2014 22:13:48 +1030

So you see that these predefined formatters follow the accepted ISO and RFC specifications and won’t actually show the localised date format the way dates are formatted in e.g. Japan or the US.

We’ll take a look at date localisation using the DateTimeFormatter in the next post in this series.

View all posts related to Java here.

Filed under Java Tagged with datetime, Java

Java Date and time API: the Instant class

September 29, 2017 Leave a comment

The Date and time API in Java 8 has been completely revamped. Handling dates, time zones, calendars etc. had been cumbersome and fragmented in Java 7 with a lot of deprecated methods. Developers often had to turn to 3rd party date handlers for Java such as Joda time.

One of many new key concepts in the java.time package of Java 8 is the Instant class. It represents a point of time in a continuous timeline where this time point is accurate to the level of nanoseconds.

The immutable Instant class comes with some default built-in values:

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Filed under Java Tagged with datetime, Java

Using the Comparator class in Java to compare objects

September 28, 2017 Leave a comment

Java 8 comes with a range of built-in implementations of the Comparator interface.

Consider the following Employee class:

public class Employee
{
    private UUID id;
    private String name;
    private int age;

    public Employee(UUID id, String name, int age)
    {
        this.id = id;
        this.name = name;
        this.age = age;
    }
        
    public UUID getId()
    {
        return id;
    }

    public void setId(UUID id)
    {
        this.id = id;
    }

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }    
    
    public int getAge()
    {
        return age;
    }

    public void setAge(int age)
    {
        this.age = age;
    }    
}

…and the following list of employees:

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How to merge two Maps in Java

September 27, 2017 Leave a comment

The Map interface has been extended with the “merge” function in Java 8. Let’s see an example on how to use it.

Consider the following Empolyee class:

public class Employee
{
    private UUID id;
    private String name;
    private int age;

    public Employee(UUID id, String name, int age)
    {
        this.id = id;
        this.name = name;
        this.age = age;
    }
        
    public UUID getId()
    {
        return id;
    }

    public void setId(UUID id)
    {
        this.id = id;
    }

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }    
    
    public int getAge()
    {
        return age;
    }

    public void setAge(int age)
    {
        this.age = age;
    }
}

Let’s say that we have the following two maps where the key is an indicator of the employees’ performance and the value is the list of employees that fall into that category:

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Concatenate strings with the StringJoiner class in Java

September 26, 2017 Leave a comment

The StringJoiner has two overloads. The simpler one accepts a delimiter:

StringJoiner sj = new StringJoiner(" | ");
sj.add("Hello").add("my").add("dear").add("world!");

System.out.println(sj.toString());

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Default interface functions in Java 8

September 25, 2017 Leave a comment

Introduction

A new feature in Java 8 is default function implementations. They are default implementations of methods of an interface. Default methods can help extending an interface without breaking the existing implementations. After all if you add a new method to an interface then all implementing types must handle it otherwise the compiler will complain.

This can be cumbersome if your interface has a large number of consumers. You’ll break their code and they will need to implement the new function – which they might not even need.

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Filed under Java Tagged with interface, Java

← Older posts

Newer posts →

Exercises in .NET with Andras Nemes

Insert a non-existent value into a Map in Java

Check available number of bytes in an input stream in Java

Various ways to read bytes from an input stream in Java

Time zones in the Java Date and Time API

Formatting dates in Java using DateTimeFormatter

Java Date and time API: the Instant class

Using the Comparator class in Java to compare objects

How to merge two Maps in Java

Concatenate strings with the StringJoiner class in Java

Default interface functions in Java 8

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