Why Timestamp cannot be used in place of Date in Java?

One of the tricky question from Java Interview is, "Can we pass a Timestamp instance to a method expecting java.util.Date?", it's a tricky question because the answer is both Yes and No. You can, of course, pass a Timestamp object to a method with the argument as Date because first, Timestamp is a subclass of java.util.Date and second it contains both date and time values which are missing in either java.sql.Date and java.sql.Time. So there is more reason to pass a Timestamp value to Date but you should not be doing that. Why? because Timestamp is not exactly Date. It's a composite type of java.util.Date and an additional nanosecond value which is fitted there to confirm database DATETIME data type, which supports nanosecond precision. If you look at the implementation of java.sql.Timestamp class, you will find that the long value supplied by Date is stored separately then this nanosecond value.

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How to Iterate through ConcurrentHashMap and print all keys and values in Java

Suppose you have a ConcurrentHashMap of String and Integer and you want to print all keys and values, how do you that? This is a common, day to day programming task for Java programmer and there are many ways to do it. The Map interface provides several view methods e.g. keySet(), values(), and entrySet() to retrieve all keys, values, and all key and value pairs as entries. You can use respective methods to print all keys, all values, or all key values pairs. For printing, you also have multiple choice e.g. you can either use enhanced for loop or Iterator, though later also provide you the facility to remove key value pairs while printing if needed. Though you should remember that these views are backed by Map, so when you remove a key-value pair from entry set it will also be removed by the ConcurrentHashMap.

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Top 10 Maven Plugins Every Java Developer Should Know

In the last couple of years, Maven has become the de-facto build tool for Java applications. Though there are challenges exists from tools like Gradle, but I think the dominance of Maven will help it to win the final battle. When I started with Maven, I was happy with just dependency management but then I come to know about maven plugins which let you customize your build up to the level you can do with ANT. These plugins provide true power to Maven to automate most of the build related task e.g. compilation, testing, packaging, deployment, and even committing the source code into the remote source repository. Many Java developers who use Maven daily are usually not aware of these essential Maven plugins mostly because  Maven just works, or someone has already done the hard work for them.

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How to find the 3rd element from end in linked list in Java

The problem to find the 3rd element from the end in a singly linked list or nth node from the tail is one of the tricky but frequently asked linked list problems in programming job interviews. The challenge here is to solve the problem in just one pass, i.e. you can not traverse the linked list again and you cannot traverse backward because it's a singly linked list. If you have practiced some linked list problems e.g. finding length, inserting elements, or deleting elements then you should be familiar with traversal. Here, we'll use the same algorithm which we have used to find the middle element of linked list in one pass. The algorithm is also known as tortoise and hare algorithm because of the speed of two pointers used by the algorithm to traverse the singly linked list.

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How to read a text file using Scanner in Java? Example Tutorial

As I told you before that there are multiple ways to read a file in Java e.g. FileReader, BufferedReader, and FileInputStream. You chose the Reader or InputStream depending upon whether you are reading text data or binary data, for example, the BufferedReader class is mostly used to read a text file in Java. The Scanner class is also another way to read a text file in java. Even though Scanner is more popular as a utility to read user input from the command prompt, you will be glad to know that you can also read a file using Scanner. Similar to BufferedReader, it provides buffering but with a smaller buffer size of 1KB and you can also use the Scanner to read a file line by line in Java. Similar to readLine(), Scanner class also have nextLine() method which return the next line of the file. Scanner also provides parsing functionality e.g. you can not only read text but parse it into correct data type e.g. nextInt() can read integer and nextFloat() can read float and so on.

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Difference between row_number(), rank() and dense_rank() in SQL Server, Oracle.

Though all three are ranking functions in SQL, also known as window function in Microsoft SQL Server, the difference between rank(), dense_rank(), and row_number() comes when you have ties on ranking i.e. duplicate records. For example, if you are ranking employees by their salaries then what would be the rank of two employees of same salaries? It depends on upon which ranking function you are using e.g. row_number, rank, or dense_rank. The row_number() function always generates a unique ranking even with duplicate records i.e. if the ORDER BY clause cannot distinguish between two rows, it will still give them different rankings, though which record will come earlier or later is decided randomly e.g. in our example two employees Shane and Rick have the same salary and has row number 4 and 5, this is random, if you run again, Shane might come 5th.

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How to calculate GCF and LCM of two numbers in Java? Example

This week's programming exercise is to write a Java program to calculate GCF and LCM of two numbers. The GCF, stands for Greatest common factor and LCM stands for Lowest common multiplier, both are popular mathematical operation and related to each other. The GCF is the largest number which divides both the number without leaving any remainder e.g. if two numbers are 24 and 40 then their GCF is 8 because 8 is the largest number which divides both 24 and 40 perfectly, without leaving any remainder. Similarly, LCM is the lowest number which is perfectly divisible by the two number, for example, if given number is 40 and 24 then their LCM is 120 because this is the lowest number which is perfectly divisible by both 40 and 24.

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