Java Collections – HashMap

Create a HashMap by parsing CSV. Declare and use a multi-map (a HashMap inside a HashMap).

“Don’t judge each day by the harvest you reap but by the seeds that you plant.”
― Robert Louis Stevenson

Check out the previous parts of this Java Collections Guide: Introduction, Part 1 – Collection, Part 2 – Sets and Part 3 – ArrayList.

1. Introduction

The HashMap implements the Map interface and provides the abstraction of a dictionary mapping of keys to values. It is a part of the Java Collections Framework but does not fall under the Iterable and Collection hierarchy. In this article, we learn how to use HashMaps.

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Java Collections – ArrayList

How to properly create and use an ArrayList including adding, retrieving and modifying items.

Check out the previous parts of this Java Collections Guide: Introduction, Part 1 – Collection and Part 2 – Sets.

1. Introduction

The ArrayList is one of the most widely used classes in the Java Collections Framework (if not the entire JDK; that position probably belongs to String). It provides a simple array-like storage of items with dynamically adjusting capacity. In this article, we present some usage patterns of the List and ArrayList.

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How to Convert CSV to JSON in Java

Convert CSV to JSON using Jackson. Use a POJO for conversion or a List & Map for intermediate storage.

“Any fool can know. The point is to understand.”
― Albert Einstein


1. Introduction

CSV to JSON conversion is easy. In this article, we present a couple of methods to parse CSV data and convert it to JSON. The first method defines a POJO and uses simple string splitting to convert CSV data to POJO, which in turn is serialized to JSON. The second method uses a more complete CSV parser with support for quoted fields and commas embedded within fields. In this method, we use the Java Collection classes to store the parsed data and convert those to JSON.

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Java Collections – Set

A Set is a container which does not allow duplicate elements. Common Set implementations include HashSet, LinkedHashSet and TreeSet.

“Quotation, n: The act of repeating erroneously the words of another.”
― Ambrose Bierce, The Unabridged Devil’s Dictionary

Check out the earlier parts of this Java Collections Series: Introduction and Part 1.

1. Introduction

A Set is a container which does not contain duplicate elements. The item method equals() is used to determine whether the object is already present in the Set. At most one null element is allowed in the Set.

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Java Collections – Part 1

Java Collections Framework: learn about adding items, iterating over a collection and removing items.

“Success is not final, failure is not fatal: it is the courage to continue that counts.”
― Winston S. Churchill

Check out the first part of this Java Collections Series: Introduction.

1. Introduction

Let us learn about Java Collections and the various operations applicable to collections in general. Since the Collection interface is implemented by most of the classes in the Collections Framework (except Map and friends), the methods described here apply to all those classes.

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Java Collections

Java Collections Framework: An Introduction to the class hierarchy.

“Any fool can know. The point is to understand.”
― Albert Einstein

1. Introduction

Java Collections Framework is an architecture for storing, managing and manipulating collections of objects. It provides many data structures and algorithms commonly used for dealing with collections. For example: searching and sorting. Many details about storing objects are abstracted away (meaning you do not have to deal with it in code). An unified interface is provided to manipulate them e.g for adding and removing entries.

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Apache POI Excel Example – Part 2

More formatting options using Java with Apache POI for Microsoft Excel spreadsheets.

1. Introduction

In Part 1 of this Apache POI Excel guide, we examined how to create an Excel spreadsheet and add data to it. We also looked at properly storing data into cells to avoid “Number Stored as Text” errors.

In this chapter, let us look at some more options for formatting data within an Excel spreadsheet.

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Java Scanner

“The only way of discovering the limits of the possible is to venture a little way past them into the impossible.”
― Arthur C. Clarke


Java provides a Scanner class that can be used as a text parser. It accepts a regular expression as a delimiter and returns tokens separated by the delimiter. Let us look at some usage scenarios of the Scanner class.

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Java HashMap Examples

1. Introduction

A HashMap is a map of keys to values which uses a hash table for implementation. The HashMap organizes the keys into buckets based on the value of the hashCode() of the key. It provides constant-time performance for get and put operations. What this means is that the time taken by get and put operations does not depend on the size of the map.

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