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A WeakHashMap is a special Map implementation where the keys of the map are stored in a java.lang.ref.WeakReference . By storing the keys in a weak reference, key-value pairs can dynamically be dropped from the map when the only reference to the key is from the weak reference. This makes the WeakHashMap an excellent implementation for a weakly referenced list, where entries that aren't used elsewhere may be dropped with no side effects. Also, just because a key may be dropped, doesn't mean it will immediately be dropped. If the system has sufficient resources, the weak key reference that isn't externally referenced could stay around for a long time. This class provides the easiest way to harness the power of weak references. It is useful for implementing "registry-like" data structures, where the utility of an entry vanishes when its key is no longer reachable by any thread. The WeakHashMap functions identically to the HashMap with one very important exception: if...

The processing of String often consists of parsing a formatted input string.  Parsing  is the division of text into a set of discrete parts, or  tokens,  which in a certain sequence can convey a semantic meaning. The  StringTokenizer  class provides the first step in this parsing process, often called the  lexer  (lexical analyzer) or  scanner .  StringTokenizer  implements the  Enumeration  interface. Therefore, given an input string, you can enumerate the individual tokens contained in it using  StringTokenizer . To use  StringTokenizer , you specify an input string and a string that contains delimiters.  Delimiters are characters that separate tokens. Each character in the delimiters string is considered a valid delimiter—for example,  ",;:"  sets the delimiters to a comma, semicolon, and colon. The default set of delimiters consists of the whitespace characters: space, tab, newline, and carriage return. StringTokenizer Example import java.util.*; class StringTokenizerDemo...

Equals and hashcode methods are two primary but yet one of most important methods for java developers to be aware of. Java intends to provide equals and hashcode for every class to test the equality and to provide a hash or digest based on content of class. Importance of hashcode increases when we use the object in different collection classes which works on hashing principle e.g. hashtable and hashmap. A well written hashcode method can improve performance drastically by distributing objects uniformly and avoiding collision. General Contracts for hashCode() in Java  1) If two objects are  equal  by equals() method then there  hashcode  returned by hashCode() method must be same. 2) Whenever  hashCode() mehtod  is invoked on the same object more than once within single execution of application, hashCode() must return same integer provided no information or fields used in equals and hashcode is modified. This integer is not required to be same during multiple execution of application th...

The two most common techniques for reusing functionality in object-oriented systems are class inheritance and object composition. Class inheritance lets you define the implementation of one class in terms of another's. Reuse by subclassing is often referred to as white-box reuse. The term "whitebox" refers to visibility: With inheritance, the internals of parent classes are often visible to subclasses. Object composition is an alternative to class inheritance. Here, new functionality is obtained by assembling or composing objects to get functionality that is more complex. Object composition requires that the objects being composed have well-defined interfaces. This style of reuse is called black-box reuse, because no internal details of objects are visible. Objects appear only as "black boxes." Inheritance and composition each have their advantages and disadvantages. Class inheritance is defined statically at compile-time and is straightforward to use, since it ...

In this tutorial we will see how to calculate date time difference using Java library. Below is the program, package in.softcare; import java.text.SimpleDateFormat; import java.util.Date; public class DateTimeCalculation { public static void main(String[] args) { String dateStart = "03/14/2013 09:29:58"; String dateStop = "03/16/2013 11:31:48"; //HH converts hour in 24 hours format (0-23), day calculation SimpleDateFormat format = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss"); Date d1 = null; Date d2 = null; try { d1 = format.parse(dateStart); d2 = format.parse(dateStop); //in milliseconds long diff = d2.getTime() - d1.getTime(); long diffSeconds = diff / 1000 % 60; long diffMinutes = diff / (60 * 1000) % 60; long diffHours = diff / (60 * 60 * 1000) % 24; long diffDays = diff / (24 * 60 * 60 * 1000); System.out.print(diffDays + " days, "); System.out.print(diffHours + " hours, "); System.out...

Let’s see how we can perform JDBC Batch Insert in Java. Although you might already knew this, I will try to explain this with different scenarios. In this note, we will see how we can use JDBC APIs like Statement and PreparedStatement to insert data in any database in batches. Also we will try to explore scenarios where we can run out of memory and how to optimize the batch operation. So first, the basic API to Insert data in database in batches using Java JDBC. Simple Batch I am calling this a simple batch. The requirement is simple. Execute a list of inserts in batch. Instead of hitting database once for each insert statement, we will using JDBC batch operation and optimize the performance. Consider the following code: Bad Code String [] queries = {     "insert into employee (name, city, phone) values ('A', 'X', '123')",     "insert into employee (name, city, phone) values ('B', 'Y', '234')",     "insert into ...