Monthly Archives: October 2013

eclipse shortcut: go to the implementation of the method

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F3 is the typical “go to implementation”. For interfaces that go to the interface definition.

Instead use Ctrl + T to see all implementations of the interface definition. You can then easily go to the one you want with the arrow keys and Enter. I believe that the first one is automatically selected so that Ctrl-T + Enter will do what you need.

can constructor be private?

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http://stackoverflow.com/questions/2816123/can-a-constructor-in-java-be-private

here are three uses of private constructors:

  • to prevent instantiation outside of the object, in the following cases:
    • singleton
    • factory method
    • static-methods-only (utility) class
    • constants-only class
      .
  • to prevent sublcassing (extending). If you make only a private constructor, no class can extend your class, because it can’t call the super() constructor. This is some kind of a synonym for final
  • overloaded constructors – as a result of overloading methods and constructors, some may be private and some public. Especially in case when there is a non-public class that you use in your constructors, you may create a public constructor that creates an instance of that class and then passes it to a private constructor.

sql unique constraint

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The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

CREATE TABLE PersonsUnique
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

CREATE TABLE PersonsUniqueMulti
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)

SQL UNIQUE Constraint on ALTER TABLE

ALTER TABLE Persons
ADD UNIQUE (P_Id)

ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)

ALTER TABLE Persons
DROP INDEX uc_PersonID

autoboxing

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http://docs.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html

As any Java programmer knows, you can’t put an int (or other primitive value) into a collection. Collections can only hold object references, so you have to box primitive values into the appropriate wrapper class (which is Integer in the case of int). When you take the object out of the collection, you get the Integer that you put in; if you need an int, you must unbox the Integer using the intValue method. All of this boxing and unboxing is a pain, and clutters up your code. The autoboxing and unboxing feature automates the process, eliminating the pain and the clutter.

The following example illustrates autoboxing and unboxing, along with generics and the for-each loop. In a mere ten lines of code, it computes and prints an alphabetized frequency table of the words appearing on the command line.

import java.util.*;

// Prints a frequency table of the words on the command line
public class Frequency {
   public static void main(String[] args) {
      Map<String, Integer> m = new TreeMap<String, Integer>();
      for (String word : args) {
          Integer freq = m.get(word);
          m.put(word, (freq == null ? 1 : freq + 1));
      }
      System.out.println(m);
   }
}

java Frequency if it is to be it is up to me to do the watusi
{be=1, do=1, if=1, is=2, it=2, me=1, the=1, to=3, up=1, watusi=1}

The program first declares a map from String to Integer, associating the number of times a word occurs on the command line with the word. Then it iterates over each word on the command line. For each word, it looks up the word in the map. Then it puts a revised entry for the word into the map. The line that does this (highlighted in green) contains both autoboxing and unboxing.

garbage collection

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  • everything managed by garbage collector is on the heap
  • garbage collects the heap that has reference to.
  • stack: contiguous, easy to go up/down, methods place frames on stack
  • heap: stack has some variable pointed to heap

int i = 5; double d = 25.5; // on stack Object o = new Car(); Public static Foo = “foo”; // on heap

TUTORIAL http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html