Memory locations - static/stack/heap/code

Published by admin on Oct 22nd, 2007 in Uncategorized with No Comments

You can separate “memory” logically into something like four categories based on when it gets allocated and how long it lives. It’s code, static, stack and heap.

The Stack

    • JVM creates a private stack at the same time as the thread
      (analagous to stack in C)
      it holds local variables and partial results and plays a part in method invocation and return
      Stack memory stores primitive types and addresses of objects

    • The Heap

    • Runtime data area from which memory for all classes and arrays is allocated
      The heap is shared amoung all Java virtual machine threads
      The heap has garbage collection run on it

    • Instance variables and objects live on the heap memory.
    Local variables live on the stack memory.
    Static variables live on the Static memory..

    Selecting the nth rows from a query

    Published by admin on Oct 22nd, 2007 in Uncategorized with No Comments

    First off, ensure that you can order the data. Selects do not guarantee ordering of results! This is a common mistake that breaks all kinds of things when iterating over data.

    Below examples show the possible implementations to get 2nd and 3rd rows.

    MySql

    PLAIN TEXT
    SQL:
    1. SELECT COLUMNS FROM TABLE ORDER BY COLUMNS LIMIT 2,3

    SQL Server/Sybase

    PLAIN TEXT
    SQL:
    1. SELECT top 3 COLUMNS FROM TABLE ORDER BY COLUMNS

    and then throw away first row.

    All Others

    PLAIN TEXT
    SQL:
    1. SELECT column1, column2, column3
    2. FROM TABLE a
    3. WHERE ( SELECT count(1)
    4. FROM TABLE b
    5. WHERE b.column1> a.column1 )
    6. BETWEEN 2 AND 3
    7. ORDER BY column1 descending

    Creating default accounts

    Published by admin on Oct 21st, 2007 in Uncategorized with No Comments

    You can customise the creation of new unix accounts very easily: just change the /etc/skel directory.

    I've just added a link for the web directory -- so that everything points to the correct locations

    Want a quieter ssh login?

    Published by admin on Oct 21st, 2007 in Uncategorized with No Comments

    Create a .hushlogin file in your home directory!
    That's all there is to it!

    Design Principles - interfaces

    Published by admin on Oct 8th, 2007 in Uncategorized with No Comments

    Key concepts

    • Program to an interface, not an implementation.
    • Use an interface to represent each behaviour.
    • Program to an inteface / supertype:

      PLAIN TEXT
      JAVA:
      1. Animal animal = new Dog();
      2. animal.makeSound();

    Delegation Pattern:

    An object outwardly expresses certain behavious but in reality delegates responsibility for implementing that behavious to an associated object in an inversion of responsibility [aka inversion of control]. The delegation pattern is the fundamental abstraction that underpins composition, mixins and aspects.

    PLAIN TEXT
    JAVA:
    1. class A {
    2.      void f() { System.out.println("A: doing f()"); }
    3.      void g() { System.out.println("A: doing g()"); }
    4.  }
    5.  
    6.  class C {
    7.      // delegation
    8.      A a = new A();
    9.  
    10.      void f() { a.f(); }
    11.      void g() { a.g(); }
    12.  
    13.      // normal attributes
    14.      X x = new X();
    15.      void y() { /* do stuff */ }
    16.  }
    17.  
    18.  public class Main {
    19.      public static void main(String[] args) {
    20.          C c = new C();
    21.          c.f();
    22.          c.g();
    23.      }
    24.  }

    Published by admin on Oct 3rd, 2007 in Uncategorized with 1 Comment
    PLAIN TEXT
    SQL:
    1. SELECT * FROM dual

    Published by admin on Oct 2nd, 2007 in Uncategorized with No Comments
    PLAIN TEXT
    CODE:
    1. #!/bin/sh echo “Hello world!”;

    test

    Published by admin on Sep 30th, 2007 in Uncategorized with No Comments

    #!/bin/sh echo "Hello world!";

    Hello world!

    Published by admin on Sep 30th, 2007 in Uncategorized with 1 Comment

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