Lecture 02

Constructors

The procedure done by constructor executing the line s2 = new String(s);

Look where s points
Follows reference to string object
Reads string
constructs new string w/ copy of characters
make s2 reference new string

3 String constructors now

new String()
"Yow!"
new String(s)

Constructor always has same name as their class, except the special constructor "stuffinquotes". That's the only exception.

Methods

s2 = s.toUppercase();       // Create a string like s, but in all upper case.

   ---     ----------
s2 |.+---->|  YOW!  |
   ---     ----------

String s3 = s2.concat("!!");             // Also written:  s3 = s2 + "!!";

   ---     ------------
s3 |.+---->|  YOW!!!  |
   ---     ------------

String s4 = "*".concat(s2).concat("*");  // Also written:  s4 = "*" + s + "*";

   ---     ------------
s4 |.+---->|  *YOW!*  |
   ---     ------------

Important facts

When Java executed the line s2 = s.toUppercase();, the String object "Yow!" did not change. Java wrote a new "pointer" into the variable s2 so that s2 points to a different object.
Unlike C, in Java Strings are immutable -- once constructed, their contents never change.

I/O Classes and Objects in Java

Object in System class for interacting with a user:

System.out is a PrintStream object that outputs to the screen. (System.out references to the PrintStream object.)
System.in is an InputStream object that reads the keyboard.
- A method called readLine to read a line, which is defined on BufferedReader
- construct an InputStream object and use a BufferedReader to read on top of it

How do we construct a BufferedReader? With an InputStreamReader.
How do we construct a InputStreamReader? With an InputStream.
- How do we construct a InputStreamReader? With an System.in is one.

More on Java API - java.io.

Why all this fuss?

InputStream objects (like System.in) read raw data from some source (like the keyboard), but don't format the data.
InputStreamReader objects compose the raw data into characters (which are typically two bytes long in Java).
BufferedReader objects compose the characters into entire lines of text.

Why break down into so many classes?

Object-oriented encourages you to write your code into decomposable pieces, so that each piece can be easily replaced.

Here's a complete Java program that reads a line from the keyboard and prints it on the screen.

import java.io.*;

class SimpleIO {
  public static void main(String[] arg) throws Exception {
    BufferedReader keybd =
          new BufferedReader(new InputStreamReader(System.in));
    System.out.println(keybd.readLine());
  }
}

Classes for Web Access

Let's say we want to read a line of text from the White House Web page. (The line will be HTML, which looks ugly. You don't need to understand HTML.)

How to read a line of text? With readLine on BufferedReader. How to create a BufferedReader? With an InputStreamReader. How to create a InputStreamReader? With an InputStream. How to create an InputStream? With a URL.

import java.net.*;
import java.io.*;

class WHWWW {
  public static void main(String[] arg) throws Exception {
    URL u = new URL("http://www.whitehouse.gov/");
    InputStream ins = u.openStream();
    InputStreamReader isr = new InputStreamReader(ins);
    BufferedReader whiteHouse = new BufferedReader(isr);
    System.out.println(whiteHouse.readLine());
  }
}

HFJ Reading

Garbage-collection heap. Objects are created in an area of memory called The Heap.
Many classes? Java Archive .jar file - based on pkzip format; can include a manifest that defines which class in that jar holds the main() method that should run.

Java API

Java API, classes are grouped into packages.
- [java.util].(ArrayList): [package name].[class name]

Two ways to use
- import
- type
  - eg. java.util.ArrayList<Dog> list = new java.util.Arraylist...

An import saves you from typing -- it's not the same as an include in C.
You get the java.lang package "pre-imported" for free.

Immutability

Strings in Java are immutable.
JVM puts a new String into memory "String Pool" - if there is already one, it doesn't create a duplicate. [Why? They are immutable so it's safe.]
Garbage Collector doesn't go there. [Why?]

String and StringBuffer/StringBuilder Methods

p. 669

string.charAt(i);
string.substring(i, j);
string1.concat(string2);
string.toUpperCase();
string1.equals(string2);
string1.equalsIgnoreCase(string2);
string.indexOf(substring); // cannot find: returns -1

02.Using objects