Java - Object & Methods

Object & Methods

String s1; //Step 1: declare a String variable.
s1 = new String(); //Step 2: assign it a value: a new empty string.

first line create a box, store references in strings.
second line construct a brand new string object.
= create a reference

A syntax sugar in Java:
String s2 = new String();

s1 = "Yow!"; also creates a constructor with Yow in the box.

now s1 forget the last one and remembers yow instead of the empty one.

s2 = s1; // reference to the same object;
s2 = new String (s1); //now 2 different, identical objects.

3 String constructors:

1. new String() constructs empty string - contains no character. 
2. "whatever"
3. new String(s1) takes a parameter s1. Makes copy of object that s1 references.

Constructors always have the same name as their class, except "stuff in quotes".

s2 = s1.toUppercase();

String s3= s2.concat("!!"); // or just s2+"!!";

String s4 = "*".concat(s2).concat("*");//"*".concat(s2) compute first

In Java, String object are immutable unlike in C: their contents never change.

Java I/O Classes

Objects in System class for interacting with a user:

System.out is a PrintStream object that outputs to the screen.

System.in is an InputStream " " reads from the keyboard.

Actually, the two mentioned above should be variables that references to the object.

readLine method is defined on BufferedReader objects.

How do we construct a BufferedReader(compose into entire lines of text)? With an InputStream.Reader.

How .. an InputStreamReader(compose into characters typically 2 bytes)? We need an InputStream 

How .. an InputStream(read raw data)? System.in is one.

Figure this out from online Java libraries API. Specifically, java.io library here.

import java.io.*;//to use the java library, other than java.lang

class SimpleIO{

  public static void main(String[] arg) throws Exception{

    BufferedReader keybd = new BufferedReader(new InputStreamReader(System.in));

    System.out.println(keybd.readLine());

  }

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

public class readURL {
public static void main(String[] arg) throws Exception{
URL u = new URL("http://www.bbc.co.uk");
InputStream s = u.openStream();
InputStreamReader inputrd = new InputStreamReader(s);
BufferedReader bbc = new BufferedReader(inputrd);
System.out.println(bbc.readLine());
}

}