Java Basic Syntax II

	Looping and Branching Constructs Arrays in Java Using Arrays Example Methods - Functions Passing Arguments Example Passing Arguments The String Object Using The String Object Example Converting Strings Quiz Exercises
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Looping and Branching Constructs

if-else	while [initialization]
if ( boolean-expression ) statement1; [ else statement2; ]	while ( boolean-expression ) { [statements] [iteration] }
switch-case	for loops
switch ( int-value ) { case int-value1: break; case int-value2: break; default: }	for ([initialization]; [ boolean-expression ]; [iteration]) { [statements] }

Arrays in Java

- Arrays in Java are considered to be objects

- Arrays can contain any simple or complex type

- c++ like declaration : type t[];

- java declaration : type[] t;

- Multidimensional Arrays : type[][]..[] t;

- Subscripts begin at zero

- Out of bounds access throws an Exception

Using Arrays Example

public class InitArrays {
  
    public static void main (String[] argv) {

       int [] numArray; // declaration 
       numArray = new int[10]; // creation
       for (int x=0; x<numArray.length; x++) {
          numArray[x] = x * 2;
       }
       for (int x=0; x<numArray.length; x++) {
          System.out.println("position " + x + 
                              " contains: " + numArray[x] );
      }
   }
}

Methods - Functions

	<modifiers> <return_type> <name> ( <args> ) <block> modifiers : public, private, protected, static, final return_type : any type and void for nothing name : Identifier - Method's name args : Argument list block : block of code starting with '{' end finishing with '}'
Examples : public int getSize(); private byte[] getBuffer(); public void printNums( int[] theArray )

Passing Arguments

by Value :

- the argument may not be changed by the method called

- The 8 datatypes are passed this way

by Reference :

- the argument may be manipulated directly by the method called

- Objects are passed this way

What would you do to pass simple datatypes by reference?

complex types by value? For the first case, you can use a wrapper, for the second you need to clone the data first.

Example Passing Arguments

public class PassingArguments {

   public void test(int x, int y, int[] array) {
       x=1; y=2; array[0] = 5;
       System.out.println("inside test method x:" + x + " y:" + y + 
                          " array[0]:" +array[0] );
   }

   public static void main(String[] argv) {
      int a = 5; int b = 10 ; 
      int[] ar = {15 };
      System.out.println(
      "Before the test call a:" + a + " b:" + b + " ar[0]:" +ar[0]);
      PassingArguments pa = new PassingArguments();
      pa.test(a,b, ar);
      System.out.println(
     "After the test call a:" + a + " b:" + b + " ar[0]:" +ar[0]); 
   }
}

Before the test call:

a:5 b:10 ar[0]:15

inside test method:

x:1 y:2 array[0]:5

After the test call:

a:5 b:10 ar[0]:5

The String Object

String, is a container type for 16-bit Unicode chars

Declaration : String str;
Creation : str = new String("hi"); or str = "hi";
Usage : String's methods, operator overloading, String-Number conversions

A String cannot change value after creation

Very frequently used methods:

int length()	boolean equals(String)
boolean startsWith(String)	String toUpperCase()
String toLowerCase()	int indexOf(String)
String subString(int begin)	String subString(int begin, int end)
char charAt(int index)

Using String Example

public class StringExample { 
   public static void main(String[] argv) {
      String str = new String("my name is Thanassis");
      System.out.println("The string contains:" + str );
      System.out.println("its length is :" + str.length() + " chars");
      System.out.println("uppercase :" + str.toUpperCase());
      System.out.println("lowercase :" + str.toLowerCase());
      System.out.print("-is- starts at :" + str.indexOf("is") );
      if (str.equals("my name is Thanasis")) 
         System.out.println("Yes")
      else System.out.println("No")
   }
}

Converting Strings

- Any simple numeric to String :

1. using String's valueOf(numeric) or

2. using Number Classes and toString()

- String to numeric :

1. using String's intValue() or floatValue() or

2. using Number Classes and parse(String)

Quiz

Which of the following statements are correct ?

int i=(int)((a<10) && (b>10));
boolean b = 1;
boolean b = (!!!!(3<4))
System.out.println((char)(260>>2));
int i= (int)(true);
Point $2nd_point = new Point(5,5);

What is the output of the following program?

for (int i=10; i>1; i--) {
for (int j=1; j<10; j++) {
if ( ((i+j)%(10)) == 0 )
System.out.println(i + “ “+ j)
}
}

Exercises

1. Write the HelloWorld program, compile and execute it. After executing successfully, try to alter both the process and the code to get as many errors as possible :) I challenge you. Can you achieve 20 different errors?

2. Write a Java class that declares variables for every available data-type, executes a simple arithmetic operation with each data-type and prints the result. Can you use the “+” operator with all the datatypes?

3. Write a program that creates an array, fills it it with 100 int numbers and finds their average value.

4. Implement a method to reverse a given String

5. Write programs to print :

- the following pattern in all possible directions with the least possible code
*
**
***
****
*****

- the ASCII table ( chars 32-255 )

- the Hypotenuse z of the following right triangle :

Hint! : z = Math.sqrt(double d) returns the square root

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