CS 596: Java Notes

CS 596 Client-Server Programming
Java Control Structures

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San Diego State University -- This page last updated January 31, 1996

Contents of Java Control Structures Lecture

Control Structures

Contains all the standard C/C++ control structures

class Control 
	{

	public static void main( String args[] ) 
		{
		int a = 5;
		int b = 10;

		if ( a > b ) System.out.println( "a is bigger ");

		if ( a > b ) 
			System.out.println( "a is bigger ");
		else
			System.out.println( "b is bigger ");

		switch ( a ) 
			{	//Controlling expression converted to int
			case  4: 
				b++;
				break;
			case  10: 
				b--;
				break;

			default:							// optional
				System.out.println( "Default action" );
			};

		while ( a < b ) 
			{
			a++;
			};

// Control Structures Continued


		for  ( int loop = 0;  loop  < a;  loop++ ) 
			{
			System.out.println( a );
			};
		
		System.out.println( loop );	// Error, loop does not
										// exist here


		do 
			{
			System.out.println( a-- );
			System.out.println( b );    
			}
		while ( a > b );


		int max  =  ( a > b ) ?  a  :  b;

		if  ( ( a > 5 ) &&  ( b < 10 ) ) 
			System.out.println( "Good" );

		a  += ( a = 5 );
		}
	}

Jump Statements

break, continue, return

class ControlStructures 
	{
	public static void main( String args[] ) 
		{
		for ( int row = 0; row < 5;  row++  )  
			{ 
			for (  int column = 0; column < 4 ;  column++ )  
				{
				System.out.println( row + "\t" + column );
				if ( ((row + column) % 2 ) == 0 )
					break;
				System.out.println( " After if " );
				}
			};

		Outer:
		for ( int row = 0; row < 5;  row++  )  
			{ 
			for (  int column = 0; column < 4;  column++ )  
				{
				System.out.println( row + "\t" + column );
				if ( ((row + column) % 2 ) == 0 )
					break Outer;
				System.out.println( " After if " );
				} 
			} 
		}
	}

Output

0 0	3 0
1 0	After if
After if	3 1
1 1	4 0
2 0	0 0

Arrays

class ArrayExamples 
	{
	public static void main( String args[] ) 
		{
		int declarationTypeA[] = new int[5];
		float[]	floatArray  =  new float[ 25 ];;

		int[]	integerArray;
		int[]	alias;
		integerArray  =  new  int[ 10 ];	// Indexed from 0 to 9

		for ( int index = 0; index < integerArray.length; index++ )
			integerArray[  index  ]  =  5;

		alias = integerArray;		// Arrays are references
		alias[  3  ]  =  10;
		System.out.println( integerArray[ 3 ] );
		
		integerArray  =  new int[ 8 ];

		System.out.println( integerArray[ 3 ]  );
		System.out.println( alias[ 3 ]  );
		System.out.println( integerArray  );

		int[]	factorial = { 1,  1,  2,  6,  24,  120,  720,  5040  };
		char[]	vowels = { 'a',  'e',  'o',  'i',  'u' };
		}
	}

Output 10
0
10

[I@5e300868

Multidimensional Arrays

class MultidimensionalArrayExample 
	{

	public static void main( String args[] ) 
		{
		int[][]	squareArray = new int [ 10 ] [ 20 ];

		squareArray[ 2 ] [ 5 ] = 25;

		int[][][]	threeDArray = new int [ 10 ] [ 20 ][ 5 ];

		int[][]	triangularArray;

		triangularArray = new int [ 30 ] [ ] ;

		for ( int row = 0; row < triangularArray.length; row ++ )
			triangularArray [  row ]  =  new int [ row ];

		for ( int row = 0; row < triangularArray.length; row ++ )
			for ( int col = 0; col < triangularArray[ row ].length; col++ )
				triangularArray[ row ][ col ] = 10;
		}
	}

Arrays and Memory Leaks

Arrays are references!

Garbage collection reclaims arrays that can not be accessed!

References are initialized to null

When done with a reference set it to null

class ArrayExamples 
	{

	public static void main( String args[] ) 
		{
		int[]	integerArray  =  new  int[ 10 ];

		integerArray  =  new  int[ 5 ];	// Memory Leak - No!

		integerArray[  1  ]  =  5;

		int[]	aliasForArray  =  integerArray;

		aliasForArray[  1  ]  =  10;

		System.out.println( integerArray[ 1 ] );	//Prints 10
		}
	}

Out-of Bounds - Exceptions

class  ArrayOutOfBounds 
	{

	public static void main( String args[] ) 
		{
		int  students[] = new int[5];

		students[  10  ]  =  1;
		System.out.println( " The End" );
		}
	}

Output
java.lang.ArrayIndexOutOfBoundsException: 10
at ArrayOutofBounds.main(All.java:8)

Out-of Bounds - With try


class  ArrayOutOfBounds 
	{

	public static void main( String args[] ) 
		{
		int  students[] = new int[5];

		try  
			{
			System.out.println( "Start Try"  );
			students[  10  ]  =  1;
			System.out.println(  "After assignment statement"  );
			}

		catch (ArrayIndexOutOfBoundsException e) 
			{
			System.err.println( "OutOfBounds: "  +  e.getMessage());
			};

		System.out.println(  " After try " );
		}
	}

Output
Start Try
OutOfBounds: 10
After try

Multiple Exceptions


class  MultipleExceptions  
	{

	public static void main( String args[] ) 
		{
		int  students[] = new int[5];
		int  classSize  =  0;

		try  
			{
			System.out.println( "Start Try"  );
			int  classAverage  =  10 / classSize;
			System.out.println( "After class average"  );
			students[  10  ]  =  1;
			System.out.println(  "After array statement"  );
			} 

		catch  (ArrayIndexOutOfBoundsException e) 
			{
			System.err.println( "OutOfBounds: "  +  e.getMessage());
			}

		catch  (  ArithmeticException  e  )  
			{
			System.err.println( " Math Error"  +  e.getMessage() );
			}
		System.out.println(  " After try " );
		}
	}

Output Start Try
Math Error/ by zero
After try

One Size Fits All Exception


class  MultipleExceptions  
	{
	public static void main( String args[] ) 
		{
		int  students[] = new int[5];
		int  classSize  =  0;

		try  
			{
			System.out.println( "Start Try"  );
			students[  10  ]  =  1;
			System.out.println(  "After array statement"  );
			
			int  classAverage  =  10 / classSize;
			} 

		catch  ( Exception  e ) 
			{
			System.err.println( "Exception:"  +  e.getMessage());
			e.printStackTrace();
			}
 
		System.out.println(  " After try " );
		}
	}

Output
Start Try
Exception:10
java.lang.ArrayIndexOutOfBoundsException: 10
at MultipleExceptions.main(All.java:11)

 After try

Beware of Order

class  MultipleExceptions  
{
	public static void main( String args[] ) 
		{
		int  students[] = new int[5];
		int  classSize  =  0;

		try  
		{
			System.out.println( "Start Try"  );
			students[  10  ]  =  1;
			System.out.println(  "After array statement"  );
			
			int  classAverage  =  10 / classSize;
		}  
		catch  ( Exception  e ) 
		{
			System.err.println( "Exception:"  +  e.getMessage());
			e.printStackTrace();
		}  
		catch  (ArrayIndexOutOfBoundsException e) 
		{
			System.err.println( "OutOfBounds: "  +  e.getMessage());
		}
		System.out.println(  " After try " );
	}
}

Compile Error All.java:21: catch not reached.
} catch (ArrayIndexOutOfBoundsException e) {
^
1 error

Select the First Exception That Applies

class  MultipleExceptions  
{
	public static void main( String args[] ) 
		{
		int  students[] = new int[5];
		int  classSize  =  0;

		try  
		{
			System.out.println( "Start Try"  );
			students[  10  ]  =  1;
			System.out.println(  "After array statement"  );
			
			int  classAverage  =  10 / classSize;
		}  

		catch  (ArrayIndexOutOfBoundsException e) 
		{
			System.err.println( "OutOfBounds: "  +  
											e.getMessage());
		}   

		catch  ( Exception  e ) 
		{
			System.err.println( "Exception:"  +  e.getMessage());
			e.printStackTrace();
		}
		System.out.println(  " After try " );
	}
}

Output Start Try
OutOfBounds: 10
After try

Final Block - For Clean Up


class  FinalBlockWithExceptionCalled  
{
	public static void main( String args[] ) 
	{
		int  students[] = new int[5];

		try  
		{
			System.out.println( "Start Try"  );
			students[  10  ]  =  1;
			System.out.println(  "After array statement"  );
		}

		catch  (ArrayIndexOutOfBoundsException e) 
		{
			System.err.println( "OutOfBounds: "  +  e.getMessage());
		}

		finally
		{
			System.out.println(  "In Final Block"  );
		}

		System.out.println(  " After try " );
	}
}

Output OutOfBounds: 10
In Final Block
After try

Final Block - Always Called


class  FinalBlockExceptionNotCalled  
{
	public static void main( String args[] ) 
	{
		int  students[] = new int[5];

		try  
		{
			System.out.println( "Start Try"  );
			students[  2  ]  =  1;
			System.out.println(  "After array statement"  );
		}  

		catch  (ArrayIndexOutOfBoundsException e) 
		{
			System.err.println( "OutOfBounds: "  +  e.getMessage());
		}   

		finally   
		{
			System.out.println(  "In Final Block"  );
		}

		System.out.println(  " After try "  );
	}
}

Output Start Try
After array statement
In Final Block
After try

Exceptions and Function Calls


class  ExceptionFunction  
{
	public  static  void  change(  int[]  course  )  
	{
		System.out.println( "Start Change"  );
		course[ 10 ]  =  1;
		System.out.println( "End Change"  );
	};

	public  static  void  main( String args[] ) 
	{
		int  students[] = new int[5];
		try  
		{
			System.out.println( "Start Try"  );
			change( students );
			System.out.println(  "After array statement"  );
		}  
		catch  (ArrayIndexOutOfBoundsException e) 
		{
			System.err.println( "OutOfBounds: "  +  e.getMessage());
		}   
		finally   
		{
			System.out.println(  "In Final Block"  );
		}
		System.out.println(  " After try "  );
	}
}

Output Start Try
Start Change
OutOfBounds: 10
In Final Block
After try

Can't Escape Finally


class  ExceptionFunction  
{
	public  static  boolean  change(  int[]  course  )  
	{
		try  
		{
			System.out.println( "Start change"  );
			course[ 10 ]  =  1;
			System.out.println( "End Change"  );
		}  
		catch  (ArrayIndexOutOfBoundsException e) 
		{
			System.err.println( "OutOfBounds: " );
			return  true;
		}   
		finally   
		{
			System.out.println(  "In Final Block"  );
		}
		return  false;
	};

	public  static  void  main( String args[] ) 
	{
		int  students[] = new int[5];
			System.out.println( "Main"  );
			System.out.println(  change( students )  );
			System.out.println(  "After Change"  );
	}
}