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Conditional: if-else
Repetitions
Flow Control
Java vs C/C++
JVM Instructions 1

Perhaps the single most powerful tool in programming is the if-then-else conditional test statement. It allows the flow of the program to proceed in different ways according to a test that results in boolean true or false value.

In Java the "then" is implied. The conditional test follows the C/C++ form with:

    if( boolean expression )
        statement 1;
    else
        statement 2;

where boolean expression encompasses any expression that returns a boolean value and statement 1 and statement 2 indicate single line statements.

Often the if-else will use code blocks of multiple statements enclosed in parentheses:

    if( boolean expression )
    {
        statement 1;
        statement 1a;
    }
    else
    {
        statement 2;
        statement 2a;
    }

The else clause can be dropped if no alternate operation needed :

    if( boolean expression ) statement;

Possible boolean expressions include the following:

• A boolean variable:
  
    public void test( boolean b)
    {
      ...
      if ( b ) y = 5;
      ...
    }
  
  
• A comparison
  
    public void test( int x )
    {
      ... 
      if ( x > 5 ) x = 5;
      ...
    }
  
  
• A boolean operation
  
    public void test( boolean b1, boolean b2)
    {
      ...
      if ( b1 || b2 ) x = 5;
      ...
    }
  
  
• An assignment to a boolean
  
    public void test( boolean b)
    {
      ...
      boolean b1;
      if ( b1 = b ) x = 5;
      ...
    }
  

Several if-else sequential tests can be done in one compound statement:

    if( boolean expression 1)
    {
        statement 1;
        statement 1a;
    } else if( boolean expression 2)
    {
        statement 2;
        statement 2a;
    }else if( boolean expression 3)
        statement 3;
    else
    {
        statement 4;
    }

Note that statement 4 is executed if the other boolean expressions all return false.

Latest update: Dec.11.2003

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