An Overview of Java

Java allows two or more statements to be grouped into blocks of code, also called code blocks.This is done by enclosing the statements between opening and closing curly braces. Once a block of code has been created, it becomes a logical unit that can be used any place that a single statement can. For example, a block can be a target for Java’s if and for statements. Consider this if statement:

if(x < y) { // begin a block
  x = y;
  y = 0;
} // end of block

Here, if x is less than y, then both statements inside the block will be executed. Thus, the two statements inside the block form a logical unit, and one statement cannot execute without the other also executing. The key point here is that whenever you need to logically link two or more statements, you do so by creating a block.

Let’s look at another example. The following program uses a block of code as the target of a for loop.

/*
  Demonstrate a block of code.
  Call this file "BlockTest.java"
*/
class BlockTest {
  public static void main(String args[]) {
    int x, y;
    y = 20;
    // the target of this loop is a block
    for(x = 0; x<10; x++) {
      System.out.println("This is x: " + x);  
      System.out.println("This is y: " + y);
      y = y - 2;
    }
  }
}

The output generated by this program is shown here:

This is x: 0
This is y: 20
This is x: 1
This is y: 18
This is x: 2
This is y: 16
This is x: 3
This is y: 14
This is x: 4
This is y: 12
This is x: 5
This is y: 10
This is x: 6
This is y: 8
This is x: 7
This is y: 6
This is x: 8
This is y: 4
This is x: 9
This is y: 2

In this case, the target of the for loop is a block of code and not just a single statement. Thus, each time the loop iterates, the three statements inside the block will be executed. This fact is, of course, evidenced by the output generated by the program.

As you will see later in this book, blocks of code have additional properties and uses. However, the main reason for their existence is to create logically inseparable units of code.