Java eLearning Kit For Dummies (2014)

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Creating a basic for-each loop

A for-each loop can process any collection of objects that you want. In the example that follows, you create a collection of char variables from a String. A collection of this sort makes it easier to see how the for-each loop works and why it’s such an important Java programming feature.

Files needed: StandardForEach.java

1. Open the editor or IDE that you’ve chosen to use for this book.

2. Type the following code into the editor screen.

// Import the required API classes.
import java.lang.String;
import java.util.Scanner;

public class StandardForEach
{
public static void main(String[] args)
{
// Create the scanner.
Scanner GetString = new Scanner(System.in);

// Obtain a string value.
System.out.print("Type any string: ");
String MyString = GetString.nextLine();

// Process each character separately in a
// for-each loop.
for(char Letter : MyString.toCharArray())
System.out.println(Letter);
}
}

This example begins by obtaining a String from the user. The String can contain anything — numbers, letters, or special characters, as long as they aren’t special characters, such as a tab.

The example then converts the String  — MyString, in this example — to a collection of char variables by calling the toCharArray() method. It places each char in Letter and then displays Letter on screen. In short, the for-each loop is processing each letter in MyString separately.

3. Save the code to disk using the filename StandardForEach.java.

4. In your command prompt, type javac StandardForEach.java and press Enter.

The compiler compiles your application and creates a .class file from it.

5. Type java StandardForEach and press Enter.

The application asks you to type any string.

6. Type Hello and press Enter.

The application outputs each letter that you’ve typed on a separate line of the output, as shown in Figure 7-7.

Figure 7-7:

Using the for-each loop with enumerations

Chapter 3 introduces you to enumeration, a special kind of structure that makes it possible to create item lists. It’s possible to use a for-each loop to list an enumeration. Instead of assuming that you know what a list contains (as the example in Lesson 3 does), you can use a for-each loop to ask what your options are. People commonly perform this task with real world lists, too, so it makes sense that Java supports this feature.

In this example, you create a Days of the Week enumeration and then use it to display the names to the user with a for-each loop. The user selects any of the entries by typing its associated number.

Files needed: UsingEnumerations.java

1. Open the editor or IDE that you’ve chosen to use for this book.

2. Type the following code into the editor screen.

// Import the required API classes.
import java.util.Scanner;

public class UsingEnumerations
{
// Create an enumeration to use in the
// example.
enum Days
{
Sunday,
Monday,
Tuesday,
Wednesday,
Thursday,
Friday,
Saturday
}

public static void main(String[] args)
{
// Create the scanner.
Scanner GetDay = new Scanner(System.in);

// Display the list of days on screen.
for (Days ThisDay : Days.values())
System.out.println(
ThisDay.ordinal() + "\t" + ThisDay);

// Display a message asking the user to select
// a day.
System.out.print("Type the number of a day: ");

// Get the number.
int Selection = GetDay.nextInt();

// Display the result.
Days TheDay = Days.values()[Selection];
System.out.println("You selected " + TheDay);
}
}

Enumerations can appear just about anywhere in your code, but this example places the enumeration within the class structure, outside of main(). The enumeration is named Days, and it contains the days of the week.

Inside main(), the for-each loop lists each day of the week in turn. An enumerated entry has both a name and an ordinal value. The example shows you both of them.

After the application displays the days of the week for the user, it asks the user to select a particular day by typing its numeric value. The code uses this value to select a particular day from Days and place it in TheDay. The output from the example is the name of the day the user has selected.

3. Save the code to disk using the filename UsingEnumerations.java.

4. In your command prompt, type javac UsingEnumerations.java and press Enter.

The compiler compiles your application and creates a .class file from it.

5. Type java UsingEnumerations and press Enter.

The application displays the days of the week. It then asks the user to type a number corresponding to a particular day.

6. Type 1 and press Enter.

The application outputs Monday as the day selected, as shown in Figure 7-8.

Figure 7-8:

Summing Up

Here are the key points you learned about in this chapter:

· The for loop repeats a series of tasks a specific number of times.

· Use the break statement to stop a for loop from continuing to process tasks.

· Use the continue statement when you want to stop a single for loop from processing but want to continue with the remaining loops.

· Nest two or more sets of for loops when you need to process multiple dimensions or create a complex processing loop series.

· A while loop continues to repeat a series of tasks until a specific condition is met.

· The do … while statement acts much like the while statement, but it has the distinction of executing the tasks within its structure at least once, no matter what condition you might set for it.

· It’s possible to repeat most forms of standard looping requirements by using the for, while, or do … while statements and your choice depends on convenience or personal taste.

· The for-next loop is special in that it works with each element in an enumeration, array, or collection.

· The for-each loop relies on a special kind of for loop to do its work.

· You use the for-each loop to process individual elements of a collection of elements one element at a time.

· Using an enumeration can make it easier to work with your application.

Try-it-yourself lab

For more practice with the features covered in this chapter, try the following exercise on your own.

1. Open the UsingEnumerations application.

2. Create a new enumeration that contains a list of colors.

Make sure you separate each color with a command and don’t include any spaces in the color names.

3. Rewrite the code to display the enumeration to the user with the for-each loop.

4. Change the code to ask the user to select a color and then display the color selection onscreen.

5. Compile the application.

6. Run the application.

7. Try each of the new color entries.

Do the new color entries work as you anticipated? If they don’t, why not?

Know this tech talk

· counter variable: A special sort of variable used to track the current count of something in the application. In most cases, a counter variable is a primitive integer type because counter variables only track whole numbers and don’t require the extra features of an object variable type.

· enumeration: A listing of like items that you can use to make your code easier to read. An enumeration makes it possible to refer to something using a human-readable term, even though the actual value is a number.

· iteration: A single loop from a series of repetitive loops. A single iteration is one pass through the code in a repetitive loop structure.

· nesting: Placing one structure within another structure of the same type. For example, nesting for loops places a subordinate for loop within a main for loop.

· repetitive loop: A method of telling your application to perform a task more than once. The repetitive loop consists of a structure that defines where the loop begins and ends, some sort of condition used to determine how long to loop, and the series of tasks contained within the structure.

· structure: A programming construct that begins with a statement that specifies the purpose of the structure. Normally curly braces ({}) appear after the statement to define the statement body. One or more lines of code appear within the structure body to define the tasks performed by the structure.