This book is now obsolete Please use CSAwesome instead.
8.2. Looping with the For-Each Loop¶
You will often loop through all of the elements of an array (to get the average or to get each one to display). You will typically do this using a for-each loop. A for-each loop is a loop that can only be used on a collection of items. It will loop through the collection and each time through the loop it will use the next item from the collection. It starts with the first item in the array (the one at index 0) and continues through in order to the last item in the array. You can step through this code using the Java Visualizer by clicking on the following link link1.
The for-each loop is shown on line 6 above. It says to loop through the array called
values and each time through the loop set the variable
val to the next item in the array. We have to specify the type of
val first since this declares a variable. The type must match the type of objects in the array.
Only use the for-each loop when you want to loop through all the values in an array or list. If you only want to loop through part of an array or list use a for loop instead. Also use a for loop instead of a for-each loop if you want to change any of the values in the array or list.
The code above wasn’t object-oriented. You may have noticed that it was declared to be static. This means that it is a class method not an object method. It is a class method since it doesn’t operate on any object fields - all data that it needs has been passed in to the method. Class methods can be called using
ClassName.methodName(). They can also be called on an object of the class. Object methods can only be called on an object of the class.
A more object-oriented way of doing this would be if the array was a field called
values in the same class as the
getAverage method. Then you don’t need to pass the array
values to the method and the method is an object (instance) method since it operates on the fields of the object. You will typically initialize fields in the constructor as shown below.
You can use the Java Visualizer to step through this code by clicking on the following link link2.
Notice that we have to create an object of the class now in the
main method. Object methods have to be called on an object of the class.
values is an object field and the method
getAverage is in the same class it can directly access the field
values. The code could have also been written as
this.values to indicate the current object’s field called
values. Every object method is passed the object the method was called on and it can be referenced using the Java keyword
Mixed up programs
The following method has the correct code to return the largest value in an integer array called <i>vals</i> (a field of the current object), but the code is mixed up. Drag the blocks from the left into the correct order on the right and indent them correctly as well. You will be told if any of the blocks are in the wrong order or not indented correctly.</p>
If you want to step through the correct code to see what it does in the Java Visualizer click on the following link link3.
Some examples of finding the largest value in an array start by setting the largest variable to 0. But, what happens if the array only contains negative numbers? What value could you set largest to and still have it work correctly even if the field
vals contained only negative numbers?