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So don t do it, and if you see it on the test, run screaming toward the nearest answer marked Compilation fails.
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You may see the words construct, create, and instantiate used interchangeably. They all mean, An object is built and placed on the heap. These words also imply that the object s constructor runs, as a result of the contruct/create/ instantiate code. You can say with certainty, for example, that any code that uses the keyword new will (if it runs successfully) cause the class constructor and all superclass constructors to run.
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In addition to being constructed with new, arrays can also be created using a kind of syntax shorthand that creates the array while simultaneously initializing the array elements to values supplied in code (as opposed to default values). We ll look
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Array Declaration, Construction, and Initialization (Exam Objective 1.1)
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at that in detail in the section on initialization. For now, understand that because of these syntax shortcuts, objects can still be created even without you ever using or seeing the keyword new.
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Constructing Multidimensional Arrays
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Multidimensional arrays, remember, are simply arrays of arrays. So a two-dimensional array of type int is really an object of type int array (int []), with each element in that array holding a reference to another int array. The second dimension holds the actual int primitives. The following code declares and constructs a two-dimensional array of type int:
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int[][] ratings = new int[3][];
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Notice that only the first brackets are given a size. That s acceptable in Java, since the JVM needs to know only the size of the object assigned to the variable ratings. Figure 1-4 shows how a two-dimensional int array works on the heap.
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Initializing an Array
Initializing an array means putting things into it. Things (why, yes that is a technical term) in the array are the array s elements, and they re either primitive values (2, a , false, etc.), or objects referred to by the reference variables in the array. If you have an array of objects (as opposed to primitives) the array doesn t actually hold the objects, just as any other nonprimitive variable never actually holds the object, but instead holds a reference to the object. But we talk about arrays as, for example, an array of five strings , even though what we really mean is, an array of five references to String objects. Then the big question becomes whether or not those references are actually pointing (oops, this is Java, we mean referring) to real String objects, or are simply null. Remember, a reference that has not had an object assigned to it is a null reference. And if you try to actually use that null reference by, say, applying the dot operator to invoke a method on it, you ll get the infamous NullPointerException. The individual elements in the array can be accessed with an index number. The index number always begins with zero, so for an array of ten objects the index numbers will run from 0 through 9. Suppose we create an array of three Animals as follows:
Animal [] pets = new Animal[3];
1: Language Fundamentals
FIGURE 1-3
A two-dimensional array on the heap
We have one array object on the heap, with three null references of type Animal, but we still do not have any Animal objects. The next step is to create some Animal objects and assign them to index positions in the array referenced by pets:
pets[0] = new Animal(); pets[1] = new Animal(); pets[2] = new Animal();
This code puts three new Animal objects on the heap and assigns them to the three index positions (elements) in the pets array.
Array Declaration, Construction, and Initialization (Exam Objective 1.1)
Look for code that tries to access an out of range array index. For example, if an array has three elements, trying to access the [3] element will raise an ArrayIndexOutOfBoundsException, because in an array of three elements, the legal index values are 0, 1, and 2. You also might see an attempt to use a negative number as an array index. The following are examples of legal and illegal array access attempts. Be sure to recognize that these cause runtime exceptions and not compiler errors! Nearly all of the exam questions list both runtime exception and compiler error as possible answers.
int[] x = new int[5]; x[4] = 2; // OK, the last element is at index 4 x[5] = 3; // Runtime exception. There is no element at index 5! int [] z = new int[2]; int y = -3; z[y] = 4; // Runtime exception.; y is a negative number
These can be hard to spot in a complex loop, but that s where you re most likely to see array index problems in exam questions.
A two-dimensional array (an array of arrays) can be initialized as follows:
int[][] scores = new int[3][]; // Declare and create an array holding three references to int arrays scores[0] = new int[4]; // the first element in the scores array is an int array of four int element scores[1] = new int[6]; // the second element in the scores array is an int array of six int elements scores[2] = new int[1]; // the third element in the scores array is an int array of one int element
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