Class Types

The name of a class can be used to specify the type of a reference. If a variable is declared as a class type, the variable either contains null or a reference to an object of that class or a subclass of that class. It is not allowed to contain any other kinds of values. For example:

class Shape { ... }
class Triangle extends Shape { ... }
...
Shape s;
Triangle t;
...
s = t;

This example declares a class called Shape and a subclass of Shape called Triangle. The code later declares a reference variable called s that can contain a reference to a Shape object and another variable called t that can contain a reference to a Triangle object. The value of s can be assigned to the value of t because an object is not only an instance of its declared class, but also an instance of every superclass of its declared class. Since instances of the Triangle class are also instances of its superclass Shape, the Java compiler has no problem with s = t.

However, saying t = s generates an error message from the compiler. Java does not allow a reference variable declared as a class type to contain a reference to a superclass of the declared class. The assignment t = s is illegal because Shape is a superclass of Triangle. The assignment can be accomplished if s is first cast to a reference to Triangle:

t = (Triangle)s;

The cast operation ensures that the object referenced by s is a class type that is either Triangle or a descendant of Triangle. When you cast an object reference to a subclass of the reference type, you are saying that you want to treat the object being referenced as an instance of the specified subclass. If the compiler cannot determine whether the argument of a cast will be of the required type, the compiler generates runtime code that ensures that the argument really is an instance of the specified subclass. At runtime, if the class of the object being referenced is not an instance of the specified subclass, a ClassCastException is thrown.

References Casts; Classes; Class Declarations; Object Allocation Expressions; Runtime exceptions

Interface Types

The name of an interface can be used to specify the type of a reference. A reference variable declared using an interface name as its type can only reference instances of classes that implement that interface. For example, Java provides an interface called Runnable. Java also provides a class called Thread that implements Runnable. This means that the following assignment is allowed:

Runnable r;
r = new Thread();

The Java compiler does not allow a value to be assigned to a variable declared using an interface type unless the compiler can be sure that the object referenced by the value implements the specified interface. Casting a reference variable to an interface type allows the variable to be assigned to that interface type, because the cast operation provides its own guarantee that the object implements the specified interface. Unless the compiler is able to determine the actual class of the object that will be referenced at runtime, the cast produces code that verifies at runtime that the object being cast really does implement the specified interface. At runtime, if the object being cast does not implement the required interface, a ClassCastException is thrown.

References Casts; Interfaces; Interface Declarations; Object Allocation Expressions; Runtime exceptions

Array Types

An array is a special kind of object that contains values called elements. Array elements are similar to variables in that they contain values that can be used in expressions and set by assignment operations. Elements differ from variables, however, in that they do not have names. Instead, they are identified by non-negative integers. The elements in an array are identified by a contiguous range of integers from 0 to one less than the number of elements in the array. The elements of an array must all contain the same type of value; the type of the array is specified when the array is created.

An array-type variable is declared as follows:

int [] a;

This declaration specifies that the variable a refers to an array of int values. Java actually allows two styles of array declarations: the one shown above and a style that is more like that used in C/C++. In other words, you can put the square brackets after the variable name instead of after the type:

int a[];

Technically, all arrays in Java are one-dimensional. However, Java does allow you to declare an array of arrays, which is a more flexible data structure than a multi-dimensional array. The additional flexibility comes from the fact that the arrays in an array of arrays do not have to be the same length. Because arrays of arrays are typically used to represent multi-dimensional arrays, this book refers to them as multi-dimensional arrays, even though that is not precisely correct.

A multi-dimensional array is declared using multiple pairs of square brackets, as in the following examples:

int [][] d2;       // Refers to a 2-dimensional array
int [][][] d3;     // Refers to a 3-dimensional array

When you declare a variable to refer to a multi-dimensional array, the number of dimensions in the array is determined by the number of pairs of square brackets. Whether the brackets follow the type name or the variable name is not important. Thus, the above variables could have been declared like this:

int [] d2[],       // Refers to a 2-dimensional array
       d3[][];     // Refers to a 3-dimensional array

The actual length of each dimension of an array object is specified when the array object is created, not when the array variable is declared. An array object is not created at the same time that an array variable is declared. An array object is created with the new operator. Here are some examples:

int j[] = new int[10];        // An array of 10 ints
int k[][] = new float[3][4];  // An array of 3 arrays of 4 floats

The arrays contained in an array of arrays can also be of different lengths:

int a[][] = new int [3][];
a[0] = new int [5];
a[1] = new int [6];
a[2] = new int [7];

Although the first new operator is creating a two-dimensional array, only the length of one dimension is specified. In this case, just the array of arrays is created. The subarrays are created by the subsequent new operators.

The expression used to specify the length of an array does not have to be a constant. Consider the following example:

int[] countArray(int n){
    int[] a = new int[n];
    for (int i=0; i<n; i++) {
        a[i]=i+1;
    }
    return a;
}

The number of elements in an array object is fixed at the time that the array object is created and cannot be changed.[2] Every array object has a public variable called length that contains the number of elements in the array. The variable length is final, which means that its value cannot be changed by assignment.

[2] The standard class java.util.Vector implements an array-like object with a length that can be changed.

The Java notion of arrays is fundamentally different than that of C/C++. Subscripting a Java array does not imply pointer arithmetic, so there is no danger of an out-of-range index accessing memory that shouldn't be accessed. Array objects in Java detect out-of-range subscripts; when they do they throw an ArrayIndexOutOfBoundsException. And unlike C/C++, arrays of type char are not strings in Java. Instead, Java uses the String class to support strings.

Although array objects are reference types, array objects are different from other kinds of objects. The Object class is the parent class of all array objects, but array objects do not really belong to a class of their own. An array object inherits all of the variables and methods of the Object class. Every array also defines the variable length, but there is no class declaration for an array type.

References Variable initializers; Array Allocation Expressions; Index Expressions; Object; String