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3.2 Reference Types

Java is an object-oriented language. An object is a collection of variables and associated methods that is described by a class. The concepts in this section that relate to objects are discussed in detail in Object-Orientation Java Style.

The name of a class can be used as a type, so you can declare an object-type variable or specify that a method returns an object. If you declare a variable using the name of a class for its type, that variable can contain a reference to an object of that class. Such a variable does not contain an actual object, but rather a reference to the class instance, or object, the variable refers to. Because using a class name as a type declares a reference to an object, such types are called reference types. Java also allows the use of an interface name to specify a reference type. In addition, array types in Java are reference types because Java treats arrays as objects.

The two main characteristics of objects in Java are that:

  • Objects are always dynamically allocated. The lifetime of the storage occupied by an object is determined by the program's logic, not by the lifetime of a procedure call or the boundaries of a block. The lifetime of the storage occupied by an object refers to the span of time that begins when the object is created and ends at the earliest time it can be freed by the garbage collector.

  • Objects are not contained by variables. Instead, a variable contains a reference to an object. A reference is similar to what is called a pointer in other languages. If there are two variables of the same reference type and one variable is assigned to the other, both variables refer to the same object. If the information in that object is changed, the change is visible through both variables.

Java references are very similar to pointers in C/C++, but they are not at all related to the C++ notion of a reference. The main difference between Java references and C++ pointers is that Java does not allow any arithmetic to be done with references. This, coupled with Java's lack of any way to explicitly deallocate the storage used by reference type values, guarantees that a reference can never point to an illegal address.

The formal definition of a reference type is:

[Graphic: Figure from the text]

It is possible to cause a reference variable to contain a reference to nothing by assigning the special value represented by the keyword null to the variable. The value null can be assigned to any reference variable without a type cast.

Java does not allow reference types to be cast to primitive data types or primitive data types to be type cast to reference types. In particular, unlike C/C++, there is no conversion between integer values and references.

The only operation that Java provides for reference-type variables is the ability to fetch the referenced object. However, Java does not provide an operator to fetch the object referenced by a reference variable. Instead, the object fetch operation is performed implicitly by the following operations:

  • A field expression that accesses a variable or method of a class or interface object

  • A field expression that accesses an element of an array object

  • A type comparison operation that uses the instanceof operator

References Array Types; ClassOrInterfaceName 4.1.6; Class Types; Field Expressions; Interface Types; null; Object-Orientation Java Style; Primitive Types; The instanceof Operator

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

Specially supported classes

Java provides special support for the String and StringBuffer classes. All string literals are compiled into String objects. The result of a string concatenation operation is a String object. An intermediate StringBuffer object is used to compute the result of a concatenation operation. Because operations on strings are generally based on the length of the string, Java does not automatically supply a NUL character (\u0000) at the end of a string literal. For the same reason, it is not customary for Java programs to put a NUL character at the end of a string.

Java also provides special support for the Object class. This class is the ultimate superclass of all other classes in Java. If a class is declared without its superclass being specified, the language automatically specifies Object as its superclass.

The throw statement in Java is special, in that it requires the use of a Throwable object.

References Object; String; StringBuffer; String Concatenation Operator +; String literals; The throw Statement; Throwable

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


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