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This chapter is from the book

3.2 Method Declarations

For the purpose of this book, we will use the following simplified syntax of a method declaration:

method_modifiers return_type method_name
                                 (formal_parameter_list) throws_clause // Method header
{ // Method body
          local_variable_declarations
          statements
}

In addition to the name of the method, the method header can specify the following information:

  • Scope or accessibility modifier (§4.7, p. 123)

  • Additional method modifiers (§4.8, p. 131)

  • The type of the return value, or void if the method does not return any value (§6.4, p. 224)

  • A formal parameter list

  • Any exceptions thrown by the method, which are specified in a throws clause (§6.9, p. 251)

The formal parameter list is a comma-separated list of parameters for passing information to the method when the method is invoked by a method call (§3.5, p. 72). An empty parameter list must be specified by ( ). Each parameter is a simple variable declaration consisting of its type and name:

optional_parameter_modifier type parameter_name

The parameter names are local to the method (§4.4, p. 117). The optional parameter modifier final is discussed in §3.5, p. 80. It is recommended to use the @param tag in a Javadoc comment to document the formal parameters of a method.

The signature of a method comprises the name of the method and the types of the formal parameters only.

The method body is a block containing the local variable declarations (§2.3, p. 40) and the statements of the method.

The mandatory parts of a method declaration are the return type, the method name, and the method body braces ({}), as exemplified by the following method declaration:

void noAction() {}

Like member variables, member methods can be characterized as one of two types:

  • Instance methods, which are discussed later in this section

  • Static methods, which are discussed in §4.8, p. 132

Statements

Statements in Java can be grouped into various categories. Variable declarations with explicit initialization of the variables are called declaration statements (§2.3, p. 40, and §3.4, p. 60). Other basic forms of statements are control flow statements (§6.1, p. 200) and expression statements.

An expression statement is an expression terminated by a semicolon. Any value returned by the expression is discarded. Only certain types of expressions have meaning as statements:

  • Assignments (§5.6, p. 158)

  • Increment and decrement operators (§5.9, p. 176)

  • Method calls (§3.5, p. 72)

  • Object creation expressions with the new operator (§5.17, p. 195)

A solitary semicolon denotes the empty statement, which does nothing.

A block, {}, is a compound statement that can be used to group zero or more local declarations and statements (§4.4, p. 117). Blocks can be nested, since a block is a statement that can contain other statements. A block can be used in any context where a simple statement is permitted. The compound statement that is embodied in a block begins at the left brace, {, and ends with a matching right brace, }. Such a block must not be confused with an array initializer in declaration statements (§3.4, p. 60).

Labeled statements are discussed in §6.4 on page 220.

Instance Methods and the Object Reference this

Instance methods belong to every object of the class and can be invoked only on objects. All members defined in the class, both static and non-static, are accessible in the context of an instance method. The reason is that all instance methods are passed an implicit reference to the current object—that is, the object on which the method is being invoked. The current object can be referenced in the body of the instance method by the keyword this. In the body of the method, the this reference can be used like any other object reference to access members of the object. In fact, the keyword this can be used in any non-static context. The this reference can be used as a normal reference to reference the current object, but the reference cannot be modified—it is a final reference (§4.8, p. 133).

The this reference to the current object is useful in situations where a local variable hides, or shadows, a field with the same name. In Example 3.1, the two parameters noOfWatts and indicator in the constructor of the Light class have the same names as the fields in the class. The example also declares a local variable location, which has the same name as one of the fields. The reference this can be used to distinguish the fields from the local variables. At (1), the this reference is used to identify the field noOfWatts, which is assigned the value of the parameter noOfWatts. Without the this reference at (2), the value of the parameter indicator is assigned back to this parameter, and not to the field by the same name, resulting in a logical error. Similarly at (3), without the this reference, it is the local variable location that is assigned the value of the parameter site, and not the field with the same name.

Example 3.1 Using the this Reference

public class Light {
  // Fields:
  int     noOfWatts;      // Wattage
  boolean indicator;      // On or off
  String  location;       // Placement

  // Constructor
  public Light(int noOfWatts, boolean indicator, String site) {
    String location;

    this.noOfWatts = noOfWatts;   // (1) Assignment to field
    indicator = indicator;        // (2) Assignment to parameter
    location = site;              // (3) Assignment to local variable
    this.superfluous();           // (4)
    superfluous();                // equivalent to call at (4)
  }

  public void superfluous() {
    System.out.printf("Current object: %s%n", this); // (5)
  }

  public static void main(String[] args) {
    Light light = new Light(100, true, "loft");
    System.out.println("No. of watts: " + light.noOfWatts);
    System.out.println("Indicator:    " + light.indicator);
    System.out.println("Location:     " + light.location);
  }
}

Probable output from the program:

Current object: Light@1bc4459
Current object: Light@1bc4459
No. of watts: 100
Indicator:    false
Location:     null

If a member is not shadowed by a local declaration, the simple name member is considered a short-hand notation for this.member. In particular, the this reference can be used explicitly to invoke other methods in the class. This usage is illustrated at (4) in Example 3.1, where the method superfluous() is called.

If, for some reason, a method needs to pass the current object to another method, it can do so using the this reference. This approach is illustrated at (5) in Example 3.1, where the current object is passed to the printf() method. The printf() method prints the string representation of the current object (which comprises the name of the class of the current object and the hexadecimal representation of the current object’s hash code). (The hash code of an object is an int value that can be used to store and retrieve the object from special data structures called hash tables.)

Note that the this reference cannot be used in a static context, as static code is not executed in the context of any object.

Method Overloading

Each method has a signature, which comprises the name of the method plus the types and order of the parameters in the formal parameter list. Several method implementations may have the same name, as long as the method signatures differ. This practice is called method overloading. Because overloaded methods have the same name, their parameter lists must be different.

Rather than inventing new method names, method overloading can be used when the same logical operation requires multiple implementations. The Java SE platform API makes heavy use of method overloading. For example, the class java.lang.Math contains an overloaded method min(), which returns the minimum of two numeric values.

public static double min(double a, double b)
public static float min(float a, float b)
public static int min(int a, int b)
public static long min(long a, long b)

In the following examples, five implementations of the method methodA are shown:

void methodA(int a, double b) { /* ... */ }      // (1)
int  methodA(int a)           { return a; }      // (2)
int  methodA()                { return 1; }      // (3)
long methodA(double a, int b) { return b; }      // (4)
long methodA(int x, double y) { return x; }      // (5) Not OK.

The corresponding signatures of the five methods are as follows:

methodA(int, double)             1'
methodA(int)                     2': Number of parameters
methodA()                        3': Number of parameters
methodA(double, int)             4': Order of parameters
methodA(int, double)             5': Same as 1'

The first four implementations of the method named methodA are overloaded correctly, each time with a different parameter list and, therefore, different signatures. The declaration at (5) has the same signature methodA(int, double) as the declaration at (1) and, therefore, is not a valid overloading of this method.

void bake(Cake k)  { /* ... */ }                 // (1)
void bake(Pizza p) { /* ... */ }                 // (2)

int     halfIt(int a) { return a/2; }            // (3)
double  halfIt(int a) { return a/2.0; }          // (4) Not OK. Same signature.

The method named bake is correctly overloaded at (1) and (2), with two different parameter lists. In the implementation, changing just the return type (as shown at (3) and (4) in the preceding example), is not enough to overload a method, and will be flagged as a compile-time error. The parameter list in the declarations must be different.

Only methods declared in the same class and those that are inherited by the class can be overloaded. Overloaded methods should be considered to be individual methods that just happen to have the same name. Methods with the same name are allowed, since methods are identified by their signature. At compile time, the right implementation of an overloaded method is chosen, based on the signature of the method call. Details of method overloading resolution can be found in §7.10 on page 316. Method overloading should not be confused with method overriding (§7.2, p. 268).

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