Review:   Method Signature

The (1) method name and the (2) parameter list together constitutes the method signature:

The method signature is used to identify a method

Therefore:   a class cannot have more than 1 method with the same method signature.

Method overloading

  • A method is overloaded when:

    • There are multiple methods inside a class that have the same method name

      (but have different method signature)

    Example: 

        public static void print(short x)
    {
        System.out.println("short: " + x);
    }
    
    public static void print(int x)
    {
        System.out.println("int: " + x);
    }
    
    public static void print(double x)
    {
        System.out.println("double: " + x);
    }

How Java select methods:   (1) method signatures match exactly

  • The Java compiler determines which method to use by matching the method signatures:

    • Java will select/use a method with the exact matching signature as the arguments if one can be found

    Examples:   

        public static void main(String[] args)
    {
        short  a = 1;
        int    b = 1;
        double c = 1;

        print(a);      // Invokes  print(short x)
        print(b);      // Invokes  print(int x)
        print(c);      // Invokes  print(double x)
    }

DEMO: demo/06-methods/05-method-signature/ExactMatch.java     ---     Step to show diff method invocations

How Java select methods:   (2) no exact matching method signatures

  • If no method with exact matching method signature can be found:

    • Java will promote the data type of the arguments to the nearest "larger" type until a matching method signature can be found 

                                char
 Data type                  \
 promotion:   byte -> short --> int -> long -> float -> double

    Examples: 

     byte a = 1;    
 print(a);    // Promotion: byte --> short  to find print(short x)
              // Invokes    print(short x)  (not print(int x))

 float c = 1; 
 print(c);    // Promotion: float --> double  to find print(double x)
              // Invokes    print(double x)

DEMO: demo/06-methods/05-method-signature/NoExactMatch.java

Method selection ambiguity

  • Method selection ambiguity:

    • If there are different ways to find a matching method signature, Java will report an error

  • Example of an ambigious selection: 

        public static void main(String[] args) 
    {
       int a = 4, b = 5;
       
       print(a,b); // (1) promote b to double --> uses print(int, double)
                   // (2) promote a to double --> uses print(double, int)
    }
    
    public static void print(int x, double y)
    {
        System.out.println("int x:" + x + " double: " + y);
    }
    
    public static void print(double x, int y)
    {
        System.out.println("double: " + x + " int: " + y);
    }

DEMO: demo/06-methods/05-method-signature/AmbiguousSelection.java

Good programming practice

  • Recommendation:

    • Do not allow Java to select a method by type promotion

  • Force the choice of the methods by yourself using casting: 

        public static void main(String[] args) 
    {
       int a = 4, b = 5;
       
       print((double)a, (int)b);  // Calls: print(double, int)
    }
    
    public static void print(int x, double y)
    {
        System.out.println("int x:" + x + " double: " + y);
    }
    
    public static void print(double x, int y)
    {
        System.out.println("double: " + x + " int: " + y);
    }

DEMO: demo/06-methods/05-method-signature/MakeExplicit.java