Overloading class methods

 

Overview

It is possible in Java to define several methods which have the same name but a recognizable difference in their parameters. This is referred to as method overloading. When a call expression is encountered, the compiler knows which of the identically named methods to call based upon the parameters being passed.

 

Overloaded class methods

For example, consider the following method headers:

public static double area(double length, double width)

public static double area(double radius)

public static double area(double top, double bottom, double height)

These represent the headers of methods that could be used to calculate the area of a rectangle, the area of a circle, and the area of a trapezoid. If the compiler encounters the call expression

area(2.3)

it knows to call the second area() method (to calculate the area of a circle) because that method only accepts one double parameter. If the compiler encounters the call expression

area(3.5, 1.7)

it knows to call the first area() method (to calculate the area of a rectangle) because that method accepts two double parameters. And, if the compiler encounters the call expression

area(4.2, 1.8, 3.4)

it knows to call the third area() method (to calculate the area of a trapezoid) because that method accepts three double parameters.

area(2)

would result in the compiler calling the second area() method (to calculate the area of a circle) because the int being passed can be widened to a double. If this is a problem, another overloaded method could be defined which only accepts a single integer parameter. Its header might be

public static double area(long radius)

If any type of integer is passed (byte, short, etc.), it would be widened to a long and passed to this overloaded method. Because of automatic widening, care must be used in defining overloaded methods.

public static void aMethod(byte parm1, float parm2)

public static int aMethod()

public static char aMethod(float parm1, byte parm2)

public static boolean aMethod(double parm1)

are the headers of four different methods that happen to have the same name. What you are NEVER allowed to do, however, is to have exactly the same signatures but return different data types. For example, the following method headers 

public static int aMethod(int x)

public static char aMethod(int y)

would result in a compile error because the compiler wouldn't know which method to call if it encountered a call expression such as

aMethod(17)

 

Sample program

The following sample program calculates the amount due for an order. Overloaded methods are used to perform the calculation differently depending upon how many items are ordered and whether or not the item is taxable.

public class App {
  public static void main(String[] args) {

    // Variables.

    byte quantity;
    double unitPrice;
    boolean isTaxable;
    double amountDue;
    char again;

    // Constants.

    final double TAX_RATE = .06;

    // Loop to process one order.

    do {

      // Get data from the user.

      Utility.separator(50, '>');
      System.out.print("Enter quantity ordered: ");
      quantity = Keyboard.readByte();
      System.out.print("Enter unit price: ");
      unitPrice = Keyboard.readDouble();
      System.out.print("Taxable item? (true or false): ");
      isTaxable = Keyboard.readBoolean();

      // Process the order data.

      Utility.skip();

      if (quantity <= 0 || unitPrice <= 0) {
        System.out.println("\tInvalid data");
      }
      else if (quantity == 1 && isTaxable) {
        System.out.println("Single item with tax: " +
          Utility.moneyFormat(amountDue(unitPrice, TAX_RATE)));
      }
      else if (quantity == 1 && !isTaxable) {
        System.out.println("Single item without tax: " +
          Utility.moneyFormat(amountDue(quantity, unitPrice)));
      }
      else if (quantity > 1 && isTaxable) {
        System.out.println("Multiple items with tax: " +
          Utility.moneyFormat(amountDue(quantity, unitPrice, TAX_RATE)));
      }
      else {
        System.out.println("Multiple items without tax: " +
          Utility.moneyFormat(amountDue(quantity, unitPrice)));
      }

      // Ask if they want to do another and repeat as requested.

      Utility.skip();
      System.out.print("Again? (Y/N): ");
      again = Keyboard.readChar();
    } while (again == 'Y' || again == 'y');
  }

  /**
  * Method to calculate the amount due for an order of multiple
  * non-taxable items. This method receives the quantity and the
  * unit price.
  */

  public static double amountDue(int qty, double price) {
    return qty * price;
  }

  /**
  * Method to calculate the amount due for an order of multiple
  * taxable items. This method receives the quantity, the unit
  * price, and the sales tax rate.
  */

  public static double amountDue(int qty, double price, double rate) {
    return qty * price * (1 + rate);
  }

  /**
  * Method to calculate the amount due for an order of a single
  * taxable item. This method receives the unit price and the
  * sales tax rate.
  */

  public static double amountDue(double price, double rate) {
    return price * (1 + rate);
  }
}

Notes:

  1. Although the overloaded methods are trivial in this sample, they demonstrate how overloaded methods work. Be sure to notice how their signatures differ and how they are called.

  2. Don't be intimidated by the level of cascading. Read cascaded code from the inside and work out. Focus on what you get back from the inner method and see how that value can be used as a parameter in calling the outer method. Being able to nest method calls inside method calls is one of the strengths of Java.

 

Review questions

  1. Based upon the following class definition, which of the methods below can be legally defined at line 4?  (choose three)

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public class MyMethods {
  public static int aMethod(int a, char b) {
  }

}
  1. public static int aMethod(int c, char d) {}

  2. public static int aMethod(char b, int a) {}

  3. public static byte aMethod(int x, char y) {}

  4. public static byte aMethod(char b, int a) {}

  5. public static void aMethod(char b, int a) {}

  1. If a class named MyClass has methods with the headers

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5
public static void aMethod(float a, boolean b)
public static void aMethod(double a, boolean b)
public static void aMethod(int a, boolean b)
public static void aMethod(long a, boolean b, char c)
public static void aMethod(int a, boolean b, char c)

Which of the methods would be called by the following expression?

aMethod(123L, true)

  1. method 1

  2. method 2

  3. method 3

  4. method 4

  5. method 5

  1. What will happen when an attempt is made to compile and execute the following program? The line numbers are for reference purposes only.

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public class App {
  public static void main(String[] args) {
    System.out.println("Result: " + aMethod(3, 2.0));
  }
  public static int aMethod(int a, int b) {
    return a * b;
  }
  public static double aMethod(int a, double b) {
    return a + b;
  }
  public static double aMethod(double a, int b) {
    return a - b;
  }
}
  1. a compile error will occur at line 3

  2. the program will compile but an error will occur at run time

  3. the program will compile and run to display "Result: 6"

  4. the program will compile and run to display "Result: 5.0"

  5. the program will compile and run to display "Result: 1.0"

  1. What will happen when an attempt is made to compile and execute the following program? The line numbers are for reference purposes only.

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public class App {
  public static void main(String[] args) {
    short x = 7;
    aMethod(x);
    System.out.println("Variable of x is now " + x);
  }
  public static int aMethod(int a) {
    System.out.println("Received an int");
    return a + 1;
  }
  public static double aMethod(double b) {
    System.out.println("Received a double");
    return b + 1;
  }
}
  1. a compile error will occur at line 4

  2. the program will compile but an error will occur at run time

  3. the program will compile and run to display:

Received an int
Value of x is now 7

  1. the program will compile and run to display:

    Received an int
    Value of x is now 8

  2. the program will compile and run to display:

    Received a double
    Value of x is now 7.0

  3. the program will compile and run to display:

    Received a double
    Value of x is now 8.0