Static methods

Figure 189. A method adding two values Slide presentation 
public class Add {

    public static void main(String[] args) {
        int i = 2, j = 5;
        int result = sum (i, j);
        System.out.println("The sum of " + i +         
              " and " + j + " is " + result);
    }
    static int sum ( int a, int b) {
        return a + b;
    }
}
The sum of 2 and 5 is 7
 sum : ℤ × ℤ → ℤ ( a , b ) ↦ a + b

Figure 190. Syntax of method definition Slide presentation 
   Method's return type:    Method's arguments:
   Method returns           Method expects two
   int values               arguments of type int
          ↘               ↙      ↙
    static int sum ( int a, int b) {
        int result = a + b;
        return result; 
    }                ↖
                      Returning a value 
                      of type int

Figure 191. The static modifier matters! Slide presentation 
public static void main(String[] args) {
     ...
    // Compile error: Non-static method 'sum(int, int)' 
    // cannot be called from a static context
    int result = sum (i,j);
         ...
}

// No static modifier
int sum ( int a, int b) {
   return a + b;
}

Figure 192. Documentation: Javadoc™ is your friend! Slide presentation 
/**
 * Calculates the sum of two integers.
 *
 * @param a the first integer to add
 * @param b the second integer to add
 * @return the sum of the two integers
 */
static int sum (int a, int b) {
    int result = a + b;
    return result;
}

Figure 193. Separating usage and definition Slide presentation
Method definition Method usage 
public class Helper {

    static int sum(int a, int b) {              
        int result = a + b;
        return result;
    }
}




public class Add {

    public static void main(String[] args) {
        int i = 2, j = 5;

        int result = Helper.sum(i, j);
        System.out.println("The sum of " + i +              
           " and " + j + " is " + result);
    }
}

exercise No. 87

Defining methods

Q:

Follow Figure 193, “Separating usage and definition ” and define the following two methods in a separate class Helpers:

  • A method square(...) expecting a double and returning its square value.

  • A method greetings(...) expecting a String.

  • A method repeat(...) expecting a String and an integer number e.g. "Test" and 3. The string shall be repeated the given number of times generating "TestTestTest" for the given example values. If 0 == count holds an empty string shall be returned.

Provide Javadoc™ based documentation as being shown in Figure 192, “Documentation: Javadoc™ is your friend! ”. Then generate HTML documentation from it and view it in a Web browser of your choice.

On completion the following sample should work (if the Helpers class resides within the same package).

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

        System.out.println("Square: " + Helpers.square(2.5));

        System.out.println(Helpers.greetings("John"));

        System.out.println("Result string: \"" + Helpers.repeat("Test", 0) + '"');
        System.out.println("Result string: \"" + Helpers.repeat("Test", 3) + '"');
    }
}
Result 
Square: 6.25
Hello John!
Result string: ""
Result string: "TestTestTest"

A:

 
/**
 * Common helper methods.
 */
public class Helpers {

    /**
     * <p>Calculates the square of a given value.</p>
     *
     * @param value
     * @return The square of the given value.
     */
    static double square(double value) {
        return value * value;
    }

     /**
     * <p>Greets a given name.</p>
     *
     * <p>Example: Given the name <code>"John"</code>, the result will be <code>"Hello John!"</code></p>
     *
     * @param name The name to be greeted.
     * @return A greeting message to the person in question, e.g. "Hello John!".
     */
    static String greetings(String name) {
        return "Hello " + name + "!";
    }

    /**
     * <p>Repeat a string a given number of times.</p>
     *
     * <p>Example: Given a string "Test" and the number 3, the result will be "TestTestTest".</p>
     *
     * @param s the string to be repeated
     * @param count the string will be repeated this number of times.
     * @return The result of concatenating the input string <code>count</code> times.
     */
    static String repeat( String s, int count) {
        String result = "";
        for (int i = 0; i < count; i++) {
            result += s;
        }
        return result;
    }
}

exercise No. 88

Defining methods

Q:

Consider:

/**
 * Returns the absolute value of a given integer.
 * @param value
 * @return The absolute value of the given integer.
 */
static int abs(int value) {
  if (value < 0) {
     return -value;
  }
}

This causes a “Missing return statement” compile time error. Explain the underlying cause and correct the implementation.

A:

There is no else {...}. Thus in case of value being zero or positive nothing is being returned contradicting the Javadoc™'s promise. We thus require e.g.: 

/**
 * Returns the absolute value of a given integer.
 * @param value
 * @return The absolute value of the given integer.
 */
static int abs(int value) {
  if (value < 0) {
     return -value;
  } else {
     return value;
  }
}
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