/*
  This is file ArraySum.java
  Java block comments go between slash-asterisk delimiters
*/

// This is a Java class - everything in Java is inside some class or another,
//  class is the outermost construct everywhere
// Usually put each class in its own file, the file name must be the class name
// This is a library class - to use it, write a main program in another class
// Declaring a class public makes it easy to use from anywhere
// Java one-line comments go from the double slash to the end of the line
public class ArraySum {
    // This is a method, like a function or procedure in other languages
    // public makes it easy to call from anywhere
    // static methods are not associated with any particular object,
    //  they are like functions in other (non-object-oriented) languages
    // this method returns an integer
    // this method takes a single parameter, an array of integers 
    //  (of any length), named a
    public static int array_sum(int a[]) {
	// This declares a local variable of type integer, named sum
	//   which is only visible inside this method
	//  Declarations can also initialize variables, initialize sum to zero
	int sum = 0;
	// This is a for loop,
	//  the usual way to repeat something a predetermined number of times 
	// The first line of the loop is of the form: for (init.; test; incr.)
	// where init is executed once before control enters the loop
	//  init can declare and initialize variables
	// test is executed each time before control enters body of loop
	//  if test is true, Java executes statements in body of loop
	//  a.length is the length of array a
	// incr is executed each time after Java completes executing the body
	//  choose incr so it eventually causes test to be false
	//  i++ means the same thing as i = i + 1, which would also work
	// Here the for statement is designed to traverse each element of a
	for (int i = 0; i < a.length; i++) {
	    // This is the body of the loop (just one statement)
	    // a[i] is the i'th element of the array a
	    // In Java, arrays are zero-indexed:in an array of length elements,
	    //  the first element is a[0] and the last is a[length-1]
	    // Each trip through the loop adds a[i] to sum
	    sum = sum + a[i]; // sum += a[i] would also work
	}
	// When control exits the loop, sum holds the sum of all elems. of a
	// sum is the return value promised in the method declaration
	return sum;
    }
    // Other methods in this class would go here ...
}










/*
  This is file ArraySumDemo.java
*/

// This class exercises methods in the ArraySumClass
// Everything in Java appears inside a class, class is the outermost construct
public class ArraySumDemo {
    // Define some test data
    // Declare an array of integers named "six_ones"
    // static variables are not associated with any particular object
    //  they are like global variables in other (non-object-oriented) languages
    static int[] six_ones = {1,1,1,1,1,1};  // declare and initialize the array
    // from Bentley's Programming Pearls column,CACM 27(9), Sept 1984, p.865
    // also Programming Pearls books, 1st edition p. 69, 2nd edition p. 77
    static int[] bentley = {31, -41, 59, 26, -53, 58, 97, -93, -23, 84};
    // The main method is the main program
    //  the main method is the *only* thing you can run from the command line
    //  always declare it exactly like this - never mind why for now
    public static void main(String argv[]) {
	// Print a line of output
	// "..." is a literal string, is printed out exactly as it appears
	//  here + is the string concatenation operator - it joins strings
	//  ArraySum.array_sum(six_ones) calls the array_sum method in the 
	//   ArraySum class, and passes it the array named six_ones
	System.out.println("The array of six ones adds up to "
			   + ArraySum.array_sum(six_ones));
	System.out.println("Bentley's array adds up to " 
			   + ArraySum.array_sum(bentley));
    }
}

---------

This command compiles ArraySum.java and produces ArraySum.class:

$ javac ArraySum.java

This command compiles ArrayDemo.java and produces ArraySumDemo.class:

$ javac ArraySumDemo.java

This command executes the main method in the ArraySumDemo class:

$ java ArraySumDemo
The array of six ones adds up to 6
Bentley's array adds up to 145  

---------------------------------------------

Your assignment: add a method to the ArraySum class named
signed_array_sum.  The first parameter is a boolean, the second
parameter is an array of integers.  When the first parameter is true,
the function returns the sum of all the elements in the array that are
positive (greater than zero).  When the first parameter is false, the
function returns the sum of all the negative elements.  In the
ArraySumDemo class, add three more well-chosen arrays of test data.
In the main method of the ArraySumDemo class, add statements to test
signed_array_sum with both values of the first parameter on six_ones,
bentley, and your three new arrays.