Test Eclipse

Open eclipse and select your workspace. If you start out with a blue screen with some circular buttons saying Welcome, Tutorial, etc., click Go to Workbench. Otherwise, you are already at the workbench. Go to File → New → Java Project. Name your project sandbox, then click the button Finish. You now have a new project!

In the Package Explorer (left side) right click the arrow next to sandbox to expand the project. Right click on the src folder and then go to new → class. Name the class TestClass. Under "Which method stubs would you like to create?" check the box next to public static void main(String[] args). Hit the Finish button. Then replace

// TODO Auto-generated method stub

by

System.out.println("Hello World!")

Save the file TestClass.java by hitting Ctrl+s or the save button (top left on the menu bar). To run your code, go to the top center of the menu bar and hit the run icon (its a green circle with a white arrow inside), or go to the menu bar and hit Run → Run. The Console should pop up on the bottom with the output of your program.

Test Cplex from Eclipse

Open Eclipse and expand the sandbox project we created previously. Create a new class called KnapsackSolver. Next we are going to connect to Cplex. Right click on sandbox in the package explorer and select new->folder. Give it the name lib and click Finish. Find the file cplex.jar in your Cplex installation. On my Windows installation, it was located at

• Windows: C:\Program Files\IBM\ILOG\CPLEX_Studio125\cplex\lib\cplex.jar
• Linux: /opt/IBM/ILOG/CPLEX_Studio125/cplex/lib/cplex.jar TODO check
• Mac: Unknown.
  Copy the file cplex.jar and paste it in the newly created lib folder in our project. Right click on sandbox in the package explorer, then select Build Path → Configure Build Path... Select the Libraries tab, then hit the Add JARs button (see here for an explanation of what all these buttons do). Select the file sandbox/lib/cplex.jar, then hit OK until we have exited all the menus.

Open up KnapsackSolver and replace the text of the file with the following:

import java.util.Arrays;
import ilog.concert.IloException;
import ilog.concert.IloIntVar;
import ilog.cplex.IloCplex;

public class KnapsackSolver {
	
	public static double[] solveKnapsack(double[] values, double[] weights, double capacity){		
		if(weights.length != values.length){
			throw new RuntimeException("values.length was " + values.length 
					+ " and weights.length was" +  weights.length 
					+ " but they must be the same");
		}
		try {
			IloCplex cplex = new IloCplex();
			IloIntVar[] x = cplex.boolVarArray(values.length);
			cplex.addLe(cplex.scalProd(x, weights), capacity);
			cplex.addMaximize(cplex.scalProd(x, values));
			cplex.solve();
			return cplex.getValues(x);			
		} catch (IloException e) {
			throw new RuntimeException();
		}
	}
		
	public static void main(String[] args){
		double[] values = new double[]{2,4,5,6};
		double[] weights= new double[]{2,3,4,7};
		double capacity = 8;
		double[] solution = solveKnapsack(values,weights,capacity);
		System.out.println(Arrays.toString(solution));
	}
}

Run the code.  The final line of output should of course be {{\[0.0, 1.0, 1.0, 0.0\]}}, up to some precision.  If you get the error

Cannot find cplex...

then your PATH was not set properly. The easiest way to deal with this is to add a virtual machine argument to the Run Configuration. Starting from the menu bar at the top, select Run → Run Configurations... and then on the left, make sure Java Application → KnapsackSolver is selected. In the center panel, pick the =Arguments tab. In the second box, titled VM Arguments, add the line:

this is a VM argument

Finally, hit Run on the bottom.