Tutorial: Getting Started with the Gurobi Java API

The example begins by importing the Gurobi classes (import com.gurobi.gurobi.*). Gurobi Java applications should always start with this line.

The first executable statement in our example obtains a Gurobi environment (using the GRBEnv constructor):

// Create empty environment, set options, and start
GRBEnv env = new GRBEnv(true);
env.set("logFile", "mip1.log");
env.start();

In this call we requested an empty environment, chose a log file (using GRBEnv.set), and started the environment (using GRBEnv.start).

Later calls to create an optimization model will always require an environment, so environment creation is typically the first step in a Gurobi application.

Once an environment has been created, the next step is to create a model. A Gurobi model holds a single optimization problem. It consists of a set of variables, a set of constraints, and the associated attributes (variable bounds, objective coefficients, variable types, constraint senses, constraint right-hand side values, etc.). The first step towards building a model that contains all of this information is to create an empty model object:

// Create empty model
GRBModel model = new GRBModel(env);

The GRBModel constructor takes the previously created environment as its argument.

The next step in our example is to add variables to the model.

// Create variables
GRBVar x = model.addVar(0.0, 1.0, 0.0, GRB.BINARY, "x");
GRBVar y = model.addVar(0.0, 1.0, 0.0, GRB.BINARY, "y");
GRBVar z = model.addVar(0.0, 1.0, 0.0, GRB.BINARY, "z");

Variables are added through the GRBModel.addVar method on a model object (or GRBModel.addVars if you wish to add more than one at a time). A variable is always associated with a particular model.

The first and second arguments to the GRBModel.addVar call are the variable lower and upper bounds, respectively. The third argument is the linear objective coefficient (zero here - we'll set the objective later). The fourth argument is the variable type. Our variables are all binary in this example. The final argument is the name of the variable.

The GRBModel.addVar method has been overloaded to accept several different argument lists. Please refer to the Gurobi Reference Manual for further details.

The next step in the example is to add the linear constraints. The first constraint is added here:

// Add constraint: x + 2 y + 3 z <= 4
expr = new GRBLinExpr();
expr.addTerm(1.0, x);
expr.addTerm(2.0, y);
expr.addTerm(3.0, z);
model.addConstr(expr, GRB.LESS_EQUAL, 4.0, "c0");

As with variables, constraints are always associated with a specific model. They are created using the GRBModel.addConstr or GRBModel.addConstrs methods on the model object.

The first argument to GRBModel.addConstr is the left-hand side of the constraint. We built the left-hand side by first creating an empty linear expression object GRBLinExpr, and then adding three terms to it using GRBLinExpr.addTerm. The second argument is the constraint sense (GRB_LESS_EQUAL, GRB_GREATER_EQUAL, or GRB_EQUAL). The third argument is the right-hand side (a constant in our example). The final argument is the constraint name. Several signatures are available for GRBModel.addConstr. Please consult the Gurobi Reference Manual for details.

The second constraint is created in a similar manner:

// Add constraint: x + y >= 1
expr = new GRBLinExpr();
expr.addTerm(1.0, x);
expr.addTerm(1.0, y);
model.addConstr(expr, GRB.GREATER_EQUAL, 1.0, "c1");

The next step in the example is to set the optimization objective:

// Set objective: maximize x + y + 2 z
GRBLinExpr expr = new GRBLinExpr();
expr.addTerm(1.0, x);
expr.addTerm(1.0, y);
expr.addTerm(2.0, z);
model.setObjective(expr, GRB.MAXIMIZE);

The objective must be a linear or quadratic function of the variables in the model. In our example, we build our objective by first constructing an empty linear expression and adding three terms to it.

The second argument to GRBModel.setObjective indicates that the optimization sense is maximization.

Now that the model has been built, the next step is to optimize it:

// Optimize model
model.optimize();

This routine performs the optimization and populates several internal model attributes (including the status of the optimization, the solution, etc.).

Once the optimization is complete, we can query the values of the attributes. In particular, we can query the VarName and X attributes (using GRBVar.get) to obtain the name and solution value for each variable:

System.out.println(x.get(GRB.StringAttr.VarName)
                   + " " + x.get(GRB.DoubleAttr.X));

We can also query the ObjVal attribute on the model to obtain the objective value for the current solution:

System.out.println("Obj: " + model.get(GRB.DoubleAttr.ObjVal));

The names and types of all model, variable, and constraint attributes can be found under the Attributes section of the documentation.

The example concludes with dispose calls:

// Dispose of model and environment
model.dispose();
env.dispose();

These reclaim the resources associated with the model and environment. Garbage collection would reclaim these eventually, but if your program doesn't exit immediately after performing the optimization, it is best to reclaim them explicitly.

Note that all models associated with an environment must be disposed before the environment itself is disposed.

Errors in the Gurobi Java interface are handled through the Java exception mechanism. In the example, all Gurobi statements are enclosed inside a try block, and any associated errors would be caught by the catch block:

try { 
  // Formulate and solve model
} catch (GRBException e) {
  System.out.println("Error code: " + e.getErrorCode() + ". " +
                     e.getMessage());
}

To build and run the example, please refer to the files in <installdir>/examples/build.

For Linux or macOS platforms, the <installdir>/examples/build directory contains an example Makefile. Typing make Mip1 will build and run this example.

For Windows platforms, this directory contains runjava.bat, a simple script to compile and run a java example. Say runjava Mip1 to run this example.

The Java example directory <installdir>/examples/java contains a number of examples. We encourage you to browse and modify them in order to become more familiar with the Gurobi Java interface. We also encourage you to read the Gurobi Example Tour for more information.