The unique keyword in Umple can be applied to an attribute to ensure that each instance of the class in the currently running program has a unique value for that attribute. The code generated by Umple will not allow this constraint to be violated.

If an attempt is made to call the constructor of a class with an argument that would result in a violation of the uniqueness constraint, an exception will be thrown. The text of the constraint will start with the phrase "Cannot create".

In the example below,

s1 is first initialized with number set to 1.
Then an attempt is made to initialize s2 with number also set to 1. This throws the exception, but it is caught using a try-catch block.
Next s3 is successfully created with number 2, but an attempt is made to change the number to 1 using the method call setNumber(). This returns false, since it is only constructors that throw the exception.
Finally, an attempt is made to create s4, with the number set to 1. This throws the exception without a try-catch block, thus the program terminates.

Programs should be designed with logic that avoids violation of uniqueness. It is not recommended that production code relies on catching this exception as a normal part of its logic.

Example

// This demonstrates code that will throw the
// uniqueness exception
class Student {
  unique Integer number;
  name;
 
  public static void main(String[] argv) {
    
    Student s1 = null;
    Student s2 = null;
    Student s3 = null;
    Student s4 = null;

    System.out.println("Initial\ns1= "+s1+
      "\ns2= "+s2+"\ns3= "+s3+"\n\n");

    // We set up the first student s1 with no problem
    s1 = new Student(1,"Abel");
    
    // For the second student we use a try catch
    // block to catch the exception
    // Ideally code would avoid creating violations
    // to start with, but the following
    // approach can be used as a safety mechanism.
    try {
      s2 = new Student(1, "Baker");
    }
    catch (RuntimeException re) {
      System.out.println("Exception Caught: "+re+"\n");
    }
    
    // We now create a student that doesn't
    // violate uniqueness
    s3 = new Student(2,"Charlie");
    
    // Uniqueness can also be detected by trying
    // to change a value
    // This does not throw the exception, since
    // the setNumber() method returns false
    boolean result = s3.setNumber(1);
    if(!result) System.out.println(
      "Could not change number as it would have"+
      " violated uniqueness\n");

    System.out.println("Final\ns1= "+s1+
      "\ns2= "+s2+"\ns3= "+s3+"\n\n");
    
    // The following will throw the exception
    // without being caught
    s4 = new Student(1, "Delta");
    
    // This statement will not be reached
    System.out.println(
      "This line should not be reached");
  }
}

Load the above code into UmpleOnline