A progress bar with an unknown maximum typically displays an animation until the task is complete. 
Note: The percentage display should not be enabled when the maximum is not known (e804 Displaying the Percentage Done on a JProgressBar Component). 

    // Create a horizontal progress bar
    int min = 0;
    int max = 100;
    JProgressBar progress = new JProgressBar(min, max);
    
    // Play animation
    progress.setIndeterminate(true);

When information on the task's progress is available, the progress bar can be made determinate: 
    int value = 33;
    progress.setValue(value);
    progress.setIndeterminate(false);