Encapsulate Field

There is a public field.

Make it private and provide accessors.

  public String _name
  private String _name;
  public String getName() {return _name;}
  public void setName(String arg) {_name = arg;}


One of the principal tenets of object orientation is encapsulation, or data hiding. This says that you should never make your data public. When you make data public, other objects can change and access data values without the owning object's knowing about it. This separates data from behavior.

This is seen as a bad thing because it reduces the modularity of the program. When the data and behavior that uses it are clustered together, it is easier to change the code, because the changed code is in one place rather than scattered all over the program.

Encapsulate Field begins the process by hiding the data and adding accessors. But this is only the first step. A class with only accessors is a dumb class that doesn't really take advantage of the opportunities of objects, and an object is terrible thing to waste. Once I've done Encapsulate Field I look for methods that use the new methods to see whether they fancy packing their bags and moving to the new object with a quick Move Method.


  • Create getting and setting methods for the field.
  • Find all clients outside the class that reference the field. If the client uses the value, replace the reference with a call to the getting method. If the client changes the value, replace the reference with a call to the setting method.
    If the field is an object and the client invokes a modifier on the object, that is a use. Only use the setting method to replace an assignment.
  • Compile and test after each change.
  • Once all clients are changed, declare the field as private.
  • Compile and test.