Why should an object hide its data?

Answer 1

As my AP Computer Science teacher eloquently said:

"You can't let strangers touch your private parts"

Therefore, if data was not hidden, a client class can modify all the variables(given that it is not a final constant) by simply calling:

className.variableName = newWrongData;

By using special methods to access the variables, the class can change its internal implementation, without breaking compatibility with other classes that are already using it.

Answer 2

In Java, you hide data by declaring it private. In that case, it is not visible outside the class. When you do that, you should provide set and get methods (they can have any name, but starting them with "set" and "get" is a common coding standard). Example:

class Person
{
private String name;

void setName(parameterName) {name = parameterName;}

String getName() {return name;}

}

By hiding the data, you may later decide to change the implementation, without breaking compatibility with existing code that uses your class. It is general recommended to hide all class fields.