java - Why use getters and setters/accessors?

What's the advantage of using getters and setters - that only get and set - instead of simply using public fields for those variables?

If getters and setters are ever doing more than just the simple get/set, I can figure this one out very quickly, but I'm not 100% clear on how:

public String foo;

is any worse than:

private String foo;
public void setFoo(String foo) { this.foo = foo; }
public String getFoo() { return foo; }

Whereas the former takes a lot less boilerplate code.

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Best Answer-推荐答案

There are actually many good reasons to consider using accessors rather than directly exposing fields of a class - beyond just the argument of encapsulation and making future changes easier.

Here are the some of the reasons I am aware of:

• Encapsulation of behavior associated with getting or setting the property - this allows additional functionality (like validation) to be added more easily later.
• Hiding the internal representation of the property while exposing a property using an alternative representation.
• Insulating your public interface from change - allowing the public interface to remain constant while the implementation changes without affecting existing consumers.
• Controlling the lifetime and memory management (disposal) semantics of the property - particularly important in non-managed memory environments (like C++ or Objective-C).
• Providing a debugging interception point for when a property changes at runtime - debugging when and where a property changed to a particular value can be quite difficult without this in some languages.
• Improved interoperability with libraries that are designed to operate against property getter/setters - Mocking, Serialization, and WPF come to mind.
• Allowing inheritors to change the semantics of how the property behaves and is exposed by overriding the getter/setter methods.
• Allowing the getter/setter to be passed around as lambda expressions rather than values.
• Getters and setters can allow different access levels - for example the get may be public, but the set could be protected.