Mr. Happy Object teaches static methods When to choose static methods over instance methods
By Tony Sintes
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November 21, 2001
When would you create static methods as opposed to instance methods? I understa nd that static methods allow you to use those methods without having to create a n instance of that class, and that class methods apply to the class rather than an object. Are these the only reasons? Could you give an example of a case where you would use a class method over an instance method?
Many Java developers find it confusing to decide when, and when not to declare a method as static. However, making the choice is simple if you have a clear unde rstanding of the difference between a class method and an instance method.
Note: You can download the source code that accompanies this article from Resour ces.
Consider the following class definition:
public class MrHappyObject {
private String _mood = _HAPPY;
private final static String _HAPPY = "happy"; private final static String _ANNOYED = "annoyed"; private final static String _ANGRY = "angry";
public void printMood() { System.out.println( "I am " + _mood ); }
First, before I get emails about it, there are more object-oriented ways to track and transition between states. However, those fancy ways would detract from the intent of the example. Now, without further ado...
printMood(), receivePinch(), and receiveHug() are all instance methods. Syntactically, you call these methods instance methods because they are not static; but the important distinction concerns why I didn't declare them as static.
Instance methods are instance methods because they rely on the state of the specific object instance. Instance methods are tied to a particular instance because the behavior that the method invokes relies upon the state of that particular instance. Consider the following example:
MrHappyObject obj1 = new MrHappyObject(); MrHappyObject obj2 = new MrHappyObject();
obj1.printMood(); obj2.printMood();
obj1.receiveHug(); obj2.receivePinch();
obj1.printMood(); obj2.printMood();
大家可以看到上面的兩個實例調用方法的結果是不一樣的。 而靜態方法調用的結果都是一樣的，它是類一級的方法，與實例的狀態沒有關系。 private static int _instantiations;
public MrHappyObject() {
_instantiations++; }
public static int instances() { return _instantiations; }
