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Java's Object Orientation, I/O
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CS 161 - Java |
Instance methods are only called through an object: an instance
of some class created by invoking new
on a class.
someInstance.instanceMethod()
Source: examples/Counter.java
/** An example of a class defining counter objects.
**/
public class Counter {
private int count = 0; // instance variable
public void increment() { // instance method
count++;
}
public int getCount() { // another
return count;
}
// for stand-alone testing
public static void main(String[] args) {
Counter[] cArray = new Counter[5];
for (int i = 0; i < cArray.length; i++ ) {
cArray[i] = new Counter();
for (int num = 0; num < 10; num++) {
cArray[i].increment();
}
}
for (int i = 0; i < cArray.length; i++) {
System.out.println("cArray[" + i + "] = " + cArray[i].getCount());
}
// What is output from the above?
}
}
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Utility methods associated with an entire class, not any one
particular instance of the class.
ClassName.classMethod()
Qualifying a method with the keyword static
makes that method a class method.
Source: examples/ClassCounter.java
/** An example of a class that defines several class methods.
**/
public class ClassCounter {
private static int count = 0; // a class variable
public static void increment() { // class method
count++;
}
public static int getCount() { // another
return count;
}
public static void main(String argv[]) {
// The normal way to call class methods, from within the class
// or from some other class is:
// Classname.methodname( [arguments if any] );
// Even if I make instances of the class, and call the static
// methods through the instance references, the results are
// not going to work the same way that instance methods would.
ClassCounter[] cArray = new ClassCounter[5];
for (int i = 0; i < cArray.length; i++ ) {
cArray[i] = new ClassCounter();
for (int num = 0; num < 10; num++) {
cArray[i].increment();
}
}
for (int i = 0; i < cArray.length; i++) {
System.out.println("cArray[" + i + "] = " + cArray[i].getCount());
}
// What is output from the above?
}
}
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If you find yourself using a lot of class methods in Java, it is a
sign that you are thinking in the old, procedural programming style,
not like an object oriented programmer.
