visual basic barcode printing 8: Inner Classes in Java

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8: Inner Classes
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Here s a quick summary of the differences between inner class instantiation code that s within the outer class (but not static), and inner class instantiation code that s outside the outer class:
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From inside the outer class instance code, use the inner class name in the
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normal way:
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MyInner mi = new MyInner();
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From outside the outer class instance code (including static method code
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within the outer class), the inner class name must now include the outer class name,
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MyOuter.MyInner
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and to instantiate, you must use a reference to the outer class,
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new MyOuter().new MyInner(); or outerObjRef.new MyInner();
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if you already have an instance of the outer class.
Referencing the Inner or Outer Instance from Within the Inner Class
How does an object refer to itself normally Using the this reference. Quick review of this:
The keyword this can be used only from within instance code.
In other words, not within static code.
The this reference is a reference to the currently-executing object.
In other words, the object whose reference was used to invoke the currently-running method.
The this reference is the way an object can pass a reference to itself to some
other code, as a method argument:
public void myMethod() { MyClass mc = new MyClass(); mc.doStuff(this); // pass a ref to object running myMethod }
Inner Classes
Within an inner class code, the this reference refers to the instance of the inner class, as you d probably expect, since this always refers to the currently-executing object. But what if the inner class code wants an explicit reference to the outer class instance the inner instance is tied to In other words, how do you reference the outer this Although normally the inner class code doesn t need a reference to the outer class, since it already has an implicit one it s using to access the members of the outer class, it would need a reference to the outer class if it needed to pass that reference to some other code as follows:
class MyInner { public void seeOuter() { System.out.println("Outer x is " + x); System.out.println("Inner class ref is " + this); System.out.println("Outer class ref is " + MyOuter.this); } }
If we run the complete code as follows:
class MyOuter { private int x = 7; public void makeInner() { MyInner in = new MyInner(); in.seeOuter(); } class MyInner { public void seeOuter() { System.out.println("Outer x is " + x); System.out.println("Inner class ref is " + this); System.out.println("Outer class ref is " + MyOuter.this); } } public static void main (String[] args) { MyOuter.MyInner inner = new MyOuter().new MyInner(); inner.seeOuter(); } }
the output is
Outer x is 7 Inner class ref is MyOuter$MyInner@113708 Outer class ref is MyOuter@33f1d7
8: Inner Classes
So the rules for an inner class referencing itself or the outer instance are as follows:
To reference the inner class instance itself, from within the inner class code,
use this.
To reference the outer this (the outer class instance) from within the inner
class code, use <NameOfOuterClass>.this (example, MyOuter.this). Member Modifiers Applied to Inner Classes A regular inner class is a member of the outer class just as instance variables and methods are, so the following modifiers can be applied to an inner class:
final abstract public private protected static except static turns it into a top-level nested class rather than an
inner class.
CERTIFICATION OBJECTIVE
Method-Local Inner Classes
A regular inner class is scoped inside another class curly braces, but outside any method code (in other words, at the same level as an instance variable is declared). But you can also define an inner class within a method:
class MyOuter2 { private String x = "Outer2"; void doStuff() { class MyInner { public void seeOuter() { System.out.println("Outer x is " + x);
Method-Local Inner Classes
} // close inner class method } // close inner class definition } // close outer class method doStuff() } // close outer class
The preceding code declares a class, MyOuter2, with one method, doStuff(). But inside dostuff(), another class, MyInner, is declared, and it has a method of its own, seeOuter()). The code above is completely useless, however, because it never instantiates the inner class! Just because you declared the class doesn t mean you created an instance of it. So if you want to actually use the inner class (say, to invoke its methods), then you must make an instance of it somewhere within the method but below the inner class definition. The following legal code shows how to instantiate and use a method-local inner class:
class MyOuter2 { private String x = "Outer2"; void doStuff() { class MyInner { public void seeOuter() { System.out.println("Outer x is " + x); } // close inner class method } // close inner class definition MyInner mi = new MyInner(); // This line must come //after the class mi.seeOuter(); } // close outer class method doStuff() } // close outer class
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