Boxing / Unboxing
Automatic type conversion
The Java 5.0, or JDK 1.5 if you like, made several updates to the Java language,
one of which was the boxing and unboxing
mechanism. Simply put, it converts or casts between primitive types and the
wrapper classes without explicit code on your part.
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In an old style code, you would have the manually wrap primitive types in
wrapper classes to add them to Collections etc., like the code below.
Vector v;
...
v.add( new Integer(111) );
...
int tmp = ((Integer)v.get(i)).intValue();
The class below implements this example.
NoBoxing.java
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The new way of handling this, using boxing and unboxing, save both some code and potential
errors. Note how integer values are added right into the Vector, and how no the intValue()
method is no longer needed.
v.add( 111 );
...
int tmp = (Integer)v.get(i);
The cast from the Object returned from the get() method is still required since the Vector
used is unchecked. That is, no generics is used to specify
the type of elements.
BoxingTest.java
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The previous examples used boxing and unboxing of integer, but of course all the Java
primitive types are supported. The final example show boxing of all types in the save
Vector. The output should be like this:
Class: java.lang.Integer, value=111
Class: java.lang.Float, value=2.2
Class: java.lang.Double, value=3.3
Class: java.lang.Short, value=4
Class: java.lang.Long, value=50
Class: java.lang.Byte, value=6
Class: java.lang.Character, value=a
Class: java.lang.Boolean, value=true
AllBoxingTest.java
It should be noted that this is an example, and that this type of code is hardly
recommended. For for on checked Collections, come back to this site soon.
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