拆箱是将JAVA中包装类的对象转换为原始数据类型的过程。它作为“java.lang”包的一部分存在，导入该包后可以在 jAVA 程序中使用。例如，将包装类Integer的对象转换为Int。这与JAVA中的自动装箱相反。它是作为 JAVA 5 开发的一部分引入的，使开发人员的工作变得更加轻松。
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语法
JAVA 中拆箱使用的语法如下：
import java.lang.*
class mainClass
{
public static void main (String args[] )
{
Integer variableName=new Integer( VariableValue);
variableName2 = variableName; //Unboxing in JAVA
}
}这里，包装类“Integer”的对象被分配给原始数据类型Int，并且该值作为参数传递到包装类的构造函数中。这在自动装箱中隐式发生。
拆箱是将对象转换为原始数据类型的过程。此过程由 JAVA 编译器自动完成，因为 JAVA 中的库在 JAVA 第五版及更高版本中支持它。要通过 JAVA 编译器进行拆箱运行，需要满足两个先决条件。下面列出了这两个先决条件：
下面是包含包装类及其相关原始数据类型的表格：
| Wrapper class | Related primitive data-type |
| Boolean | boolean |
| Integer | Int |
| Float | float |
| Character | char |
| Byte | byte |
| Long | long |
| Short | short |
| Double | double |
The data flow and explanation of unboxing in JAVA is explained properly with examples in the below section.
Some examples are provided below will provide a conceptual clarity of the unboxing technique.
Code:
public class test1
{
public static void main(String args[])
{
Integer var1=new Integer(50);
if( var1 > 10) // Unboxing using comparator.
{
int var2=var1 + 10;
int var3=var2;
System.out.println(" The value of variable using unboxing functionality is JAVA is :"+ var3);
} else
{
int var2=var1 - 10; //Unboxing using assignment operator.
int var3=var2;
System.out.println(" The value of variable using unboxing functionality is JAVA is :"+ var3);
}
}
}Output:
Explanation: In this program, The main class is declared as “test1” as it contains the main() method. The program execution starts with the main() function. An object of the wrapper class “Integer” is created with the name “var1” and assigned with the value “50”. You should focus on the syntax of assigning value to variable “var1”, which is different in comparison to autoboxing. Here, the object is used instead of the data type for declaration and assignment purposes. Once the assignment is done, unboxing is done for that object.
Here, a comparison operator is used to unboxing the object. “If” logic checks if the value of “var1” is more than 10 or not. If not, then the control flows to another part of the program, starting with the “else” keyword and the whole code snippet under if loop will be skipped. In the else section, there is no comparator operator, so it enters the control logic. Assignment operator “=” does the unboxing part in case else is invoked. You can change the value of “var1” by changing the parameter provided to the wrapper class’s constructor (“Integer()” in this example). Finally, value is added or subtracted and printed as per logic.
Code:
public class test2
{
public static void main(String args[])
{
Character charName = 'M'; // Autoboxing.
char charName2 = charName; // Unboxing
System.out.println("The process used here is auto-unboxing to display the character : "+ charName2 );
}
}Output:
Explanation: Here, unboxing is done using the assignment operator. The data flow and control execution will work, as explained in the previous example. Here one thing to notice is that we have not used the object of a wrapper class to declare and assign the value to the variable “charName”. Although unboxing is done on “charName” using the assignment operator.
Code:
public class test3 {
public static void main (String args[]){
Integer varName = new Integer("1000");
int varName2 = varName.intValue();
System.out.println("Variable name is printed after unboxing using a built-in function is : " + varName2);
}
}Output:
Explanation: This works similar to the previous example with an added function called “intValue(). This function should extract the value from the variable “varName” and assign it to another variable named “varName2”. The function “intValue()” explicitly returns the value of object ”varName”. This is exactly what the compiler does in the backend. You should try removing this function and see the results to compare it with example number 2.
Some of the primitive advantages of unboxing in JAVA is presented in the form of the list below:
Unboxing is the reverse of autoboxing in JAVA. It is converting the wrapper class’s object into a primitive data type. Although we have the functionality of declaring and assigning variables using primitive data type directly (That is called the autoboxing feature of JAVA), objectification of wrapper class and then assigning this object to a primitive data type is used several times when standardization of code is important. This reveals the true object-oriented property of JAVA. It is used mostly in big projects for easy maintenance.
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