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JAVA Programming
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 1000000; i++) {
list.add(i);
}
In this code, each time an int value is added to the List, it is autoboxed into an Integer object. This can result in a significant amount of memory usage and additional overhead due to the creation and garbage collection of so many objects.
Unboxing, on the other hand, involves converting a wrapper class to its corresponding primitive type. This can also have a performance impact, especially when dealing with large collections of objects. For example, consider the following code:
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 1000000; i++) {
list.add(i);
}
int sum = 0;
for (Integer num : list) {
sum += num;
}
In this code, each time an Integer value is retrieved from the List, it is unboxed into an int value. This can result in additional method calls and overhead, which can impact performance.
To avoid the performance impact of autoboxing and unboxing, it is often recommended to use primitive types directly whenever possible, and to use specialized collections such as IntArrayList from the fastutil library or TIntArrayList from the trove4j library, which store primitive values directly and avoid the need for autoboxing and unboxing.
int num = 10;
Integer obj = num; // autoboxing
int result = obj + 20; // unboxing
In this code, the autoboxing happens when the int value num is assigned to the Integer variable obj. The unboxing happens when the Integer value in obj is added to the int value 20 using the + operator, and the result is assigned to the int variable result.
The compiler generates the appropriate bytecode to handle autoboxing and unboxing. However, because of the additional overhead involved in creating and manipulating objects, autoboxing and unboxing can have a small impact on performance, especially in tight loops or other performance-critical code. In general, it’s best to use autoboxing and unboxing judiciously and only when necessary, and to be aware of their potential impact on performance.
import java.util.ArrayList;
public class AutoboxingUnboxingExample {
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>();
// Autoboxing
for (int i = 0; i < 10; i++) {
list.add(i); // adding primitive int to ArrayList, autoboxed to Integer
}
// Unboxing
int sum = 0;
for (Integer value : list) {
sum += value; // unboxing Integer to primitive int
}
System.out.println("Sum of values in list: " + sum);
}
}
In this example, we create an ArrayList of Integer objects, and then use autoboxing to add primitive int values to the list. We then use unboxing to retrieve the values from the list and sum them up as primitive int values. The autoboxing and unboxing are handled automatically by the compiler.
@MyAnnotation
public class MyClass {
// class definition
}
In this example, @MyAnnotation is an annotation that has been applied to the MyClass class. The annotation itself does not do anything; it simply provides additional information to the compiler or other tools that process the code.
Annotations can also have arguments or values, which are specified in parentheses after the annotation name. For example:
@MyAnnotation(name = "myName", value = 42)
public class MyClass {
// class definition
}
In this example, @MyAnnotation is an annotation that has two arguments: name, which has a value of “myName”, and value, which has a value of 42.
Annotations can be used for a variety of purposes, such as:
Providing hints to the compiler or other tools about how to process the code
Documenting the code for developers or other tools
Enforcing constraints or rules on the code, such as checking for null values or verifying security permissions
Generating code or other resources based on the annotated code
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