Java Auto装箱和拆箱 (Auto-boxing and Unboxing)

In this tutorial, we will learn about Java automatic boxing and unboxing with examples.

Java automatic boxing-The original type of the wrapper object

InAutomatic boxing inThe Java compiler will automatically convert the primitive type to its corresponding wrapper class object. For example,

int a =　56;
//　Auto-boxing
Integer aObj = a;

When using Java collections,Auto-boxinghas a great advantage.

Example1: Java automatic boxing

import java.util.ArrayList;
class Main {
　　　public static void main(String[] args) {
　　　　　　ArrayList<Integer> list = new ArrayList<>();
　　　　　　//Auto-boxing
　　　　　　list.add(5);
　　　　　　list.add(6);
　　　　　　System.out.println("ArrayList: "　+　list);
　　　}
}

Output Result

ArrayList: [5,　6]

In the above example, we created an array list of Integer type. Therefore, the array list can only contain Integer objects.

Note this line,

list.add(5);

Here, we pass the primitive type value. However, sinceAuto-boxing, the primitive values will be automatically converted to Integer objects and stored in the array list.

Java unboxing-Wrapper objects for primitive types

InUnboxing inThe Java compiler will automatically convert the wrapper class object to its corresponding primitive type. For example,

//　Auto-boxing
Integer aObj =　56;
//　Unboxing
int a = aObj;

LikeAutomaticBoxingLikeUnboxingCan also be used with Java collections.

Example2：Java Unboxing

import java.util.ArrayList;
class Main {
　　　public static void main(String[] args) {
　　　　　　ArrayList<Integer> list = new ArrayList<>();
　　　　　　//Auto-boxing
　　　　　　list.add(5);
　　　　　　list.add(6);
　　　　　　System.out.println("ArrayList: "　+　list);
　　　　　　//　Unboxing
　　　　　　int a = list.get(0);
　　　　　　System.out.println("Value at index 0: "　+　a);
　　　}
}

Output Result

ArrayList: [5,　6]
Value at index 0:　5

In the above example, please note the following line:

int a = list.get(0);

Here, the get() method returns the object at index 0. However, sinceUnboxingThe object is automatically converted to the primitive type int and assigned to the variable a.