Java中的包装类

Java中哪些需要包装类

1. 它们将原始数据类型转换为对象。如果我们希望修改传递给方法的参数，则需要对象（因为基元类型是按值传递的）。
2. java.util包中的类只处理对象，因此在这种情况下包装类也有帮助。
3. Collection框架中的数据结构（如ArrayList和Vector）仅存储对象（引用类型）而不存储基本类型。
4. 需要一个对象来支持多线程中的同步。

Java原始数据类型及其对应的包装类

Java中的自动装箱和拆箱

自动装箱：将原始类型自动转换为相应包装类的对象称为自动装箱。例如 - 将int转换为Integer，将long转换为Long，将double转换为Double等
示例：

// Java program to demonstrate Autoboxing

import java.util.ArrayList;
class Autoboxing
{
    public static void main(String[] args)
    {
        char ch = 'a';;

        // Autoboxing- primitive to Character object conversion
        Character a = ch;

        ArrayList<Integer> arrayList = new ArrayList<Integer>();

        // Autoboxing because ArrayList stores only objects
        arrayList.add(25);

        // printing the values from object
        System.out.println(arrayList.get(0));
    }
}

输出：

25

拆箱：这只是自动装箱的逆向过程。自动将包装类的对象转换为其对应的基元类型称为拆箱。例如 - 将Integer转换为int，Long转换为long，将Double转换为double等。

// Java program to demonstrate Unboxing
import java.util.ArrayList;

class Unboxing
{
    public static void main(String[] args)
    {
        Character ch = 'a';

        // unboxing - Character object to primitive conversion
        char a = ch;

        ArrayList<Integer> arrayList = new ArrayList<Integer>();
        arrayList.add(24);

        // unboxing because get method returns an Integer object
        int num = arrayList.get(0);

        // printing the values from primitive data types
        System.out.println(num);
    }
}

输出：

24

Java履行Implementation

// Java program to demonstrate Wrapping and UnWrapping
// in Java Classes
class WrappingUnwrapping
{
    public static void main(String args[])
    {
        //  byte data type
        byte a = 1;

        // wrapping around Byte object
        Byte byteobj = new Byte(a);

        // int data type
        int b = 10;

        //wrapping around Integer object
        Integer intobj = new Integer(b);

        // float data type
        float c = 18.6f;

        // wrapping around Float object
        Float floatobj = new Float(c);

        // double data type
        double d = 250.5;

        // Wrapping around Double object
        Double doubleobj = new Double(d);

        // char data type
        char e='a';

        // wrapping around Character object
        Character charobj=e;

        //  printing the values from objects
        System.out.println("Values of Wrapper objects (printing as objects)");
        System.out.println("Byte object byteobj:  " + byteobj);
        System.out.println("Integer object intobj:  " + intobj);
        System.out.println("Float object floatobj:  " + floatobj);
        System.out.println("Double object doubleobj:  " + doubleobj);
        System.out.println("Character object charobj:  " + charobj);

        // objects to data types (retrieving data types from objects)
        // unwrapping objects to primitive data types
        byte bv = byteobj;
        int iv = intobj;
        float fv = floatobj;
        double dv = doubleobj;
        char cv = charobj;

        // printing the values from data types
        System.out.println("Unwrapped values (printing as data types)");
        System.out.println("byte value, bv: " + bv);
        System.out.println("int value, iv: " + iv);
        System.out.println("float value, fv: " + fv);
        System.out.println("double value, dv: " + dv);
        System.out.println("char value, cv: " + cv);
    }
}

输出：

Values of Wrapper objects (printing as objects)
Byte object byteobj:  1
Integer object intobj:  10
Float object floatobj:  18.6
Double object doubleobj:  250.5
Character object charobj: a
Unwrapped values (printing as data types)
byte value, bv: 1
int value, iv: 10
float value, fv: 18.6
double value, dv: 250.5
char value, cv: a