Dry goods｜Java packaging concepts and usage

In order to make the basic data types have the characteristics of objects, Java provides a wrapper class for each basic data type, so that we can manipulate the basic data types like operating objects.

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Packaging concept

The basic data types are very convenient to use, but there is no corresponding method to manipulate the data of these basic data types. You can use a class to pack the basic data types and define a method in the class . This class is called a wrapper class. We can use the class To manipulate these basic types of data.

1. Packing

Packing : Pack basic types of data into packaging classes (basic types of data -> packaging classes)

Construction method:
Integer(int value) Constructs a newly allocated Integer object, which represents the specified int value.
Integer(String s) Constructs a newly allocated Integer object, which represents the int value indicated by the String parameter.
The passed string must be a basic type of string, otherwise an exception "100" will be thrown. Correct "a".
Static method:

        static Integer valueOf(int i) 返回一个表示指定的 int 值的 Integer 实例。
        static Integer valueOf(String s) 返回保存指定的 String 的值的 Integer 对象。

Code example:

public class 包装类的概念 {
    
    
    public static void main(String[] args) {
    
    
        //装箱:把基本类型的数据,包装到包装类中(基本类型的数据->包装类)
        //构造方法
        Integer in1 = new Integer(1);//有划线表示方法以及过时
        System.out.println(in1);
      

        Integer in2 = new Integer("1");
        System.out.println(in2);
       //静态方法
        Integer in3 = Integer.valueOf(1);
        System.out.println(in3);


        Integer in4 = Integer.valueOf("1");
        System.out.println(in4);

Two, unboxing

Unboxing: Take out the data of the basic type from the package class (package class -> data of the basic type).
Member method:
int intValue() returns the value of the Integer as an int.

 //拆箱:在包装类中取出基本类型的数据(包装类->基本类型的数据)
        int i = in1.intValue();
        System.out.println(i);

Three, automatic unpacking and automatic packing

Automatic boxing and automatic unboxing:
basic types of data and packaging classes can be automatically converted between
new features after JDK1.5

Auto-boxing : directly assign an integer of type int to a packaging class
code example

public class Hello Worid {
    
    
    public static void main(String[] args) {
    
    
        /*
            自动装箱:直接把int类型的整数赋值包装类
            Integer in = 1; 就相当于 Integer in = new Integer(1);
         */
        Integer in = 1;

        /*
            自动拆箱:in是包装类,无法直接参与运算,可以自动转换为基本数据类型,在进行计算
            in+2;就相当于 in.intVale() + 2 = 3
            in = in.intVale() + 2 = 3 又是一个自动装箱
         */
        in = in+2;

        ArrayList<Integer> list = new ArrayList<>();
        /*
            ArrayList集合无法直接存储整数,可以存储Integer包装类
         */
        list.add(1); //-->自动装箱 list.add(new Integer(1));

        int a = list.get(0); //-->自动拆箱  list.get(0).intValue();
    }
}

Fourth, the mutual conversion between basic types and string types

Mutual conversion between basic types and string types
Basic types -> String (String)
1. Basic type value + "" The simplest method (commonly used in work)

2. The static method toString (parameter) of the wrapper class, not the toString() overload of the Object class

static String toString(int i) Returns a String object representing the specified integer.

3. The static method valueOf of the String class (parameter)
static String valueOf(int i) returns the string representation of the int parameter.

String (String)->Basic Type
Use the static method parseXXX("string") of the wrapper class;

Integer类: static int parseInt(String s)

Double类: static double parseDouble(String s)

public class 基本类型与字符串类型之间的相互转换 {
    
    
    public static void main(String[] args) {
    
    


        //基本类型->字符串(String)
        //方法一
        int i1 = 100;
        String s1 = i1+"";
        System.out.println(s1+200);//打印输出为 100200
        //方法二
        String s2 = Integer.toString(100);
        System.out.println(s2+100);//打印输出为 100200
        //方法三
        String s3 = String.valueOf(100);
        System.out.println(s3+200);//打印输出为 100200



        // 字符串(String)->基本类型
        // Integer类: static int parseInt(String s)
        int i = Integer.parseInt(s1);
        System.out.println(i-10);//打印输出为 100200
        int a = Integer.parseInt("a");// NumberFormatException  异常 英文字符转换为整型
        System.out.println(a);
        

    }
}