Java basics (packaging class)

Packing

Concept : A class that encapsulates basic data types into object types is called a wrapper class (the advantage is that it can provide more functions for manipulating basic values).

Java provides 8 basic types of corresponding packaging classes, as shown in the figure:

Convert string to basic type:

Note: parseXXX (String s); where XXX represents the basic type, the parameter is a string that can be converted to the basic type, if the string cannot be converted to the basic type, the problem of number conversion will occur NumberFormatException .

There are 3 ways to convert basic values ​​into strings:

1. The basic type can be directly connected with "", 34 + ""
2. Call the valueOf method of String; String.valueOf (34) ;

     3. Call the toString method in the wrapper class; Integer.toString (34);

JDK5 features: automatic packing and unpacking

1. Automatic unboxing: objects are converted to basic values
2. Automatic packing: basic values ​​are converted into objects

Integer i = 4; // Automatic packing. Equivalent to Integer i = Integer.valueOf (4);

i = i + 5; // The right side of the equal sign: convert the i object into a basic value (automatic unboxing) i.intValue () + 5; After the addition operation is completed, box again and convert the basic value into an object

1. Demonstration of automatic boxing (byte constant pool) details

When the value is within the byte range, automatic boxing is performed, and the existing space is used instead of creating a new object space.

Integer a = new Integer(3);

Integer b = new Integer(3);

System.out.println(a==b);//false

System.out.println(a.equals(b));//true

Integer x = 127;

Integer y = 127;

// When jdk1.5 is automatically boxed, if the value is within the byte range, the object space will not be newly created but the existing space will be used.

System.out.println(x==y); //true

System.out.println(x.equals(y)); //true

char provides the packaging class Character class

Common methods:

1	isLetter () : whether it is a letter
2	isDigit () : whether it is a numeric character
3	isWhitespace () : whether it is a blank character
4	isUpperCase () : whether it is an uppercase letter
5	isLowerCase () : whether it is lowercase
6	toUpperCase () : Specify the capitalization of letters
7	toLowerCase (): Specify the lowercase letter
8	toString (): Returns the character string form, the length of the string is only1

 

 

 

 

 

 

 

 

Case: Counting the number of various character strings

        int bigCount = 0;
        int smallCount = 0;
        int numberCount = 0;
        String s1= "wwwTTTcccyyy193yyy";
        // 把字符串转换为字符数组
        char[] chs = s1.toCharArray();
        // 遍历字符数组,获取每一个字符
        for (int x = 0; x < chs.length; x++) {
            // 判断该字符是
            if (Character.isUpperCase(chs[x])) {
                bigCount++;
            } else if (Character.isLowerCase(chs[x])) {
                smallCount++;
            } else if (Character.isDigit(chs[x])) {
                numberCount++;
            }
        }
        // 输出结果即可
        System.out.println("大写字符:" + bigCount + "个");
        System.out.println("小写字符:" + smallCount + "个");
        System.out.println("数字字符:" + numberCount + "个");