Java Basics--Character (character) class

The Character class is used to manipulate a single character, it wraps the char value in an object.

In fact, in Java, char is a primitive data type, and Character is a wrapper class for char. Through the Character class, you can use a series of methods to manipulate characters. When creating a Character object, you can use the constructor of the Character class to pass in a char value to initialize the object, such as:

Character ch = new Character('a')。

Also, in some cases, the Java compiler does boxing (converting char to Character) and unboxing (converting Character to char) automatically.

For example, when passing a parameter of type char to a method that requires a parameter of type Character, the compiler will automatically convert the parameter of type char into a Character object. This feature is called boxing, which in turn is called unboxing.

// 原始字符 'a' 装箱到 Character 对象 ch 中
Character ch = 'a';
 
// 原始字符 'x' 用 test 方法装箱
// 返回拆箱的值到 'c'
char c = test('x');

escape sequence 

A character preceded by a backslash (\) represents an escape character, which has a special meaning to the compiler.

The following list shows the escape sequences for Java:

escape sequence	describe
\t	Insert a tab key here in the text
\b	Insert a back key here in the text
\n	Newline here in the text
\r	Insert a carriage return here in the text
\f	Insert a page break here in the text
\'	Insert single quotes here in the text
\"	Insert double quotes here in the text
\\	Insert a backslash here in the text

example 

public class Main {

    public static void main(String[] args) {
        // 使用转义序列来插入特殊字符
        System.out.println("Hello\tWorld");  // 在 Hello 和 World 之间插入一个制表符(tab)
        System.out.println("Hello\bWorld");  // 在 Hello 后面插入一个后退键(backspace)，删除了一个 'o'
        System.out.println("Hello\nWorld");  // 在 Hello 后面插入一个换行符(newline)，换到下一行输出
        System.out.println("Hello\rWorld");  // 在 Hello 后面插入一个回车符(carriage return)，将光标移到行首
        System.out.println("Hello\fWorld");  // 在 Hello 和 World 之间插入一个换页符(form feed)，清空屏幕并开始新一页
        System.out.println("She said \'Hello\'");  // 在字符串中插入单引号，使用转义序列 \' 表示
        System.out.println("He said \"Hello\"");  // 在字符串中插入双引号，使用转义序列 \" 表示
        System.out.println("C:\\Program Files\\Java");  // 在字符串中插入反斜杠，使用转义序列 \\ 表示两个反斜杠
    }
}

The result of compiling and running the above example is as follows:

Hello	World
HelloWorld
Hello
World
Hello
World
HelloWorld
She said 'Hello'
He said "Hello"
C:\Program Files\Java

Character method 

The following are some common methods of the Character class:

1. isLetter(char ch): Checks if a character is a letter.
2. isDigit(char ch): Checks if a character is a number.
3. isWhitespace(char ch): Check if a character is a whitespace character, such as space, tab, newline, etc.
4. isUpperCase(char ch): Checks if a character is an uppercase letter.
5. isLowerCase(char ch): Checks if a character is a lowercase letter.
6. toUpperCase(char ch): Convert characters to uppercase.
7. toLowerCase(char ch): Convert characters to lowercase.
8. toString(char ch): Convert a character to a string.
9. isLetterOrDigit(char ch): Checks if a character is a letter or a number.
10. isJavaIdentifierStart(char ch): Checks whether a character can be used as the starting character of a Java identifier.
11. isJavaIdentifierPart(char ch): Checks if a character can be part of a Java identifier.
12. digit(char ch, int radix): Convert the character to the corresponding integer value according to the specified base.
13. compare(char x, char y): Compare the size of two characters.

For a complete list of methods, please refer to  the java.lang.Character API  specification.

example

public class Main {
    public static void main(String[] args) {
        char ch = 'A';

        // 检查字符是否为字母
        System.out.println(Character.isLetter(ch)); // 运行:true

        // 检查字符是否为数字
        System.out.println(Character.isDigit(ch)); // 运行:false

        // 检查字符是否为空白字符
        System.out.println(Character.isWhitespace(ch)); // 运行:false

        // 检查字符是否为大写字母
        System.out.println(Character.isUpperCase(ch)); // 运行:true

        // 检查字符是否为小写字母
        System.out.println(Character.isLowerCase(ch)); // 运行:false

        // 将字符转换为大写形式
        System.out.println(Character.toUpperCase(ch)); // 运行:A

        // 将字符转换为小写形式
        System.out.println(Character.toLowerCase(ch)); // 运行:a

        // 将字符转换为字符串
        System.out.println(Character.toString(ch)); // 运行:A

        // 检查字符是否为字母或数字
        System.out.println(Character.isLetterOrDigit(ch)); // 运行:true

        // 检查字符是否可以作为 Java 标识符的起始字符
        System.out.println(Character.isJavaIdentifierStart(ch)); // 运行:true
        
        // 检查字符是否可以作为 Java 标识符的部分字符
        System.out.println(Character.isJavaIdentifierPart(ch)); // 运行:true

        // 将字符按照指定进制转换为对应的整数值
        System.out.println(Character.digit(ch, 16)); // 运行:10

        // 比较两个字符的大小
        System.out.println(Character.compare('A', 'B')); // 运行:-1
    }
}