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Common Java classes

1、 String related classes

1. String class

public class StringTest { 
    String str = new String("good");
    char[] ch = { 't','e','s','t' };
    
    public void change(String str,char ch[]) {
        str = "test ok";
        ch[0] = 'b';
    } 
    
    public static void main(String[] args) {
        StringTest ex = new Stringtest();
        ex.change(ex.str,ex.ch);
        System.out.println(ex.str);
        System.out.println(ex.ch);
    }
}
/*
结果为
good
best
因为：
	1、传递给形参的都是引用数据类型，由值传递机制可知，把变量的地址传递给了形参。
	2、str的地址传递给形参：形参要改变数据内容，就需要另外开辟内存，但是没有影响到str
	3、数组的地址传递给形参：形参要改变数据内容，直接在上面操作，导致ch数据改变
*/

2. StringBuffer class

3. StringBuilder class

2、 Date time API before jdk8

1、java. Lang.system class

Returns the time difference between the current time and 0:0:0:0 on January 1, 1970, in milliseconds: public static long currenttimemillis()

2、java. util. Date class

3、java. text. Simpledateformat class

4、java. util. Calendar Class

3、 New date time API in jdk8

1、LocalDate、LocalTime、LocalDateTime

2. Instantaneous: instant

3. Formatting and parsing

4. Other APIs

4、 Java comparator

1. Explain

2. Natural sorting: examples of using the comparable interface

3. Custom sorting: comparator interface

4. Comparison between the two

5、 Other common classes

1. System class

2. Math class

3. BigInteger class

4. BigDecimal class