Simple understanding of string

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1.String There are two ways to create ：

 String s="fjy";
 char[] Array={
    'f','j','y'};
 String s1=new String(Array);

The first method is stored in the string pool , Create only once

2. Two ways to splice strings ：

Public class TestString {
    
    public static void main(String[] args) {
    
        String s="fjy";
        char[] Array={
    'f','j','y'};
        String s1=new String(Array);
        StringBuilder sb = new StringBuilder();
        StringBuffer sb2=new StringBuffer();
        for (int i = 0; i <10000; i++) {
    
            System.out.println(sb.append(s));
            //System.out.println(sb2.append(s));
        }
    }
}​

3. Define the regular expression of the string （ Double quotation marks are required on both sides ）

String regex="[0-9]{17}[0-9x]";

The regular expression to determine whether the ID number is entered correctly has not been written

Integer
new Integer（int a）
Integer.valueof(int a):
Integer.valueof(int a) This way of creating objects is quite special , When a stay -128-127 Between time , In the pool
Double
new Double(double a)
Double.valueof(double a)
There is no difference between the above two creation methods
Integet.parseInt(" “); The return value is int type
Double.parseDouble(” "); The return value is double type
Automatic packing and unpacking ：
Integer.valueof(5)->Integer
Integer int1=new Integer(5);
int1.intValue()->int
BigDecimal
BigDecimal We need to pay attention to ：
a:BigDecimal There are two constructors for , One is to store Double type , The other is to store String type . Generally not used Double type , Because the operation is not accurate
b:divide This method is special , There are three parameters , Respectively ： Divisor , Retained decimals , rounding （ Round to the nearest , The floor , Ceiling, etc ）

System.out.println(" Please enter two numbers ");
        Double a=new Scanner(System.in).nextDouble();
        Double b=new Scanner(System.in).nextDouble();
        BigDecimal bigDecimal1 = new BigDecimal(a + "");
        BigDecimal bigDecimal2 = new BigDecimal(b + "");
        System.out.println(bigDecimal1.add(bigDecimal2));
        System.out.println(bigDecimal1.subtract(bigDecimal2));
        System.out.println(bigDecimal1.multiply(bigDecimal2));
        System.out.println(bigDecimal1.divide(bigDecimal2,3, RoundingMode.HALF_UP));