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Phương thức String.format() và printf() trong Java

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1- printf()

Trong lập trình Java, rất thường xuyên bạn bắt gặp đoạn mã System.out.printf(..). Thực ra, phương thức printf(..) được định nghĩa trong cả hai lớp PrintStreamPrintWriter, cách sử dụng của chúng là tương tự nhau, printf là viết tắt của "Print + Format".
printf methods

// Definition of the printf methods in the PrintWriter class:

public PrintStream printf(Locale l, String format, Object... args)

public PrintStream printf(String format, Object... args)

// Definition of the printf methods in PrintWriter class:

public PrintWriter printf(Locale l, String format, Object... args)

public PrintWriter format(String format, Object... args)
System.out.printf(..) là một trường hợp đặc biệt, được sử dụng để ghi một văn bản có định dạng ra màn hình Console, trong đó System.out thực chất là một đối tượng kiểu PrintStream.
System class

package java.lang;

public final class System  {

   public static final PrintStream out = ...;

}
Trước khi bắt đầu, hãy xem một ví dụ đơn giản:
Printf_ex1.java

package org.o7planning.printf.ex;

public class Printf_ex1 {

    public static void main(String[] args) {
        String firstName = "James";
        String lastName = "Smith";
        int age = 20;
        
        System.out.printf("My name is %s %s, I'm %d years old.", firstName, lastName, age);
    }
}
Output:

My name is James Smith, I'm 20 years old.

2- Quy tắc định dạng


%[flags][width][.precision]conversion-character
Các tham số: flags, width, precision là tuỳ chọn.

3- conversion-character


%[flags][width][.precision]conversion-character
conversion-character Description Type
d decimal integer byte, short, int, long
f floating-point number float, double
b Boolean Object
B will uppercase the boolean Object
c Character Capital String
C will uppercase the letter String
s String Capital String
S will uppercase all the letters in the string String
h hashcode - A hashcode is like an address. This is useful for printing a reference Object
n newline - Platform specific newline character - use %n instead of \n for greater compatibility  
conversion-character: n

System.out.printf("One%nTwo%nThree");
Output:

One
Two
Three
conversion-character: s

System.out.printf("My name is %s %s", "James", "Smith");
Output:

My name is James Smith
conversion-character: S

System.out.printf("My name is %S %S", "James", "Smith");
Output:

My name is JAMES SMITH
conversion-character: b

System.out.printf("%b%n", null);
System.out.printf("%b%n", false);
System.out.printf("%b%n", 5.3);
System.out.printf("%b%n", "Any text");
System.out.printf("%b%n", new Object());
Output:

false
false
true
true
true
conversion-character: B

System.out.printf("%B%n", null);
System.out.printf("%B%n", false);
System.out.printf("%B%n", 5.3);
System.out.printf("%B%n", "Any text");
System.out.printf("%B%n", new Object());
Output:

FALSE
FALSE
TRUE
TRUE
TRUE
conversion-character: d

System.out.printf("There are %d teachers and %d students in the class", 2, 25);
Output:

There are 2 teachers and 25 students in the class
conversion-character: f

System.out.printf("Exchange rate today: EUR %f = USD %f", 1.0, 1.2059);
Output:

Exchange rate today: EUR 1.000000 = USD 1.205900
conversion-character: c

char ch = 'a';
System.out.printf("Code of '%c' character is %d", ch, (int)ch);
Output:

Code of 'a' character is 97
conversion-character: C

char ch = 'a';
System.out.printf("'%C' is the upper case of '%c'", ch, ch);
Output:

'A' is the upper case of 'a'
conversion-character: h
Printf_cc_h_ex.java

package org.o7planning.printf.ex;

public class Printf_cc_h_ex {

    public static void main(String[] args) {

        // h (Hashcode HEX)
        Object myObject = new AnyObject("Something");

        System.out.println("Hashcode: " + myObject.hashCode());
        System.out.println("Identity Hashcode: " + System.identityHashCode(myObject));
        System.out.println("Hashcode (HEX): " + Integer.toHexString(myObject.hashCode()));
        System.out.println("toString : " + String.valueOf(myObject));

        System.out.printf("Printf: %h", myObject);
    }
}

class AnyObject {
    private String val;

    public AnyObject(String val) {
        this.val = val;
    }

    @Override
    public int hashCode() {
        if (this.val == null || this.val.isEmpty()) {
            return 0;
        }
        return 1000 + (int) this.val.charAt(0);
    }
}
Output:

Hashcode: 1083
Identity Hashcode: 1579572132
Hashcode (HEX): 43b
toString : org.o7planning.printf.ex.AnyObject@43b
Printf: 43b

4- width


%[flags][width][.precision]conversion-character
width - chỉ định khoảng không gian (số ký tự) tối thiểu để giữ chỗ cho việc in ra nội dung của một đối số tương ứng.

printf(format, arg1, arg2,.., argN)

5- flags


%[flags][width][.precision]conversion-character
flag Description
- left-justify (default is to right-justify )
+ output a plus ( + ) or minus ( - ) sign for a numerical value
0 forces numerical values to be zero-padded (default is blank padding )
, comma grouping separator (for numbers > 1000)
  space will display a minus sign if the number is negative or a space if it is positive
flag: -

String[] fullNames = new String[] {"Tom", "Jerry", "Donald"};
float[] salaries = new float[] {1000, 1500, 1200};
// s
System.out.printf("|%-10s | %-30s | %-15s |%n", "No", "Full Name", "Salary");

for(int i=0; i< fullNames.length; i++) {
    System.out.printf("|%-10d | %-30s | %15f |%n", (i+1), fullNames[i], salaries[i]);
}
Output:
flag: +

// flag: +
System.out.printf("The world's economy increased by %+f percent in 2020. %n", -3.3);
System.out.printf("China's economy increased by %+f percent in 2020. %n", 2.3);

System.out.println();

// without flag: +
System.out.printf("The world's economy increased by %f percent in 2020. %n", -3.3);
System.out.printf("China's economy increased by %f percent in 2020. %n", 2.3);
Output:

The world's economy increased by -3.300000 percent in 2020.
China's economy increased by +2.300000 percent in 2020.

The world's economy increased by -3.300000 percent in 2020.
China's economy increased by 2.300000 percent in 2020.
flag: 0  (zero)

// flag: 0  & with: 20
System.out.printf("|%020f|%n", -3.1);

// without flag & with: 20
System.out.printf("|%20f|%n", -3.1);

// flag: - (left align) & with: 20
System.out.printf("|%-20f
Output:
flag: ,  (comma)

// flag: ,
System.out.printf("%,f %n", 12345678.9);

// flag: ,
System.out.printf("%,f %n", -12345678.9);
Output:

12,345,678.900000
-12,345,678.900000
flag:  (space)

// flag:  (space)
System.out.printf("% f %n", 12345678.9);
System.out.printf("% f %n", -12345678.9);  

System.out.println();

// flags:  , (space + comma)
System.out.printf("% ,f %n", 12345678.9);
System.out.printf("% ,
Output:

 12345678.900000
-12345678.900000

 12,345,678.900000
-12,345,678.900000

6- precision


%[flags][width][.precision]conversion-character

System.out.printf("%f %n", 12345678.911);
System.out.printf("%f %n", -12345678.911);

System.out.println();

// flag: , (comma)
System.out.printf("%,f %n", 12345678.911);

// flag: , (comma)
System.out.printf("%,f %n", -12345678.911);

System.out.println();

// flags:  , (space + comma)
System.out.printf("% ,f %n", 12345678.911);

// flags:  , (space + comma)
System.out.printf("% ,f %n", -12345678.911);

System.out.println();

// flag: , (comma) & precision: .2
System.out.printf("%,.2f %n", 12345678.911);

// flag: , (comma) & precision: .3
System.out.printf("%,.3f %n", -12345678.911);

System.out.println();

// flags:  , (space + comma) & precision: .2
System.out.printf("% ,.2f %n", 12345678.911);

// flags:  , (space + comma) & precision: .3
System.out.printf("% ,.3f %n", -12345678.911);
Output:

12345678.911000
-12345678.911000

12,345,678.911000
-12,345,678.911000

 12,345,678.911000
-12,345,678.911000

12,345,678.91
-12,345,678.911

 12,345,678.91
-12,345,678.911

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