Методы String.format() и printf() в Java

printf()
Правила форматирования
conversion-character
width
flags
precision

1. printf()

В программировании на Java вы часто сталкиваетесь с кодом System.out.printf(..). На самом деле метод printf(..) определен как в классе PrintStream, так и в классе PrintWriter, их использование аналогично, printf означает "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(..) - это особый случай, используемый для записи форматированного текста на экран Console, где System.out по сути является объектом типа PrintStream.

System class

package java.lang;

public final class System  {

   public static final PrintStream out = ...;

}

Прежде чем мы начнем, давайте рассмотрим простой пример:

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. Правила форматирования

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

Параметры: flags, width, precision являются опциональными.

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 - задает минимальное пространство (количество символов), которое требуется для печати содержимого соответствующего аргумента.

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

Методы String.format() и printf() в Java

1. printf()

2. Правила форматирования

3. conversion-character

4. width

5. flags

6. precision

Java Basic

Java Basic

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