Table des matières
Méthodes Java String.format() et printf()
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Dans la programmation Java, vous rencontrez souvent le code System.out.printf(..). En réalité, la méthode printf(..) est définie dans les classes PrintStream et PrintWriter, leur mode d'utilisation est similaire, printf signifie "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(..) est un cas particulier, utilisé pour écrire un texte formaté sur l'écran Console, où System.out est essentiellement un objet de type PrintStream.
System class
package java.lang;
public final class System {
public static final PrintStream out = ...;
}
Avant de commencer, observer l'exemple suivant:
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.
%[flags][width][.precision]conversion-character
Les paramètres: flags, width, precision sont facultatifs.
%[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 : [email protected]
Printf: 43b
%[flags][width][.precision]conversion-character
width - spécifie la quantité minimale d'espace (nombre de caractères) à conserver pour imprimer le contenu d'un argument correspondant.
printf(format, arg1, arg2,.., argN)
%[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
%[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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