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Java String.format() and printf() methods

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1- printf()

In Java programming, you often come across System.out.printf(..) code. Actually, method printf(..) is defined in both PrintStream and PrintWriter classes, their usage is similar, printf stands for "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)
  • TODO Link!
System.out.printf(..) is a special case, used to write a formatted text to the Console screen, where System.out is essentially an object of type PrintStream.
System class
package java.lang;

public final class System  {

   public static final PrintStream out = ...;

}
Before we start, let's see a simple example:
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- Format rules

%[flags][width][.precision]conversion-character
These p​​​​arameters: flags, width, precision are optional.

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 - specifies the minimum amount of space (number of characters) to hold for printing the contents of a corresponding argument.
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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