String is immutable?

Question

Just have a problem with this, how do we rewrite the above code so that only a single String object is created?

String is immutable, isnt studentDetails already one single String object

public void displayString(Student[] students)
{
   String studentDetails = "";
   for (Student stu : students)
   {
       studentDetails += stu.getFirstName();
       studentDetails += " ";
       studentDetails += stu.getLastName();
       studentDetails += " ";
       studentDetails += stu.getAge();
       studentDetails += "\n";  
   }
   System.out.println("Student Details are: ");
   System.out.println(studentDetails);
}

@DanielM [The StringBuilder class should generally be used in preference to `[StringBuffer]`](https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuffer.html). — Andy Turner, May 29 '19 at 12:32
side bar: consider allowing `Student` to describe itself by moving `displayString()` to `Student`, or perhaps just override the `Student#toString()` method. — Andrew S, May 29 '19 at 12:44

score 5 · Answer 1 · answered May 29 '19 at 12:24

5

String object is immutable, while the variable studentDetails is not.

When you call studentDetails += stu.getFirstName(), you created a new String object and assign it to variable studentDetails

answered May 29 '19 at 12:24

xingbin

27,410
9
53
103

Eran · Answer 2 · 2019-05-29T12:26:17.927

3

Use StringBuilder:

public void displayString(Student[] students)
{
   StringBuilder studentDetails = new StringBuilder();
   for (Student stu : students)
   {
       studentDetails.append(stu.getFirstName());
       studentDetails.append(' ');
       studentDetails.append(stu.getLastName());
       studentDetails.append(' ');
       studentDetails.append(stu.getAge());
       studentDetails.append('\n');  
   }
   System.out.println("Student Details are: ");
   System.out.println(studentDetails.toString());
}

String is immutable, isnt studentDetails already one single String object

No, since each time you append two Strings with +=, a new String is created.

edited May 29 '19 at 12:26

answered May 29 '19 at 12:24

Eran

387,369
54
702
768

1

The principle is sound but that will create 4 `String` objects, not one. (Edit: not sure if OP's specification to create a single `String` object might be a bit of XY though). – Mena May 29 '19 at 12:26
1

@Mena yes, I noticed that and replaced the single character Strings with chars. Still, there would be a second `String` ("Student Details are: "). I think the only way to avoid that is to append the characters of that constant String one at a time to the start of the StringBuilder, but that would be ridiculous. – Eran May 29 '19 at 12:28
@Mena I see only one. How do you count? – kumesana May 29 '19 at 12:28
@kumesana there are two in the latest version of the answer (see Eran's comment above). In the previous version the char literals were `String` literals. So you had one for the whitespace, one for the newline, one for the "Student details..." and one for the `toString` invocation. – Mena May 29 '19 at 12:31
Those are literals, not really created when running this code... I suppose it's a matter of semantics. – kumesana May 29 '19 at 12:54

DanielM · Answer 3 · 2019-11-03T18:16:27.933

1

String is immutable - Use StringBuilder or StringBuffer (thread-safe) instead

public void displayString(Student[] students)
{
   StringBuilder studentDetails = new StringBuilder();
   for (Student stu : students)
   {
       studentDetails.append(stu.getFirstName());
       studentDetails.append(" ");
       studentDetails.append(stu.getLastName());
       studentDetails.append(" ");
       studentDetails.append(stu.getAge());
       studentDetails.append("\n");  
   }
   System.out.println("Student Details are: ");
   System.out.println(studentDetails.toString());
}

edited Nov 03 '19 at 18:16

answered May 29 '19 at 12:25

DanielM

3,598
5
37
53

3

synchronization is useless here. Prefer StringBuilder. – kumesana May 29 '19 at 12:25

score 0 · Answer 4 · answered May 29 '19 at 12:26

Yes, String object is immutable, that is why one need to work with StringBuilder or StringBuffer. Below are steps shown to work with StringBuilder

Create variable of type StringBuilder
Use append() method to concnat String
Call toString() to convert it into String

score 0 · Answer 5 · answered May 29 '19 at 12:26

yes, strings are immutable, just use a StringBuilder, or a stringBuffer if thread safe is required....

   StringBuilder studentDetails = "";
   for (Student stu : students)
   {
       studentDetails.append(stu.getFirstName());
       studentDetails.append(" ");
       studentDetails.append(stu.getLastName());
       studentDetails.append(" ");
       studentDetails.append(stu.getAge());
       studentDetails.append("\n");  
   }

score 0 · Answer 6 · answered May 29 '19 at 12:32

0

Yes , The string object in java is immutable. The any change in the string will create the new string object. Whereas, the stringBuilder and StringBuffer are mutable

answered May 29 '19 at 12:32

Ravikumar Navale

1

String is immutable?

6 Answers6