Replace a char in a String

[Ls chin]

Hi, this is my first post here. I'm having problem understanding 'replace'. Why doesn't this code work? Is there a syntax error in it?

public class TestA {
    public static void stringReplace(String text) {
        text = text.replace('j', 'c');
    }
    
    public static void main(String[] args) {
        String textString = new String("java");
        
        stringReplace(textString);
        
        System.out.println(textString);
    }
}

The output is still: java. How do I make the code work?

Thank you.

[Henry Wong]

http://faq.javaranch.com/java/CallByReferenceVsCallByValue

[marc weber]

Welcome to JavaRanch!

The thing to consider here is that variables representing method parameters are local to the method. So "text" is local to the method stringReplace. At first, it references the same String object as "textString." But inside the method, "text" is reassigned to reference a new String produced by text.replace('j', 'c'). This assignment has no effect on the original String that's still referenced by "textString."

[Ls chin]

Thank you, Henry & marc,

Java object references are passed by value, i.e. it pass a 'copy' of the value to the method, therefore the original value is not changed. Ahhh, I see. No wonder it still prints "java".

I refactored the code to make it print "cava" by changing the void method to return a String value and then print that return value. Is that the correct way to replace a char in a String?

public class TestA {
    public static String stringReplace(String text) {
        text = text.replace('j', 'c');
        return text;
    }
    public static void main(String[] args) {
        String textString = new String("java");
      
        stringReplace(textString);
        System.out.println(textString);
        
        System.out.println(stringReplace(textString));
    }
}

Thank you.

[Rob Spoor]

Except for the useless first call to stringReplace that's just the way to go.

[Stephen Davies]

Originally posted by LS chin:
Hi, this is my first post here. I'm having problem understanding 'replace'. Why doesn't this code work? Is there a syntax error in it?

public class TestA {
    public static void stringReplace(String text) {
        text = text.replace('j', 'c');
    }
    
    public static void main(String[] args) {
        String textString = new String("java");
        
        stringReplace(textString);
        
        System.out.println(textString);
    }
}

The output is still: java. How do I make the code work?

Thank you.

Am I missing something here, doesn the String class have a replace method?

 
 
public class Test{
 
    public String myString = new String("java");

public static void main(String[] args) {

  Test a = new Test();
  String newString = a.myString.replace('j','c');
          System.out.println(newString);
 
}
 
}
 

..or am I way off, and this is not what your trying (are you trying to override the replace Method=?

[Ls chin]

Originally posted by Stephen Davies:
Am I missing something here, doesn the String class have a replace method?

..or am I way off, and this is not what your trying (are you trying to override the replace Method=?

You're right. String class does have a replace method. Your code looks very neat.

The code that I gave in the OP came from an SCJP mock exam. I think that question was designed to "purposely" confuse us - making us think that the output would be "cava" but in fact, it is still "java".

The misleading part is this line:
stringReplace(textString);

- which serves no purpose, I would think.

[Rusty Shackleford]

Originally posted by Henry Wong:
http://faq.javaranch.com/java/CallByReferenceVsCallByValue

That really isn't the reason why it didn't work. The reason is that String is immutable and a call to replace returns a new String.

To give an example of the same scenario where it would work:

 public class test
{
 
public static void add(Int i)
        {
 
i.add();
}
 
public static void main(String[] args)
{
Int val = new Int(1);
   System.out.println(val);
add(val);
System.out.println(val);
}
 

}   

 public class Int
{
   int i=0;
   public Int(int i)
   {
this.i=i;
 
   }
 
   public void add()
   {
i++;
   }
 
           public String toString()
   {
return ""+i;
   }
}  

This returns:

1
2

If String were mutable it would have worked, despite being passed by value.

The same code using StringBuffer:

  public class test
{
 
public static void replace(StringBuffer str)
        {
 
str.replace(0,1,"C");
}
 
public static void main(String[] args)
{
StringBuffer sb = new StringBuffer("Java");
System.out.println(sb);
replace(sb);
System.out.println(sb);
}
 

}  

This prints
Java
Cava

It is true that Java is always pass by value but that is not the solution to the question.
[ August 08, 2008: Message edited by: Rusty Shackleford ]

[Ls chin]

Originally posted by Rusty Shackleford:
If String were mutable it would have worked, despite being passed by value.

Thanks for the examples. I learnt something new again today. So, String objects are immutable. Ahh, I see.

Thanks.

It's interesting that there are so many ways to do the same thing in Java.