Using String.replace for copys of final Strings [duplicate]

Question

I have the following Test:

public void testStringReplace()
    {    
        final String placeholder = "$ph$";
        final String template = "<test>" + placeholder + "</test>";
        final String result = "<test>Hello!</test>";

        String copyOfTemplate = template;
        copyOfTemplate.replace(placeholder, "Hello!");

        if(!copyOfTemplate.equals(result));
            fail();
    }

The test always fails, but why? How do I have to define copyOfTemplate, to be able to change it? Or am I missing some other detail here?

copyOfTemplate = copyOfTemplate.replace(placeholder, "Hello!"); — Eran
– Eran, Commented Oct 11, 2018 at 9:24
if(!copyOfTemplate.equals(result)); <-- Remove that semicolon at the end! — MC Emperor
– MC Emperor, Commented Oct 11, 2018 at 9:25

Michael · Accepted Answer · 2018-10-11 09:29:13Z

3

String is immutable so calling

copyOfTemplate.replace(placeholder, "Hello!");

without assigning it to anything effectively does nothing. It returns a new string with the replacement, which you're ignoring. Any half-decent IDE will warn you about this:

Also, String copyOfTemplate = template doesn't really do anything either. It's not a copy. It's just a new variable pointing to the same underlying string. There is no method to copy a string because, again, strings are immutable so a copy becomes useless.

You want

String copyOfTemplate = template.replace(placeholder, "Hello!");

I suggest reading the Oracle tutorial on strings. It seems like you've missed some of the fundamentals.

answered Oct 11, 2018 at 9:29

Michael

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Comments

dehasi · Accepted Answer · 2018-10-11 09:29:43Z

0

As strings are immutable, opyOfTemplate.replace(placeholder, "Hello!"); produces a new string. So, you have to review your variable

copyOfTemplate = copyOfTemplate.replace(placeholder, "Hello!");

answered Oct 11, 2018 at 9:29

dehasi

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Comments

Turtle · Accepted Answer · 2018-10-11 09:29:56Z

0

I see two main problems in your code:

Your use of String#replace

copyOfTemplate.replace(placeholder, "Hello!"); returns a new string, it doesn't upadate it. You have to assign it to a new variable.
The if statement at the end

if(!copyOfTemplate.equals(result)); Since you added a semicolon, the if does nothing, and you always reach the fail() method. it's as if you wrote:
```
if(!copyOfTemplate.equals(result)) {

}
fail();
```

answered Oct 11, 2018 at 9:29

Turtle

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Comments

Derrick · Accepted Answer · 2018-10-11 09:32:57Z

0

Commented some of the mistakes in your code below -

 final String placeholder = "$ph$";
final String template = "<test>" + placeholder + "</test>";
final String result = "<test>Hello!</test>";

String copyOfTemplate = template;
copyOfTemplate = copyOfTemplate.replace(placeholder, "Hello!"); 
// you cannot change existing object because string is immutable

if(!copyOfTemplate.equals(result)) // semi colon removed
    fail();

answered Oct 11, 2018 at 9:32

Derrick

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Comments

Deepak Jain · Accepted Answer · 2018-10-11 09:35:10Z

0

Please use below code

public static void main(String[] args) {

        final StringBuilder placeholder = new StringBuilder("$ph$");
        final StringBuilder template = new StringBuilder("<test>" + placeholder + "</test>");
        final StringBuilder result = new StringBuilder("<test>Hello!</test>");

        replaceString(template, placeholder.toString(), "Hello!");
        System.out.println(template);

    }

    public static void replaceString(StringBuilder sb, String toReplace, String replacement) {
        int index = -1;
        while ((index = sb.lastIndexOf(toReplace)) != -1) {
            sb.replace(index, index + toReplace.length(), replacement);
        }
    }

answered Oct 11, 2018 at 9:35

Deepak Jain

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