Replace

In Java programs we often need to modify strings before using them. We find that there are many forms of the replace() method.

Java method notes

With replaceFirst and replaceAll we specify what we want to replace. Powerful Regex-based methods are also available.

An example

Here we use the version of replace() that replaces all instances of one char value with another. So we change all "a" letters to the underscore "_" character.

public class Program {
    public static void main(String[] args) {

        String value = "Java";

        // Replace letter with underscore.
        String result = value.replace('a', '_');
        System.out.println(result);
    }
}
J_v_

CharSequence, Strings

We also can call replace() with CharSequence arguments. A CharSequence is an interface that is implemented by the String class. So we can replace Strings.

public class Program {
    public static void main(String[] args) {

        String animals = "cat cat dog";

        // Replace all CharSequences (strings) with another.
        String result = animals.replace("cat", "mouse");
        System.out.println(result);
    }
}
mouse mouse dog

ReplaceFirst

This method only replaces the first occurrence of the string found. We also can use Regex with this method. Please see the replaceAll example for tips on Regex replacing.

public class Program {
    public static void main(String[] args) {

        String pets = "cat cat dog";

        // Replace first instance of "cat".
        String result = pets.replaceFirst("cat", "bird");
        System.out.println(result);
    }
}
bird cat dog

ReplaceAll, Regex

This program uses a regular expression to replace all matches of a pattern with a string. The "\d" codes mean "digit." We match 3 digits surrounded by "a" and "z."

public class Program {
    public static void main(String[] args) {

        String codes = "a100z a200z a300z b100z";

        // Replace all matches with a string.
        String result = codes.replaceAll("a\\d\\d\\dz", "a---z");
        System.out.println(result);
    }
}
a---z a---z a---z b100z

Replace end

With the "$" metacharacter, we can match the end of a string. This allows us to use replaceFirst to replace an ending string.

public class Program {
    public static void main(String[] args) {

        String pets = "cat cat cat";

        // Use metacharacter to ensure the replacement is at the end.
        String result = pets.replaceFirst("cat$", "dog");
        System.out.println(result);
    }
}
cat cat dog

Optimization

Sometimes we can use logic to avoid calling replace() when it is not needed. If many strings do not need to be processed, indexOf() or contains() can help.

public class Program {

    public static String replaceIfNeeded(String value) {
        // Test the string to see if any of the replacements can succeed.
        if (value.contains("<span>")) {
            value = value.replace("<span><b>", "*");
            value = value.replace("<span><i>", "/");
        }
        if (value.contains("</span>")) {
            value = value.replace("</b></span>", "*");
            value = value.replace("</i></span>", "/");
        }
        return value;
    }

    public static void main(String[] args) {

        // ... All replace calls are used on this string.
        String result = replaceIfNeeded("<span><b>Cat</b></span>");
        System.out.println(result);

        // ... No replace calls are needed.
        result = replaceIfNeeded("Dog");
        System.out.println(result);
    }
}
*Cat*
Dog

Regex or string

Should we use a Regex or a String in replace? If only one constant pattern needs replacing, a String is simpler. But if differences may occur, a Regex is clearer.

StringBuilder

Replace on StringBuilder is different from the String method. With StringBuilder, we indicate a range with indexes, and then replace those characters with another string.

With replace, we gain powerful ways to change Strings. Each time, a new String is created. So all the data must be copied. With StringBuilder this inefficiency is reduced.