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Showing content from https://howtodoinjava.com/java/string/java-string-replace-method/ below:

Replace All Occurrences in Text

The String.replace() API searches for a literal substring and replaces each occurrence with the replacement string. The search for substring starts from the beginning of the string i.e. index 0.

Note that a similar method String.replaceAll() searches and replaces all substrings using regular expressions.

The replace() method is an overloaded method and comes in two versions:

public String replace(char oldChar, char newChar);

public String replace(CharSequence target, CharSequence replacement);

• The first method accepts the char types. It searches the string for specified oldChar and replaces each occurrence of oldChar with the newChar.
• The second method accepts the String types. It searches the string for the specified target substring and replaces each occurrence with the replacement.

The following Java program replaces all occurrences of the letter ‘o’ (lower case) with the letter ‘O’ (upper case).

String message = "Hello world !!";

Assertions.assertEquals("HellO wOrld !!", message.replace('o', 'O'));

The following Java program replaces all occurrences of the substring “Hello” with the new String “Hi”.

String message = "Hello world !!";

Assertions.assertEquals("Hi world !!", message.replace("Hello", "Hi"));

Regular expressions are not allowed as method arguments. If we use a regex pattern, it will be treated as a literal string.

In the following program, the regex [H] pattern will match character H is regex is supported. But replace() does not support regex so there are no matches found.

String message = "Hello world !!";

Assertions.assertEquals("Hello world !!", message.replace("[H]", "h"));

The 'null' is not allowed as both method arguments. It will throw NullPointerException.

Assertions.assertThrows(NullPointerException.class, () -> {
  message.replace(null, "O");
});

Assertions.assertThrows(NullPointerException.class, () -> {
  message.replace("o", null);
});

Happy Learning !!

References:
Java String Doc

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