Java.lang.string.replace() method in Java

Strings in Java are objects that are supported internally by a char array. Since arrays are immutable, and strings are also a type of exceptional array that holds characters, therefore, strings are immutable as well. 

The String class of Java comprises a lot of methods to execute various operations on strings such as compare(), concat(), equals(), split(), length(), replace(), compareTo(), substring(), etc. Out of these methods, we will be focusing on the Java replace() method. 

Java String replace() 

This method returns a new string resulting from replacing all occurrences of old characters in the string with new characters. 

Syntax of String replace() in Java

public String replace(char oldch, char newch)

Parameters

• oldch: the old character.
• newch: the new character.

Return Value

• It returns a string derived from this string by replacing every occurrence of oldch with newch.

Examples of Java String replace()

1. Example Java String replace(char old, char new)

Below is the implementation of the above-mentioned method:

Java

// Java code to demonstrate the
// working of replace()

public class rep1 {
    public static void main(String args[])
    {

        // Initialising String
        String Str = new String("Welcome to geeksforgeeks");

        // Using replace to replace characters
        System.out.print("After replacing all o with T : ");
        System.out.println(Str.replace('o', 'T'));

        // Using replace to replace characters
        System.out.print("After replacing all e with D : ");
        System.out.println(Str.replace('e', 'D'));
    }
}

Output

After replacing all o with T : WelcTme tT geeksfTrgeeks
After replacing all e with D : WDlcomD to gDDksforgDDks

2. Example Java String replace(String target, String replacement)

We can implement replace() method with CharSequence just like with char.

Below is the implementation of the above method:

Java

// Java Program to implement
// replace() method
import java.io.*;

class GFG {
    public static void main(String[] args)
    {
        String s1 = "GeeksforGeeks";

        // orignal string
        System.out.println(s1);

        // Replace Geeks with Gfg
        String replaceString = s1.replace("Geeks", "GfG ");

        // New String
        System.out.println(replaceString);
    }
}

Output

GeeksforGeeks
GfG forGfG 

Exceptions with replace() Method

1. NullPointerException

The null regular expression is not accepted by the replace() method, it raises the NullPointerException.

Java

// Java Program to implement
// Java replaceAll() method
import java.io.*;

// Driver Class
class GFG {
    // Main function
    public static void main(String[] args)
    {
        String str = "GeeksforGeeks";
        int size = str.length();

        System.out.println(str);
        String target = null;

        // replacing null with GFG
        str = str.replace(target, "GFG");

        System.out.println(str);
    }
}

Output

Exception in thread "main" java.lang.NullPointerException
    at java.base/java.lang.String.replace(String.java:2142)
    at GFG.main(GFG.java:12)

Java String replaceFirst()

Just like replace() method this method replaces the first substring of this string that matches the given regular expression with the given replace_str. 

Syntax of Java String replaceFirst()

public String replaceFirst(String regex, String replace_str)

Parameters

• regex: the regular expression to which this string is to be matched.
• replace_str: the string that would replace the found expression.

Return Value

• This method returns a resulting String.

Example of Java String replaceFirst()

Below is the implementation of the above topic:

Java

// Java code to demonstrate the
// working of replaceFirst()

// Driver Class
public class rep3 {
    // main function
    public static void main(String args[])
    {
        // Initialising String
        String Str = new String("Welcome to geeksforgeeks");

        // original string
        System.out.print("Original String : ");
        System.out.println(Str);

        // Using replaceFirst to replace regex with
        // replace_str Replaces 1st occurrence of geeks with
        // ASTHA
        System.out.print(
            "After replacing 1st occurrence of regex with replace_str : ");
        System.out.println(
            Str.replaceFirst("geeks", "ASTHA"));
    }
}

Output

Original String : Welcome to geeksforgeeks
After replacing 1st occurrence of regex with replace_str : Welcome to ASTHAforgeeks