Programming Tutorials

concat(), replace(), and trim() Strings in Java

By: Ivan Lim in Java Tutorials on 2007-09-12  

concat()

You can concatenate two strings using concat(), shown here:

String concat(String str)

This method creates a new object that contains the invoking string with the contents of str appended to the end. concat() performs the same function as +. For example,

String s1 = "one";
String s2 = s1.concat("two");

puts the string "onetwo" into s2. It generates the same result as the following sequence:

String s1 = "one";
String s2 = s1 + "two";

replace()

The replace() method replaces all occurrences of one character in the invoking string with another character. It has the following general form:

String replace(char original, char replacement)

Here, original specifies the character to be replaced by the character specified by replacement. The resulting string is returned. For example,

String s = "Hello".replace('l', 'w');

puts the string "Hewwo" into s.

trim()

The trim() method returns a copy of the invoking string from which any leading and trailing whitespace has been removed. It has this general form:

String trim()

Here is an example:

String s = " Hello World ".trim();

This puts the string "Hello World" into s.

The trim() method is quite useful when you process user commands. For example, the following program prompts the user for the name of a state and then displays that state's capital. It uses trim() to remove any leading or trailing whitespace that may have inadvertently been entered by the user.

// Using trim() to process commands.
import java.io.*;

class UseTrim {
    public static void main(String args[])
            throws IOException {
        // create a BufferedReader using System.in
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String str;
        System.out.println("Enter 'stop' to quit.");
        System.out.println("Enter State: ");
        do {
            str = br.readLine();
            str = str.trim(); // remove whitespace
            if (str.equals("Illinois"))
                System.out.println("Capital is Springfield.");
            else if (str.equals("Missouri"))
                System.out.println("Capital is Jefferson City.");
            else if (str.equals("California"))
                System.out.println("Capital is Sacramento.");
            else if (str.equals("Washington"))
                System.out.println("Capital is Olympia.");
            // ...
        } while (!str.equals("stop"));
    }
}

Here is a sample output

Enter 'stop' to quit.
Enter State:
Washington
Capital is Olympia.
Missouri
Capital is Jefferson City.
stop