Strings and Text

Introduction

Text is everywhere: names, messages, file contents. In Java, text is an object of the class String. It has many useful methods, and a few traps that catch every beginner. Let's learn both.

String methods

Java

public class Main {
    public static void main(String[] args) {
        String text = "  Hello, Java World  ";

        System.out.println(text.length());
        System.out.println(text.trim());
        System.out.println(text.toUpperCase());
        System.out.println(text.trim().toLowerCase());
        System.out.println(text.trim().charAt(0));
        System.out.println(text.indexOf("Java"));
        System.out.println(text.contains("World"));
        System.out.println(text.trim().substring(7));
        System.out.println(text.trim().replace("Java", "Python"));
    }
}

21
Hello, Java World
  HELLO, JAVA WORLD  
hello, java world
H
9
true
Java World
Hello, Python World

Positions start at 0. substring(7) takes everything from position 7 onwards, and substring(0, 5) takes positions 0 to 4 (the end is not included).

Strings are immutable

A String can never change after it is created. Methods like toUpperCase() return a new String and leave the original alone. A common mistake is to forget to use the result:

Java

public class Main {
    public static void main(String[] args) {
        String name = "asha";

        name.toUpperCase();                 // result thrown away
        System.out.println(name);

        name = name.toUpperCase();          // keep the result
        System.out.println(name);
    }
}

asha
ASHA

Quick quiz

What does text.toUpperCase() do to the original String text?

Comparing strings: equals, not ==

This is the most important String rule in Java. == checks whether two variables refer to the same object. equals() checks whether the text is the same.

Java

public class Main {
    public static void main(String[] args) {
        String a = "java";
        String b = new String("java");

        System.out.println(a == b);
        System.out.println(a.equals(b));
        System.out.println("JAVA".equalsIgnoreCase(a));
        System.out.println("apple".compareTo("banana") < 0);
    }
}

false
true
true
true

a and b hold the same letters, but == says false, because they are two different objects. Always compare text with .equals(). compareTo gives a negative number if the first comes before the second alphabetically.

Joining and splitting

Java

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        String first = "Ada";
        String last = "Lovelace";

        System.out.println(first + " " + last);
        System.out.println(first.concat("!"));
        System.out.println(String.join("-", "a", "b", "c"));

        String csv = "red,green,blue";
        String[] colours = csv.split(",");
        System.out.println(colours.length);
        System.out.println(Arrays.toString(colours));
        System.out.println("ha".repeat(3));
        System.out.println("".isEmpty() + " " + "   ".isBlank());
    }
}

Ada Lovelace
Ada!
a-b-c
3
[red, green, blue]
hahaha
true true

StringBuilder

Because Strings cannot change, building a long text with + in a loop creates many throwaway objects. A StringBuilder is a text you can change, which is much more efficient.

Java

public class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();

        for (int i = 1; i <= 5; i++) {
            sb.append(i);
            if (i < 5) {
                sb.append(", ");
            }
        }

        System.out.println(sb.toString());
        System.out.println(sb.reverse());
        System.out.println(sb.length());
    }
}

1, 2, 3, 4, 5
5 ,4 ,3 ,2 ,1
13

Formatting output

String.format (or System.out.printf) builds text from a template with placeholders. Common ones: %s for text, %d for whole numbers, %.2f for a decimal with 2 places, %n for a new line.

Java

public class Main {
    public static void main(String[] args) {
        String name = "Asha";
        int items = 3;
        double total = 12.5;

        System.out.printf("%s bought %d items for %.2f%n", name, items, total);
        System.out.println(String.format("|%8s|%-8s|", "right", "left"));
        System.out.println(String.format("%05d", 42));
        System.out.println(String.format("%,d", 1234567));
    }
}

Asha bought 3 items for 12.50
|   right|left    |
00042
1,234,567

Text blocks

For several lines of text, use triple quotes (Java 15 and newer). Line breaks and the indentation relative to the closing quotes are kept.

Java

public class Main {
    public static void main(String[] args) {
        String poem = """
            Roses are red,
            Violets are blue,
              Java is fun,
            And so are you.
            """;
        System.out.print(poem);
    }
}

Roses are red,
Violets are blue,
  Java is fun,
And so are you.

Quick quiz

How should you check whether two Strings contain the same text?

Try it yourself

On your computer, write a program with a method static boolean isPalindrome(String word) that returns true when the word reads the same backwards. Hint: build a reversed copy with new StringBuilder(word).reverse().toString() and compare with equals. Test it with "level" and "java".

Summary

Strings are objects with methods such as length, substring, indexOf, replace, split and trim. They never change: methods return new Strings.
Compare text with equals(), not ==.
StringBuilder builds text efficiently in loops.
String.format and printf use placeholders like %s, %d and %.2f. Text blocks """ hold multi-line text.

Strings and Text

In this lesson you will

Introduction

String methods

Strings are immutable

Comparing strings: equals, not ==

Joining and splitting

StringBuilder

Formatting output

Text blocks

Try it yourself

Summary

Lesson complete