String Tools & Equality

String methods let you inspect or transform text. trim() returns text without leading and trailing ordinary whitespace; isEmpty() tests whether its length is zero. toLowerCase() and toUpperCase() return case-converted text. A String’s contents do not change in place: store the returned value if you want to use it later. We use simple English text in these examples.

A String variable refers to a text object. Two variables can refer to the same object, or to separate objects with equal text. Think of two labels attached to one notebook versus two notebooks containing the same words.

Use equals to compare String contents. == asks whether both variables refer to the very same object, which is a different question. Java may reuse literal strings, so == can appear to work in one example and then fail with text constructed at runtime. The mistake is the comparison rule, not a random behavior of the input reader.

Choose rules before cleanup

Removing outer spaces can be helpful, but do not silently remove internal spaces or change case unless the task calls for it. A person’s topic title can preserve its spelling while an answer comparison deliberately ignores surrounding spaces. Keep the original and cleaned values separate when you need to explain the transformation. Check empty text as well as a normal word.

public class Main {
    public static void main(String[] args) {
        String raw = " Java Basics ";
        String clean = raw.trim();

        System.out.println(raw.length());
        System.out.println(clean.length());
        System.out.println(clean);
        System.out.println("   ".trim().isEmpty());
    }
}

13
11
Java Basics
true

trim returns a result and leaves raw unchanged. Internal space between Java and Basics remains. Blank input consisting of spaces becomes empty after this particular cleanup rule.

In the next example, new String("Java") deliberately makes a separate text object. Ordinary text declarations do not need this extra step; we use it here only to make the difference between identity and contents visible.

public class Main {
    public static void main(String[] args) {
        String literal = "Java";
        String separate = new String("Java");

        System.out.println(literal == separate);
        System.out.println(literal.equals(separate));
    }
}

false
true

new String deliberately creates a different object for this comparison. The contents are still Java. Do not memorize which strings happen to share identity; use equals when the question is about contents.

charAt(index) returns one char at a zero-based position. A char is a single character value in these English examples; a literal such as 'L' uses single quotes. A String uses double quotes and can hold several characters.

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

        System.out.println(topic.charAt(0));
        System.out.println(topic.charAt(topic.length() - 1));
        System.out.println(topic.toLowerCase());
    }
}

L
s
loops

charAt returns a char at a zero-based position. A char literal uses single quotes, such as ‘L’; a String uses double quotes. Empty text has no valid character position, just as an empty array has no element.

A reference can also be null, meaning it refers to no object. This is different from "", which is an actual String with no characters. Calling a method through null fails. Non-null means an actual object is supplied. The following exercise assumes an actual array containing actual Strings, including the wanted value.

Your turn

public class Main {
    public static void main(String[] args) {
        System.out.println(containsTopic(new String[] {"Java", "Loops"}, "Loops"));
    }

    static boolean containsTopic(String[] topics, String wanted) {
        return false; // Replace the placeholder.
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(containsTopic(new String[] {"Java", "Loops"}, "Loops"));
    }

    static boolean containsTopic(String[] topics, String wanted) {
        for (String topic : topics) {
            if (topic.equals(wanted)) {
                return true;
            }
        }

        return false;
    }
}

true

Change it and explain

Show that the first element can fail to match while the second succeeds. This case catches an early false return inside the loop. Explain why a search that returns true for every nonempty list would still pass one poorly chosen test.

Day 6 will teach you to reject missing text before calling String methods. For now, the exercise supplies actual Strings so you can focus on content comparison.