String vs StringBuilder in Java - Practice Questions

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Challenge - 5 Problems

💻 code output

intermediate

Output of String concatenation in loop

What is the output of the following Java code?

Java

public class Test {
    public static void main(String[] args) {
        String s = "";
        for (int i = 0; i < 3; i++) {
            s += i;
        }
        System.out.println(s);
    }
}

A null012

B 0 1 2

C 0123

D 012

💻 code output

intermediate

Output of StringBuilder append in loop

What is the output of this Java code using StringBuilder?

Java

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 3; i++) {
            sb.append(i);
        }
        System.out.println(sb.toString());
    }
}

A 0 1 2

B 012

C null012

D 0123

💻 code output

advanced

Performance difference in string concatenation

Consider these two code snippets. Which one is faster for large loops and why?

1) Using String with += in a loop

2) Using StringBuilder with append in a loop

A StringBuilder is faster because it modifies the same object without creating new ones.

B String with += is faster because it is simpler to write.

C Both have the same speed because Java optimizes string concatenation automatically.

D String with += is faster because it uses less memory.

💻 code output

advanced

Output of StringBuilder reverse method

What is the output of this Java code?

Java

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("hello");
        sb.reverse();
        System.out.println(sb.toString());
    }
}

A olleh

B hello

C null

D HELLO

🧠 conceptual

expert

Immutability and thread safety of String vs StringBuilder

Which statement correctly describes the difference between String and StringBuilder in Java regarding immutability and thread safety?

A Both String and StringBuilder are immutable and thread-safe.

B String is mutable and thread-safe; StringBuilder is immutable and not thread-safe.

C String is immutable and thread-safe; StringBuilder is mutable and not thread-safe.

D Both String and StringBuilder are mutable and not thread-safe.