Strings and Methods — Interview Questions¶

String a = "Hello";
String b = new String("Hello");

System.out.println(a == b);       // false — different objects
System.out.println(a.equals(b));  // true  — same content

String s = "hello";
s.toUpperCase();           // creates "HELLO" but s is still "hello"
s = s.toUpperCase();       // now s points to "HELLO"

String a = "Hello";  // creates in pool
String b = "Hello";  // reuses from pool
System.out.println(a == b);  // true — same pool object

// StringBuilder — fast, not thread-safe
StringBuilder sb = new StringBuilder();
sb.append("Hello").append(" World");

// StringBuffer — slower, thread-safe
StringBuffer sbuf = new StringBuffer();
sbuf.append("Hello").append(" World");

String s1 = new String("Hello");
String s2 = s1.intern();
String s3 = "Hello";

System.out.println(s1 == s3);  // false — s1 is on heap
System.out.println(s2 == s3);  // true  — both from pool

String ascii = "Hello";       // LATIN1: 5 bytes for data
String emoji = "Hello \uD83D\uDE00"; // UTF16: 14 bytes for data

// ❌ Compiles regex 10,000 times
for (String line : lines) {
    if (line.matches("\\d{4}-\\d{2}-\\d{2}")) { ... }
}

// ✅ Compiles regex ONCE
private static final Pattern DATE_PATTERN = Pattern.compile("\\d{4}-\\d{2}-\\d{2}");

for (String line : lines) {
    if (DATE_PATTERN.matcher(line).matches()) { ... }
}

new StringBuilder().append(a).append(b).toString()

String s = "\u2003Hello\u2003"; // Em-space (Unicode)
System.out.println(s.trim().equals("Hello"));   // false — trim doesn't recognize em-space
System.out.println(s.strip().equals("Hello"));   // true  — strip handles Unicode

// Also available:
" hello ".stripLeading();  // "hello "
" hello ".stripTrailing(); // " hello"

// ❌ Password as String
String password = "secret123";
// Cannot clear! "secret123" stays in memory

// ✅ Password as char[]
char[] password = {'s','e','c','r','e','t','1','2','3'};
// ... authenticate ...
Arrays.fill(password, '\0'); // zero out after use

mapper.configure(JsonFactory.Feature.CANONICALIZE_FIELD_NAMES, true);

@GetMapping(value = "/export", produces = "text/csv")
public ResponseEntity<StreamingResponseBody> exportCsv() {
    StreamingResponseBody stream = outputStream -> {
        BufferedWriter writer = new BufferedWriter(
            new OutputStreamWriter(outputStream, StandardCharsets.UTF_8));

        writer.write("id,name,email\n");

        // Stream from database in chunks
        try (Stream<User> users = userRepository.streamAll()) {
            users.forEach(user -> {
                try {
                    writer.write(user.getId() + "," +
                                 escape(user.getName()) + "," +
                                 escape(user.getEmail()) + "\n");
                    writer.flush(); // flush periodically
                } catch (IOException e) {
                    throw new UncheckedIOException(e);
                }
            });
        }
        writer.flush();
    };

    return ResponseEntity.ok()
        .header("Content-Disposition", "attachment; filename=export.csv")
        .body(stream);
}