Strings in depth: interview questions & scenarios

Model answer

String is immutable: every "change" returns a new object, which makes it safe to share, cache and use as a map key. StringBuilder and StringBuffer are mutable buffers (a byte[] with spare capacity) for building text step by step and calling toString() once at the end.

The only real difference between the two builders is that StringBuffer (Java 1.0) makes every method synchronized, while StringBuilder (Java 5) has no locks. Use StringBuilder by default; strings are almost always built in a local variable on one thread.

Model answer

String a = "PAID";
String b = "PAID";
String c = new String("PAID");
String d = c.intern();

System.out.println(a == b);       // true
System.out.println(a == c);       // false
System.out.println(a.equals(c));  // true
System.out.println(a == d);       // true

Both literals resolve to the same pooled object, so a == b. new always creates a fresh object, so a == c is false even though the content matches. intern() returns the pooled instance for "PAID", which is a.