1.15 String Manipulation

A String is an object that holds a sequence of characters. You can create one with a literal, like "hello", or with the constructor, new String("hello"). String is in java.lang, so no import is needed.

Strings are immutable: once a String object exists, its characters can never change. Methods like substring don't change the original. They return a brand-new string. To keep the result, you must store it, often back in the same variable: name = name.substring(1);.

The + and += operators join, or concatenate, strings into a new string. If one side of + is a string, the other side is converted to a string automatically, whether it's a number, a boolean or an object. An object is converted by calling its toString method, which every class inherits from Object. Many classes override toString: they replace the inherited version with their own method that has the same signature, so the text describes that object. That's why printing an ArrayList shows its elements in square brackets. You won't write your own toString on the exam.

+ works left to right, which leads to a classic trap:

JavaSystem.out.println("Sum: " + 2 + 3); System.out.println(2 + 3 + " is the sum"); System.out.println("Sum: " + (2 + 3));

The first line prints Sum: 23: the string comes first, so 2 and then 3 are each tacked on as text. The second prints 5 is the sum, because 2 + 3 is added as numbers before any string appears. The third uses parentheses to force the addition and prints Sum: 5.

Each character has an index, its position, starting at 0. In "computer", c is at index 0 and r is at index 7. The valid indexes run from 0 to length() - 1. Using an index outside that range throws a StringIndexOutOfBoundsException.

There's no method on the Quick Reference that gives you a single character, so you take a one-letter substring: s.substring(i, i + 1) is the character at index i, as a String.

substring(from, to) gives to - from characters. to may equal length(), and substring(length()) is the empty string "", not an error. Going past length() is an error.

Compare strings with equals, not ==. == checks whether two variables refer to the same object (2.6), so two separately created strings with the same letters can make == false while equals is true. With compareTo, rely only on the sign of the result, not its exact value. Java puts every uppercase letter before every lowercase one, so "Zoo".compareTo("apple") is negative. Watch for that when words mix capital and small letters.