String Manipulation: Revision Notes (Computer Science A 1.15)

Short on time? This page is the recap. For explanations and worked examples, use the full study guide.

Recap

A String object represents a sequence of characters. It is created with a string literal or by calling the String constructor. The class is in java.lang, so it is available by default.
A String is immutable: once created, its attributes cannot be changed. Methods called on it do not change its content.
+ and += concatenate and produce a new String. A primitive concatenated with a String is implicitly converted to a String. A String concatenated with any object implicitly calls that object’s toString method.
A String has indices from 0 to one less than its length. Using an index outside the range throws a StringIndexOutOfBoundsException.
substring(index, index + 1) is a one-character string.

Method	Result for `"microscope"` (length 10)
`length()`	`10`
`substring(2, 6)`	`"cros"` (indices 2 to 5)
`substring(6)`	`"cope"`
`substring(0, 1)`	`"m"`
`substring(10)`	`""` (legal)
`indexOf("sco")`	`5`
`indexOf("z")`	`-1`
`equals("microscope")`	`true`
`compareTo("microscope")`	`0`

Write the string with its indices under every character (spaces count).
For substring(from, to) mark from and the character just before to.
Use indexOf to find positions instead of counting.
Remember immutability: an unstored result is lost; a reassignment makes a new string.
Check 0 <= from <= to <= length(); otherwise the answer is a StringIndexOutOfBoundsException.

To “change” one letter, build a new string from the parts around it: s.substring(0, 3) + "x" + s.substring(4).

substring(from, to) includes the character at from and excludes the one at to.
compareTo is guaranteed only in its sign: below 0, zero or above 0.
Compare string contents with equals, not ==.
A String variable that holds null causes a NullPointerException when a method is called on it.
A string identical to the single character at index is substring(index, index + 1).

Calling word.substring(1); and expecting word to change. Store the result.
Off-by-one errors with substring(from, to): forgetting that to is excluded.
Using index length() for a character (the last index is length() - 1).
Misreading "a" + 1 + 2. Work left to right.
Treating -1 from indexOf as an index.
Comparing strings with ==.
Expecting a particular value from compareTo instead of its sign.

What is "banana".substring(1, 4)? ("ana")
What is "banana".indexOf("na")? (2)
What does 3 + 4 + "x" evaluate to? ("7x")
What happens with "abc".substring(1, 5)? (StringIndexOutOfBoundsException)
After String s = "go"; s += "al"; what does s refer to? (A new string, "goal".)