The String methods used on the AP CS A Exam are listed at the top of the AP CS A Java Quick Reference.
String(String) constructor and equals method
String objects are commonly constructing by enclosing a sequence of characters in double quotes. String objects can also be constructed by explicitly calling the String constructor that takes a String. The behavior is subtly different because of the Java String Pool. The String constructor that takes a String parameter explicitly constructs a new String object, even if an identical object already exists in the Pool.
The equals method returns true if the implicit and explict parameters represent the same sequence of characters; otherwise, it returns false. All comparisons of String objects for equality should use the equals method (or the compareTo method).
The Java String Pool makes it possible to write examples in which the == operator and the equals method appear to behave identically. It is not necessary to recognize such examples on the AP CS A Exam. The == operator should not be used to compare String objects for equality, even though it appears to work in some examples.
The equals method accepts null as an explicit parameter/argument, and returns false. As with all method calls, null can not be an implicit parameter.
String(String) constructor and equals method examples
String str1 = "Brandon";
String str2 = "Brandon";
String str3 = new String("Brandon");
String str4 = "Horn";
System.out.println(str1.equals(str2)); // prints true
System.out.println(str1.equals(str3)); // prints true
System.out.println(str1.equals(str4)); // prints false
System.out.println(str1.equals(null)); // prints false
// The examples below do NOT show the correct way to compare
// String objects, even though some output the correct result.
System.out.println(str1 == str2); // prints true
System.out.println(str1 == str3); // prints false
System.out.println(str1 == str4); // prints false
System.out.println(null.equals(str1)); // compile time error
length method
The length method returns the number of characters in a String. It does not return the index of the last character.
length examples
String name = "Brandon";
// 0123456
String empty = "";
System.out.println(name.length()); // prints 7
System.out.println(empty.length()); // prints 0
substring(int, int) method
The 2 parameter substring method returns a new String containing the characters starting at the first index up to but not including the second index. Since the character at the second index is not included, the second index can be 1 larger than the last valid index.
If the 2 parameter substring method is called with the same index value for both parameters, the behavior depends on the index. If the index if valid or no more than 1 larger than the last valid index, the method returns an empty String. If the index is invalid, other than as described, the method throws an exception.
String objects are immutable. A call to substring returns a new String. It does not modify the implicit parameter. All methods of String that appear to modify the implicit parameter actually return a new String.
substring(int, int) examples
String name = "Brandon";
// 0123456
String empty = "";
System.out.println(name.substring(2, 5)); // prints "and"
System.out.println(name.substring(2, 3)); // prints "a"
System.out.println(name.substring(4, 4)); // prints ""
System.out.println(name.substring(6, 7)); // prints "n"
System.out.println(name.substring(7, 7)); // prints ""
System.out.println(name.substring(0, name.length())); // prints "Brandon"
System.out.println(empty.substring(0, 0)); // prints ""
// Each of the statements below throws a StringIndexOutOfBoundsException.
System.out.println(name.substring(6, 8));
System.out.println(name.substring(7, 8));
System.out.println(name.substring(8, 8));
System.out.println(name.substring(-1, 0));
System.out.println(name.substring(-1, -1));
System.out.println(empty.substring(0, 1));
substring(int) method
The 1 parameter substring method returns a new String containing the character(s) starting at the parameter index to the end of the String.
The 1 parameter substring method allows the index to be 1 larger than the last valid index in the String, without throwing an exception. If the parameter index is the length, the 1 parameter substring method returns an empty String.
As with substring(int, int), substring(int) does not modifiy the implicit parameter. It returns a new String.
substring(int) examples
String name = "Brandon";
// 0123456
String empty = "";
System.out.println(name.substring(5)); // prints "on"
System.out.println(name.substring(6)); // prints "n"
System.out.println(name.substring(7)); // prints ""
System.out.println(name.substring(0)); // prints "Brandon"
System.out.println(empty.substring(0)); // prints ""
// Each of the statements below throws a StringIndexOutOfBoundsException.
System.out.println(name.substring(8));
System.out.println(empty.substring(1));
indexOf method
The indexOf method finds where one String (the explicit parameter/argument) starts inside another String (the implicit parameter). indexOf returns -1 if the explicit parameter is not found in the implicit parameter.
The String method contains is not used on the AP CS A multiple choice. The indexOf method is used to check if a String contains another String.
indexOf examples
String name = "Brandon";
// 0123456
String empty = "";
System.out.println(name.indexOf("a")); // prints 2
System.out.println(name.indexOf("n")); // prints 3
System.out.println(name.indexOf("and")); // prints 2
System.out.println(name.indexOf("j")); // prints -1
System.out.println(name.indexOf("")); // prints 0
System.out.println(empty.indexOf("")); // prints 0
System.out.println(empty.indexOf("j")); // prints -1
System.out.println(name.indexOf("and") != -1); // prints true
System.out.println(name.indexOf("n") >= 0); // prints true
System.out.println(name.indexOf("or") != -1); // prints false
compareTo method
The compareTo method is used to compare String objects for order. See compareTo on the AP CS A Exam for a detailed explanation and examples.
String concatenation
The + operator performs concatenation when at least one of its operands is a String.
The += operator performs concatenation when the left operand is a String.
The char data type is not used on the AP CS A Exam. When both operands to the + operator are of type char, the operator adds the unicode values. Use String objects and substring calls instead of char values and charAt calls to avoid this issue (and others) on the AP CS A free response.
String concatenation examples
"1" + "2" evaluates to "12".
"1" + 2 + 3 evaluates to "123".
The order operations is ("1" + 2) + 3. The result of "1" + 2 is a String.
1 + "2" + 3 evaluates to "123".
The order operations is (1 + "2") + 3. The result of 1 + "2" is a String.
1 + 2 + "3" evaluates to "33".
The order of operations is (1 + 2) + "3". The result of 1 + 2 is an int.
1 + (2 + "3") evaluates to "123".
String str = "";
str += 2;
str += " fish";
System.out.println(str); // prints "2 fish"
Practice String manipulation with AP CS Tutor Brandon Horn.
String immutability
String name = "Brandon";
name.toUpperCase();
System.out.println(name); // prints Brandon
name = name.toUpperCase();
System.out.println(name); // prints BRANDON
String objects are immutable. Once a String object has been created, it cannot be changed. All of the methods of String that appear to change the String actually return a new String.
As demonstrated above, it is possible to assign an existing variable to refer to the new String.
It is common for beginning programmers to work with String objects before understanding much about the difference between primitive types and objects. Primitive types vs references exercises demonstrates the difference.
char values
The char data type is not part of the AP Java Subset and is not used on the AP CS A Exam. I discourage students from using the char type in AP CS free response answers since it can lead to common errors (in particular, adding the Unicode values of 2 char values).
The pages below contain information about the char data type.
Help & comments
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