1.10 Calling Class Methods

1.10 Calling Class Methods

A class method is called through a class name because it belongs to the class itself, not to any particular object. In Java, the call Math.max(8, 13) asks the Math class to perform an operation; no individual Math object is needed.

Class methods versus instance methods

The dot in a method call does not by itself tell you what kind of method is being called. Identify the expression before the dot:

• Math.max(8, 13) uses the class name Math, so max is a class method.
• name.length() uses the variable name, which refers to a String object, so length is an instance method.
• robot.move() uses the variable robot, so move is an instance method.

Class method: A method associated with a class and called using the class name.
Instance method: A method associated with one object and called using a reference to that object.

The call must also match an available method signature: the method name, number of arguments, and argument types must be appropriate. If no matching method exists, the program does not compile.

int larger = Math.max(8, 13);       // valid: returns 13
double root = Math.sqrt(81.0);      // valid: returns 9.0
int answer = Math.max(4.2, 7.1);    // does not compile

The last statement fails because Math.max does not provide a version that accepts two double arguments and returns an int. A class method call can therefore be evaluated in two stages:

1. Compile-time check: Does the class provide a compatible method?
2. Execution trace: If it compiles, what value does the method return?

This directly develops 1.10.A: Develop code to call class methods and determine the result of the call.

Using the standard library

Java’s standard library supplies classes whose methods solve common problems. The Java Class Constructors and Methods Reference is not merely a list to memorize: use it to check the exact method name, parameter types, return type, and behavior.

For example, Math contains class methods for numerical operations:

int low = Math.min(14, 9);       // 9
double magnitude = Math.abs(-3.5); // 3.5
double randomValue = Math.random(); // 0.0 inclusive to 1.0 exclusive

Because these methods are associated with Math, writing new Math() is unnecessary and is outside the intended pattern. The class name acts like a shared toolbox label: choose a tool, supply its arguments, and receive the result.

String objects and String methods

A String object represents a sequence of characters. It can be created with a string literal or with the String class constructor. Since String belongs to java.lang, it is available without an import statement.

String first = "Blue";
String second = new String("Sky");

The two variables refer to String objects containing the character sequences "Blue" and "Sky". Methods such as length, substring, indexOf, equals, and compareTo operate on a String object:

String label = "BlueSky";

int count = label.length();          // 7
String part = label.substring(0, 4); // "Blue"
boolean same = label.equals("BlueSky"); // true

These are not class-method calls in the form String.length(). They are instance-method calls because label identifies the particular String object whose characters should be examined. Confusing String.length() with label.length() is a common compile-time error: the class name does not identify an individual sequence.

Combining strings

Strings can be concatenated—joined end to end—with + or +=. Concatenation creates a new String object; it does not alter an existing String object.

String city = "San";
String place = city + " " + "Jose";
city += " Diego";

System.out.println(place); // San Jose
System.out.println(city);  // San Diego

The original "San" sequence was not edited. The expression city + " Diego" produced a new String, and the assignment changed which object city refers to. A primitive value can also be concatenated with a String; Java implicitly converts the primitive to text.

int score = 27;
String message = "Score: " + score; // "Score: 27"

Skills in action

This topic exercises 2.C: Write program code involving procedural abstractions, especially when selecting and calling a library method; 3.C: Determine the result or output based on code that contains procedural abstractions, by tracing returned values; 4.A: Describe the behavior of a code segment or program; and 4.B: Describe the initial conditions that must be met for a code segment to work as intended or described.

Misconception check: “Every method after a class name is valid if the method name sounds reasonable.” Not so. Math.sqrt(25) compiles, but String.length() does not: the first is a class-method call, while the second incorrectly tries to call an instance method without an object.

Retrieval check: For each expression, identify whether it compiles and whether it calls a class or instance method: Math.abs(-6), "hello".length(), and String.length(). The first two compile; Math.abs is a class-method call, "hello".length() is an instance-method call, and String.length() does not compile.