3.1.2Attributes and methods

• Attributes (also called fields, properties or instance variables) store the state of an object.
• Methods define the behaviour of an object.

public class Student {
    // attributes
    String name;
    int marks;

    // methods
    void display() {
        System.out.println(name + " scored " + marks);
    }

    String getGrade() {
        if (marks >= 75) return "A";
        else if (marks >= 65) return "B";
        else if (marks >= 50) return "C";
        else return "F";
    }
}

A method has a return type (void means it returns nothing), a name, a parameter list and a body.

Attributes in detail

An attribute is declared inside the class but outside every method. Every object gets its own copy, so two Student objects each have their own name and marks. If you do not give an attribute a value, Java gives it a default value:

Local variables (variables declared inside a method) do not get default values. Using one before you give it a value is a compile error: variable total might not have been initialized.

Four kinds of variable

Methods in detail

String getGrade()                    // return type String, no parameters
void setMarks(int newMarks)          // returns nothing, one parameter
double average(int a, int b, int c)  // returns a double, three parameters

• The return type says what kind of value the method sends back. A method with a return type other than void must end every path with a return statement that gives a value of that type.
• The parameters are the inputs that the method needs. The values you pass when you call it are called arguments: in average(70, 80, 90), a, b and c are the parameters, and 70, 80 and 90 are the arguments.
• The name and parameter list together are called the method's signature.
• return ends the method immediately. That is why getGrade() above can use if ... return without nested else blocks.

Inside a method you can use the object's attributes directly. display() uses name and marks without receiving them as parameters, because they belong to the object the method was called on.