Class and Instantiation

• Every method is associate with some class. Every file contains a class.
• To run a class, we must define a main method.
  • Not all the classes have a main method!(But we can have a test drive class for it.) (Dependent files will be Compiled automatically.)

Terminology

1. Instance variable: instance attribute.
2. Constructor(name should be same as class name).
3. Non-static method or static method: Belong to instance or class.

4. To create an array of objects:
- First use the new keyword to create the array.
- Then use new again for each object that you want to put in the array.

So the creation of array in Java needs new and so does the creation of instances. （By the way,when defining an array, the length could be a variable.）

Static and Non-Static

A class may have a mix of static and non-static members.

• A variable or method defined in a class is also called a member of that class.
• Static members are accessed using class name, e.g. Dog.binomen.
• Non-static members cannot be invoked using class name: Dog.makeNoise()
• Static methods must access instance variables via a specific instance, e.g. d1.

public class Dog {
    public int weightInPounds;
    public static String binomen = "Canis familiaris";
 
    public Dog(int startingWeight) {
        weightInPounds = startingWeight;
    }
 
    public static Dog maxDog(Dog d1, Dog d2) {
        if (d1.weightInPounds > d2.weightInPounds)
            return d1;
        return d2;
    }
 
    public void makeNoise() {
        if (weightInPounds < 10)
            System.out.println("yipyipyip!");
        else if (weightInPounds < 30)
            System.out.println("bark.");
        else
            System.out.println("woof. woof.");
    }
}

All in all, different from Python, the class attribute and instance attribute in Java should be declared with different keyword: static. And they can’t be called by another, static is for class, vice versa.

By the way, the Java version self is this and can be omitted if there’s no variable in same name.
One more thing is that this in Java’s method is seen as constant so we can’t change it.